This essay is intended to bring to light a few of the issues surrounding
the use of metal for boats. You can access any of the specific topics
via the links above.
While the pros and cons of various metals expressed here are quite
relevant to one's choice of hull material, they are also central to the
actual process of designing and building in metal, whether one chooses in
favor of steel, aluminum, copper nickel, monel, stainless, or what have
you...
The following is therefore not solely aimed at potential metal boat
owners, but also at boat builders and designers who may wish to make better
use of metal as a structural material for boats.
One of the primary choices one will face when considering metal is just
which metal to use, where to use it, and what metals are best suited to each
vessel type. To begin the discussion, here are a few brief thoughts with
regard to steel versus aluminum…
If an existing boat design is being considered, in other words a vessel
that already has a fixed hull shape, then we can very generally observe the
following: In terms of sea kindliness, some boats may be better if
built in steel, due mainly to the extreme lightness of aluminum, which in
some hulls may result in a slightly more harsh motion. This is the
case to a greater degree with larger boats or very beamy boats.
Provided that the design has adequate displacement and stability to carry
the added structural weight, many of these types will have a more gentle
motion at sea if built in steel.
On the other hand, somewhat narrower or lighter displacement boats will
often be best if constructed of aluminum. They'll generally have a
narrower waterplane, and so less inherent shape stability. Therefore,
due to their relatively narrower waterline, they will be likely to have an
easy motion at sea if built of aluminum.
One can say in general that it is a relatively easy matter to adapt a
steel vessel design to being built in aluminum, since the result will be a
greater degree of stability (less structural weight, more ballast).
One can also say that a design optimized for aluminum construction will not ordinarily be able to be built in steel, due to excessive weight of
structure, unless of relatively heavy displacement already.
If we were to start from scratch and create a new design, in other words
if we were to begin with a blank sheet of paper, these same considerations
will apply. In this case however, we have the chance to optimize the
hull form to take best advantage of the preferred material.
With steel, we must design a hull with sufficient displacement to
carry the structure. At 490 pounds per cubic foot, the weight adds up
very quickly indeed. For smaller vessels, say below around 35 feet,
this makes for a very burdensome and heavy hull. In larger sizes, say
above 40 feet, one can make excellent use of steel. Above 45 feet the
material really begins to come into its own. Above around 50 feet, a
steel hull can actually be quite light for her length (by traditional
cruising vessel standards).
I have somewhat arbitrarily given the lower limit of a good steel vessel
as being around 35 feet of length. This is of course not a fixed
limit. The boundary of what can be built in steel is less a matter of
boat length than it is a matter of shape and displacement. With proper
design, one can successfully create a steel boat for coastwise cruising or
serious blue water sailing possibly down to around 28 to 30 feet LOD.
Adequate displacement must be maintained to carry the structure, and thus
draft and beam may not be decreased below a certain point. Therefore,
roughly below around 30 feet the boat will be very much less optimum for the
task, due to having an overly heavy and relatively less graceful shape in
order to achieve a relatively large displacement to carry the structure.
There will be that much less carrying capacity remaining for fuel, water,
and the desired number of sandwiches and beer...!
For small vessels of say less than around 40 feet, one can make a very
convincing argument in favor of aluminum. At 168 pounds per
cubic foot, we can easily make use of greater plate thickness without much
of a weight penalty, and still have a sleek and graceful hull shape.
Generally, when built to the same strength standard as a steel vessel, a
bare aluminum hull "as fabricated" will weigh roughly 1/3 less than an
equivalent steel hull. As an added bonus, the lighter weight of
aluminum will permit a given hull form to be built with greater strength
than the same hull in steel. In other words, given the same weight
budget the aluminum structure will be able to have a higher strength than
the same design in steel.
After consideration of the following summary of the various metals that
are well suited for hull building, we can additionally say that any design
optimized for steel construction can be very readily adapted to the
use of Copper Nickel or Monel for the hull without making
changes to the hull shape, since overall weights will turn out to be within
a similar range and the placement of internal framing will usually be
identical or extremely similar.
We can also say that any design that has been optimized for aluminum
construction could be adapted to the use of Titanium for hull
structure without hull shape changes.
Overall, we will observe that whether originally designed for steel or
aluminum any of the yacht designs on these pages can be adapted to
the use of any of the other metals discussed below. If the hull
material is of a substantially different weight, then hull form changes will
usually be either desired or required.
For example, all of my designs intended for aluminum structure are
optimized for the lightness of that material. Therefore converting an
aluminum design to steel or to any of the copper alloys would require a
change of hull form in order to support the added weight. Often this can be
achieved simply by just adding beam. If beam is already fairly large for the
size of vessel, then one might prefer to deepen the hull.
Certainly any of these options will add cost, however there are
benefits. Overall, the primary objective is to create a hull that
behaves well, performs well, is sufficiently strong for its intended
purpose, and that adequately supports the weights of structure and the loads
intended.
More detail on these possibilities will follow...
Mild Steel: Due to fabrication issues, one cannot
readily make use of less than 10 gauge mild steel plating (0.134 inch, or 3.5
mm). Even 10 gauge mild steel plating can be very problematic to keep
fair. It will have much greater distortion levels while welding than
plate of a greater thickness. Of course, with a few essential metal
boat building tricks learned, it is not much trouble to avoid distortion
altogether in a 10 gauge steel hull. For an amateur builder however,
working in 10 gauge mild steel without knowledge of those tricks, the result
will often be uncontrollable distortion, sometimes to the point of being
quite an awful thing to behold.
The natural temptation then is to consider the use of greater plating
thickness. Of course, if one were to begin a design from scratch,
displacement may be designed to adequately carry a greater steel plating
thickness. However if one were tempted to simply apply greater
thickness of steel plating to an existing design intended for, say, 10 gauge
steel, the result will be a grossly over weight vessel which will neither
float at her intended waterline, nor be able to carry the required amount of
ballast. In the case of 10 gauge mild steel designs intended for
amateur construction, it will be vastly preferable to use a few more
longitudinals, strategically placed to help control distortion. Other
devious means may be employed as well, such as temporary external long's,
etc.
The upshot of the above is that, although one can design and build very
fine steel boats down to around 35 feet (give or take a few feet), these
smaller vessels will necessarily make use of 10 gauge mild steel plate and
they will therefore necessarily require much greater skill in building.
If the vessel can be large enough, say over 45 feet, or of heavy enough
displacement, then one can make use of 3/16 inch mild steel plating (just
under 5 mm). This thickness will be far easier to use since it will
resist distortion all the better.
Corten Steel: One twist to these considerations is that for smaller steel vessels which
must use 10 gauge steel for plating, we can make very good use of Corten
steel. Corten has about 40% greater yield strength than mild steel.
This means that 10 gauge Corten plate will resist welding distortion and
denting more or less the same as 3/16" mild steel plate.
In my view,
its higher yield strength is the primary justification for the use of Corten steel for metal
boats, rather than for any possible corrosion benefits. Although
Corten tends to rust much more slowly than mild steel, hulls built of
mild steel or of Corten steel must always be painted everywhere both
inside and out. Corten is just
as easy to weld and cut as mild steel, so aside from the slightly greater
cost of Corten, it is to be recommended for all steel vessels having a steel
plate thickness of less than 3/16 inch.
"Corten" is simply a brand name for one of the High Strength Low Alloy
steels (HSLA). The HSLA steels that are suitable for boat building
are usually referred to by their ASTM alloy number.
"Corten" is
often referred to as A-242, which is a general "sheet" specification for the
more specific A-606 Type 4 sheet spec. This same alloy is also called
A-588 when referring to plate (3/16 inch thickness and over).
An alloy sometimes specified for low temperature applications is
A-441, a designation that is now discontinued. Actually
referred to generically as "X-ten" steel, the new spec for this alloy
is A-607 when referring to sheet, or A-572 and A-572-M when referring to
plate. These "X-ten" alloys contain a small amount of vanadium
(A-572), or they may contain both vanadium and manganese (A-572-M).
The addition of these alloying elements in HSLA steels achieves
greater strength by producing a more refined microstructure as compared
with plain carbon steel (mild steel). The alloying elements
provide a smaller crystalline grain size and a fine dispersion of
alloyed carbides, thus provide higher yield strength without sacrificing
ductility.
In General: Steel is a bit more rugged than aluminum, being tougher and much more
abrasion resistant. The various HSLA steels, even more so. Welds in steel are
100% the strength of the surrounding plates, whether mild steel or Corten.
Steel is less prone to electrolysis problems than aluminum, and one can make
use of regular copper bottom paint on a steel hull.
Aluminum is light, strong, corrosion resistant, non sparking,
conducts electricity and heat well, and is readily weldable by MIG or TIG
processes. In terms of ease of construction, aluminum is excellent. It
can be cut with carbide tipped power tools, dressed with a router, filed and
shaped easily, and so forth. Aluminum is light, clean, and easy to
work with.
Aluminum is much faster to work with than steel, so there is quite a
labor savings. For example, welding aluminum is a very quick process.
In terms of thickness, 3/16 inch (around 5 mm) is generally considered the
minimum plate thickness for MIG welding. However, if pulsed MIG
welding is available one can make excellent use of 5/32 inch plating (4 mm),
particularly for deck and house structures.
Since aluminum provides the option to make use of much greater plate
thickness within a given weight budget, not only can strength be greater
than with steel, but as a result the distortion levels are relatively easily
managed.
Aluminum alloys for use on boats are generally limited to the 5000 and
the 6000 series. These two alloy groups are very corrosion resistant
in the marine environment due to the formation of a tough aluminum oxide.
These alloys are subject to pitting, but the pitting action slows as the
oxide film thickens with age.
Aluminum alloys are subject to crevice corrosion, since they depend on
the presence of Oxygen to repair themselves. What this means is that
wherever aluminum is in contact with anything, even another piece of
aluminum or zinc, it must be cleaned, properly prepared, and painted with an
adhesive waterproof paint like epoxy, and ideally also protected with a
waterproof adhesive bedding such as Sikaflex or 3M-5200.
Paint preparation is critical. Thorough cleaning, and abrasive grit
blasting will provide the best surface for adhesion of paint or bedding.
Alternately, a thorough cleaning and then grinding with a coarse 16 grit
disk will provide enough tooth for the paint to stay put.
Aluminum is very active galvanically, and will sacrifice itself to any
other metal it contacts either directly or indirectly. Aluminum is
anodic to everything except zinc and magnesium, and must be electrically
isolated from other metals. A plastic wafer alone as an isolator is
not sufficient. Salt water must be prevented from entering the
crevice, so properly applied epoxy paint, adhesive bedding, and a
non-conductive isolator should all be used together.
In aluminum, welds done in the shop are at best around 70% of the
strength of the plate (in the 5000 series). Usually, one will
compensate for the reduction in strength in the heat affected zone either by
providing a backup strip at any plate joint, and welding the plate joint
thoroughly on both sides, or by providing additional longitudinal members to
span any butt welds in the plating.
Ideally, plating butts will be located in the position of least stress.
For most general plating, this is ordinarily at one quarter of the span
between frames. In other words, with proper engineering and design,
the strength of aluminum in the heat affected zone is a non issue.
Aluminum hulls require special bottom paint. Organo-tin based
anti-fouling paints can no longer be used as bottom paint except in such
diluted formulations as to be very nearly useless. Currently, the best
antifouling paint for aluminum hulls is called "No-Foul EP-21" made by the
E-Paint Company (800-258-5998).
No-Foul EP-21 is an update of the original "No-Foul ZDF" which makes use
of a controlled release of hydrogen peroxide to prevent fouling.
Practical Sailor magazine did a controlled study of a large variety of
anti-fouling paints over several years, during which they discovered that
No-Foul ZDF outperformed ALL other antifouling paints during the first year
of immersion in all waters. They also discovered that No-Foul ZDF
performs significantly less well than the other AF paints during the second
year... The conclusion? Refreshing the No-Foul coatings annually
will result in a top performing system, as well as frequent inspection
intervals for the hull.
The new formulation for No-Foul EP-21 is considered to be an improvement
due to the addition of an environmentally preferred booster biocide that
helps control slime and grass. Another improvement is the change from
a vinyl binder to an epoxy. This makes the paint easier to apply and
allows it to be applied over a wider variety of existing paints.
Another big savings with aluminum is that it is ordinarily not necessary
to sand blast or paint the inside of the hull. Generally, due to its very
good conductivity one must insulate an aluminum hull extremely well.
The most common insulation is blown-in polyurethane foam, although one can
make an excellent case for the use of cut-sheet foams, such as Ensolite and
Neoprene. In the latter case, it may become desirable to lightly blast
the aluminum, and provide an epoxy primer coating prior to insulating.
On the exterior, except on the bottom and in places where things are
mounted onto the hull surface, it is completely unnecessary to paint an
aluminum hull. This represents such a large cost savings that if left
unpainted, building in aluminum will often cost LESS than building the same
vessel in steel.
We have already seen that a point in favor of aluminum is it's ability to
be used to design a much lighter weight boat than would be possible in
steel. This is a performance advantage as well as a cost advantage.
Not only will the lighter displacement boat be relatively less costly to
build, it will also be much less costly to push through the water.
This savings in hull structural weights is augmented then by being able to
achieve greater range under power, by being able to carry less fuel to
achieve the same range as a heavier vessel, and by being able to be powered
by a smaller engine.
One might argue that with a lighter boat there will possibly be less room
below, the lighter boat being narrower on the waterline, and possibly less
deep. With proper planning, this need not be an issue.
On the plus side, even though an aluminum boat may cost slightly more
than a steel vessel to build, an aluminum boat will have a much higher
re-sale value than a steel boat.
I am occasionally asked, "What about building a boat in Stainless?"
A structure built in stainless will weigh approximately the same as one
built in mild steel, although on occasion one may be able to make use of
somewhat lighter scantlings due to the somewhat higher strength of
stainless. There are several major drawbacks to the use of stainless,
not the least of which is cost. Stainless of the proper alloy will
cost nearly six times the price of mild steel!
Even if it were not so costly, stainless has numerous other problems:
- Stainless is quite difficult to cut, except by plasma arc.
- Stainless work hardens when being formed and can become locally
tempered such as when being drilled.
- Stainless deforms rather extremely when heated either for cutting or
for welding.
- Stainless, even in the low carbon types, is subject to carbide
precipitation in the heat affected zone adjacent to the weld, which
creates an area that is much more susceptible to corrosion as well as to
cracking.
- Stainless is subject to crevice corrosion when starved of oxygen.
This can be prevented only by sandblasting and painting the surfaces,
just as it would be done for aluminum, wherever an object is to be
mounted onto the stainless surface. The same applies to the back
side of any stainless fittings which are applied to hull surfaces.
If the above issues with stainless can be properly accounted for in the
design of the vessel and in the building of it, then stainless can be a
viable hull construction media.
Type 316-L stainless is generally the preferred alloy. Type 316-L is a
low carbon alloy, and is used in welded structures to help prevent carbide
precipitation in the heat affected zone. When available, the use of
type 321 or 347 stainless will be of considerable benefit in preventing
carbide precipitation, since there are other alloying elements (tantalum,
columbium, or titanium) which help keep the carbides in solution during
welding.
In my view, as a builder the main battle one will face is the rather
extreme distortion levels when fabricating with stainless. Stainless
conducts heat very slowly and has a high expansion rate. Both of these
characteristics conspire against maintaining fairness during weld-up.
Short arc MIG welding will be an imperative. In fact Pulsed MIG will
probably be desired in order to sustain the right arc characteristics
while lowering the overall heat input.
Another material which should be considered along with steel, stainless,
and aluminum is Copper Nickel. One can ignore paint altogether with CuNi, inside, outside, top and bottom. Copper Nickel acts as its own
natural antifouling. In fact, bare Copper Nickel plate performs better
than antifouling paint! Being a mirror-smooth surface, any minor
fouling is very easily removed. Copper Nickel is very easy to fabricate,
being both easy to cut and to weld using the MIG process.
Besides not having to paint CuNi and its natural resistance to
fouling, CuNi is also easy to cut and weld, it has relatively great heat conductivity,
it is extremely ductile, and it is therefore very favorable with regard to distortion while welding.
There are two alloys of Copper Nickel which are the most common: 70/30 CuNi, and 90/10 CuNi. The numbers represent the relative amounts of
Copper and Nickel in the alloy. Having a greater amount of Nickel,
70/30 CuNi is the stronger of the two. Having the higher Nickel
content, 70/30 is also the more expensive of the two.
In the US, 90/10 CuNi is currently priced (February 2007) around USD $8.50 per
pound, and 70/30 CuNi around USD $13.00 per pound, both based on a
minimum order of greater than 15,000 pounds.
The main issues with CuNi are not only those of cost, but also of
strength. For example, the ultimate strength of 90/10 Cu Ni is about
one third less than that of mild steel, and the yield strength about half
that of mild steel. In practice, this means that a hull built of Cu Ni
will need to make use of heavier scantlings. CuNi, being slightly
heavier than steel per cubic foot, the CuNi hull structure will end up
being slightly heavier than an equivalent steel hull structure.
In most materials, we will usually
"design to yield." This means that the ultimate failure strength of a
material is more or less ignored, and the yield strength is instead used as
the guide for determining scantlings. For example, if we were to
desire a 90/10 CuNi structure having the same yield strength as there
would be with a similar steel structure, then we might be tempted to
actually double the scantlings. Naturally this would result in quite a
huge weight penalty, BUT....
In practice, a CuNi structure need not be taken to this extreme.
Using the ABS rules to calculate the scantlings, an all 90/10 Cu Ni structure
will have around 25% more weight than a similar structure in steel.
It is best to use the same plate thickness as with steel, and
compensate for the lower yield strength by spacing the longitudinals more
closely, say using approximately half the stringer spacing prescribed for the same
thickness of mild steel plating.
It is unlikely that one would choose CuNi for the internal framing,
primarily because of its relatively low strength and the relatively much
larger scantlings and weight that would result. In other words, there is
no reason not to make use of CuNi for the hull skin only in order to take
full advantage of its benefits, but to use a stronger and less expensive
material for all the internal framing.
What is the best choice for the internal framing...? Probably type
316-L
Stainless. As long as the various attributes of stainless are kept in mind,
this is a combination having considerable merit. Here is why...
- Stainless can be readily welded.
- One can easily make a weld between stainless and Cu Ni.
-
Scantlings of stainless internal framing would not need to be
increased, in fact they would be less than those required for mild
steel.
-
The weight of stainless internal framing would therefore be
roughly 10% less than with mild steel, or approximately equal to the
weight of a Corten steel internal structure.
-
316-L Stainless costs (February 2007) around USD $4.50 per
pound based on a minimum order
of 10,000 pounds. Therefore the cost of stainless is roughly half
that of 90/10 Cu Ni, and about
one third the cost of
70/30 Cu Ni... Combined with there being much lighter scantlings,
the overall cost factor would be reduced considerably.
To reduce costs still further, it would be ideal to make use of NC plasma cutting
or water jet cutting for the entire frame of the boat.
We can see from the above that if one's budget can tolerate the
higher first-cost to build with CuNi, that the hull dimensions will be
slightly greater than would be the case for
a mild steel structure. We can also see that Copper Nickel plating,
combined with 316L Stainless internal framing will provide a nearly ideal combination for the entire hull and
superstructure.
Are there still more options...?
Fiberglass...! Compared to the weight and cost of an all CuNi /
Stainless structure, both cost and weight could be reduced by using fiberglass for
the deck and house structures, or possibly just for the house structures.
A
cold moulded wooden deck and / or superstructure is also a possibility.
Even with GRP or composite wood for the house structures, it probably
would be most advantageous to plate the deck with Cu Ni. In so
doing, one could then use CuNi for all the various deck fittings: stanchions, cleats, bitts,
etc. Pipe fittings are readily available in either alloy of CuNi, so
this would be a natural.
The resulting integral strength and lack of maintenance would be an outstanding plus.
While the expense of Copper Nickel may seem completely crazy to
some, given a bit of extra room in the budget and the will to be
completely free from all requirements for painting, this is the bee's knees....!
The savings
realized by not having to paint the entire vessel inside and out - EVER
- will go
quite a long way toward easing the cost differential.
Per existing research on a number of commercial vessels, their
operators have shown a very favorable economic benefit over the life of
a Copper Nickel vessel, primarily due to there being a much longer
vessel life; far less cost for docking; zero
painting costs; no maintenance; no corrosion; few if any repairs; etc.
Monel 400 is an alloy of around 65% Nickel, around 30% Copper, plus small
percentages of Manganese, Iron and Silicon. Monel is extremely
ductile, and therefore will take considerable punishment without failure.
Monel is easily welded, and Monel has extraordinary resistance to corrosion,
even at elevated temperatures.
Monel is much stronger than mild steel, stronger than Corten, and
stronger than the usual varieties of stainless. As a result of this
greater strength, Monel could be used for the entire structure. As
compared to a similar steel structure, Monel will therefore permit lighter
scantlings and would allow one to create a lighter overall structure
than with steel. Alternately one could use the same scantlings in
order to achieve a vessel having greater strength.
To reduce costs even more, one could use
the same strategy as with CuNi, i.e. use Monel just for the plating, and
then use 316-L Stainless for the internal framing. This is
probably the sweet spot, offering light scantlings and extraordinary
freedom from on-going maintenance costs.
If cost is not an
important factor, an all Monel structure may well be the ultimate boatbuilding material of
all time.
With very high strength, extreme nobility on the galvanic scale,
virtual immunity to corrosion in sea water and in the atmosphere, and about
half the weight of steel, there are only a few reasons
why Titanium would not be considered the "perfect" hull material, not
the least of which is its cost. A few of the other considerations
are as follows:
Among the higher strength Titanium alloys there is little spread between
the yield point and the failure point. This reveals a limited plastic range.
However, elongation before failure is fairly high compared to, say steel.
Another characteristic is "stiffness" expressed through the modulus of
elasticity. For steel, it is 29 million psi. For aluminum, it is 10 million
psi. For Titanium, it is 15 million psi. This indicates behavior that is
somewhat closer to aluminum in terms of material rigidity.
In other words,
before it is made to yield (the point at which a material is deformed so far it will not return to its original shape
when released), Titanium will flex about
twice as much as steel, but about 50% less than aluminum. Interestingly, Ti
has about the same modulus of elasticity (stiffness) as Silicon Bronze, but Ti has less stiffness
than copper nickel, which has an elastic modulus of 22 million pounds.
Yet another consideration is the welding of Titanium, which is
somewhat of a mixed bag due to
several of the material's properties.
The melting point of Titanium is above that of steel (3,042 deg F, vs 2,500 deg
F) and about three times that of aluminum (1,135 deg F). Titanium forms a
very tough oxide immediately on exposure to the air, so welding must be done only after
thorough cleaning of the weld zone, and the welding process must assure a complete inert gas
shroud of the weld zone both on the side being
welded and on the opposite side. The weld must continue to be shielded until
the metal cools below 800 degrees.
These factors may provide considerable difficulty, but they are surmountable by thorough
attention to detail, good technique, and aggressive measures to assure post-weld shielding. These
factors will however dramatically
increase fabrication costs over those for, say aluminum.
Among the other material properties that contribute to ease of
fabrication of any metal are its heat conductivity, and its thermal
expansion rate. Aluminum expands twice as much as steel per
degree of temperature change, and is three times as conductive thermally. The thermal conductivity of aluminum is a big help, but the expansion makes
trouble in terms of distortion. As a benefit though, an equivalent aluminum structure will have
greater thickness and thus locally greater yield strength, so the score is more
or less even between steel and aluminum, with aluminum having a slightly
greater tendency toward distortion while welding.
With Titanium, this latter consideration will be the overriding factor in
determining the minimum practical thickness for plating. Thermal
conductivity is given as 4.5 BTU / Sq Ft / Hr/ Deg F / Ft for Titanium. For
steel, it is 31, for aluminum it is 90. Thermal expansion is given as
.0000039 in / in / deg F for Titanium, about 50% the expansion of steel and
about 30% that of aluminum. These figures seem to indicate that the material
would be fairly stable while welding, but that welds would take a much
longer time to cool as compared to steel and vastly longer compared to
aluminum. In other words, the heat would not dissipate - it would remain
concentrated in the weld zone.
Based on these factors, as a very rough guess, a thickness of around
3/32" may possibly be the minimum practical hull thickness for a
welded structure in Titanium. As a comparison, the minimum thickness for other materials
(mainly due to welding ease and distortion issues) is 10 gauge for mild steel
(.1345"), and 5/32" for aluminum.
An interesting Titanium alloy is the experimental alloy 5111 (5% Al;
1% SN; 1% Zr; 1% V; 0.8% MO), described as "a near
alpha alloy having excellent weldability, seawater stress corrosion cracking
resistance and high dynamic toughness." It has a high elongation
before failure, a "medium" overall strength of about twice that of mild
steel, and has a slightly greater spread between its yield point and failure
point than the "high" strength Titanium alloys.
Although I believe Titanium would be an outstanding hull material, it
would require extreme care during construction, thus labor costs would be
very high. If those factors can be mitigated, i.e. if cost
is not an issue, then Titanium may possibly
be the "ultimate" in terms of heirloom boat hull materials...!
Relative Cost
If we ignore the cost of the hull materials themselves for a moment and
consider what may impact costs in other ways, we can observe the
following... Vessel construction costs will vary more or less directly
with displacement, assuming a given material, and a given level of finish
and complexity in the design. Since displacement varies as the cube of
the dimensions, we can see that the costs for a vessel will increase
exponentially with size.
It often seems that the inherent good sense of a vessel is inversely proportional to
its size...!
With regard to the complexity of a
vessel the same can be said. Complexity in whatever form affects cost
perhaps to the fourth power...! Assuming a given budget, a simpler
boat can just plain afford to be done better and to be made larger!
Estimating actual construction costs is relatively straightforward.
A reliable construction cost estimate will be possible though only in
relation to a specific design, hull material, degree of finish, complexity,
building method, whether the hull is computer cut - AND only with a well
articulated vessel specification, a complete equipment list, and a set
of drawings that show the layout and the structure.
Assuming we are considering vessels of equal size and complexity, when
all is said and done, and if painted to the same standard on the exterior,
an aluminum vessel may possibly be around 10% more expensive to build than
the same vessel in steel. If the aluminum vessel is left unpainted on
the exterior except where necessary, many yards can build for less in
aluminum than in steel. This may come as a surprise, however this has
been verified via quite a number of recent construction estimates for
vessels of my design.
Maintenance will be less costly on an aluminum boat. Taken as a
whole, any increased hull construction costs for an aluminum hull will shrink
into insignificance in the context of the entire life of the boat.
Of course a Copper Nickel, Monel, or Titanium vessel will be
considerably more
costly than one built in steel or aluminum, however in terms of longevity a boat
built with any of those metals will provide the ultimate as a family
heirloom...
For more information, please review our comprehensive web article on
Boat Building Costs.
The materials of construction need not dictate the aesthetics of a
vessel. Much can be done to make a metal boat friendly to the eye.
On the interior for example with the use of a full ceiling and well done
interior woodwork, there will generally be no hint that you're even aboard a
metal boat.
On the exterior, if metal decks are preferred for their incredible
strength and complete water tightness, one can make the various areas more
inviting by devious means. An example would be the use of removable
wood gratings in way of the cockpit. Fitted boat cushions made of a
closed cell foam work equally well to cover the metal deck in the cockpit
area.
Many metal boats we encounter seem "industrial" in their appearance.
In my view, classic and traditional lines, if attended to faithfully, nearly
completely eliminate that industrial look. With a bit of classic
gracefulness introduced by the designer, a metal boat will be every bit as
beautiful as a boat of any other material.
My design work often tends to be drawn toward fairly traditional
aesthetics, which some may regard as being somewhat old fashioned.
What I have done in these designs however, is to take maximum advantage of
up to date materials and current knowledge of hydrodynamics, while bringing
forth the look and feel of a classic boat. In so doing, my overall
preference is to provide a boat that is very simple, functional, and rugged,
while carrying forth a bit of traditional elegance.
Everyone's needs are different of course, and when considering a new
design one should remember that nearly anything is possible. The eventual
form given to any vessel will be the result of the wishes of the owner, the
accommodations the boat must contain, the purpose for which it is intended,
and the budget which has been offered for its creation.
I am always interested in the question of efficiency and performance
versus the many other considerations that go into shaping a hull.
With metal hulls, there is always a question of whether a vessel should be
rounded or "chine" shaped.
Presuming we compare two vessels are of equally good design, whether they
are rounded or single chine will not have much impact on their performance,
i.e. they will be more or less equivalent. Here are a few
considerations that may be of some benefit when considering the choice
between rounded or single chine hull shapes...
- If one were to take a single chine hull form and simply introduce a
fairly large radius instead of the chine, the newly rounded vessel's
wetted surface would be less; displacement would be less; and initial
stability would be less, making the comparison somewhat skewed.
- If instead one were to design the two vessels so that they had
exactly the same length and displacement, exactly the same sail area and
rig, no "reverse" to the garboard area, and with hull forms as similar
to each other as one could make them, one would quickly observe the
following:
- In terms of interior hull space, a chine hull form would be
slightly less wide at sole level and slightly wider at the waterline level, so less room to walk around but larger seats and
berths.
- The single chine hull form would have slightly greater initial
stability (greater shape stability), and would therefore have slightly
greater sail carrying ability at typical heel angles under sail.
- The single chine hull form will have greater roll dampening (faster
roll decay).
- The rounded hull form will have a slightly more gentle rolling
motion.
- The chine hull form will have slightly greater wetted surface.
- This implies that the rounded hull form will have slightly less
resistance at slow speeds due to wetted surface dominating the
resistance at slow speeds.
- The chine hull can be designed to equalize or reverse that
resistance equation at higher speeds due to wake differences
resulting from the chine hull being able to have a slightly flatter run.
Aside from these generalities, relative performance would be difficult to
pre-judge. We can however observe the following:
- Given the same sail area, when sailing at slow speeds in
light airs, one might see the rounded hull form show a slight advantage
due to having slightly less wetted surface.
- When sailing fast, a chine hull form will be more likely to
exhibit greater dynamic lift, especially when surfing.
- Especially in heavier air, one might even see a slight advantage to
windward with the chine hull.
Given that those observations do not reveal any special deficiency with
regard to a single chine hull we can additionally observe the following:
- When creating a new design, wetted surface is one of the determining
factors of sail area.
- Having slightly greater wetted surface, a single chine hull will
ideally be given slightly more sail area, so its slightly greater wetted
surface will become a non-issue.
- If the chine hull is given slightly more sail area, it will
therefore be subject to a slightly greater heeling force.
- The single chine hull form will have inherently greater "shape
stability" to resist that heeling force.
- One can therefore expect the sail carrying ability to be essentially
equalized.
Among the above considerations, the one that seems to favor the rounded
hull form most definitively is that of having a slightly more gentle rolling
motion. In other words, a slower "deceleration" at the end of each
roll. On the other hand, rolling motions will decay more quickly with
a single chine hull form. Even these factors can be more or less
equivocated via correct hull design.
Regarding the notion of a rounded metal hull, in my view they can be
excellent! As we have seen, one cannot claim that a rounded hull form
is inherently better in terms of performance without heavily qualifying that
claim. The primary trade-offs between a rounded hull and a chine type
of hull form for metal boats therefore turn out to be purely a matter of
cost and personal preference.
I have designed several rounded hulls for construction in metal.
These are true round bottom boats designed with the greatest ease of plating
in mind. Some are double ended, some have a transom stern, others have
a fantail stern. Another hull shape I have developed is a rounded hull
with a canoe stern where the shape of the stem nicely balances the shape of
the stern.
Having a very easily plated shape, any of these rounded hull forms can be
economically built. These rounded shapes require plate rolling only in
a few places and are elsewhere designed to receive flat sheets without fuss.
These are not "radius chine" boats. They are simply easily plated
rounded hulls.
With any of these types, the keel is attached as an appendage, there
being no need when using metal to create a large rounded garboard area for
the sake of strength, as would be the case with a glass or a wooden hull.
This achieves both a more economically built structure, as well as a better
defined keel for windward performance under sail and for better tracking
under power.
Plating on these rounded hull types will be arranged in strips of a
limited width running lengthwise along the hull. Usually the topsides
can be one sheet wide, the rounded bilge one sheet, and the bottom one much
larger sheet width.
The plating is most easily done if the edges are "joggled" so that one
plate fits nicely over another along the edge. Many vessels have been
built using this method. They benefit from the additional strength provided
by the joggled lap. Some vessels can be designed without
longitudinals, simplifying the framing. The joggled laps can be very
attractive if lined off correctly (as one would do with a wooden boat's
planking).
Examples of these rounded hull types among my designs are
Jasmine, Lucille,
Benrogin, Greybeard,
Fantom and amongst my prototypes such as
Josephine and Caribe . While these may possibly be thought similar to a "radius"
chine shape, they are in fact true rounded hull forms. In other
words, the turn of the
bilge is not a radius but is instead a free form curve between bottom and
topsides. Both bottom and topsides have gently rounded sectional
contours that blend nicely into the curve at the turn of the bilge.
With the exception of the turn of the bilge, all of the plating on these designs is developable and will bend in
very readily making these vessels just as easily constructed as any radius
chine shape. In other words, 85% to 90% of the vessel is able to be
plated without pre-forming using flat metal sheets.
What's the difference...?
In my view the visual difference between radius chine and rounded hull
forms is significant, yet the cost is the same. Due to the gentle
transverse curvature given to the surfaces above and below the turn of
the bilge, the appearance is a vast improvement over the relatively
crude radius chine shape - and at no penalty whatever in terms of labor
cost or ease of construction.
Radius Chine Metal
Hulls
Looking around at typically available metal boat designs offered
elsewhere, we quickly observe that the "radius chine" construction method
has become fairly common. Here, a simple radius is used to intersect
the "flat" side and bottom plates. Although the radius chine shape takes
fairly good advantage of flat plate for most of the hull surface, it is not a more economical construction method than the easily plated
rounded hulls described above - nor is it nearly as attractive.
One reason for the popularity of the radius chine is that nearly any
single chine boat can be converted to a radius chine. This is often
done without any re-design of the hull by simply choosing an appropriate
radius, and using rolled plate for that part of the hull. Radius chine
construction does add quite a few extra hours to the hull fabrication
as compared to single chine hull forms.
In my experience there is no benefit whatever to employ a radius chine
shape over that of an easily plated rounded hull form. The radius
chine hull will always be easily recognized for what it is... a radius
chine shape rather than a true rounded hull. By contrast a
gently rounded hull form will be vastly more appealing visually.
Chine Hull Forms
Aesthetics are of course a highly personal thing.
As we can observe with many designs a single chine can look
quite appealing, especially when used with a more classic / traditional style. A
few single chine examples among my sailing designs are the
36' Grace, the 42' Zephyr, the
44' Redpath, the 56' Shiraz, along with a
number of others such as the prototype designs for a
51' Skipjack, or the
55' Wylde Pathaway.
As supplied, metal plate is always flat. When building a boat using
flat sheet material, it makes the most sense to think in terms of
sheet material and how one may optimize a hull design to suit the materials,
without incurring extra labor. I am attracted to the single chine
shape for metal boats. In my view the single chine shape represents
the most "honest" use of the material.
In this regard I feel traditional styling has much to offer, keeping in
mind of course the goals of seakindliness, safety, and of excellent
performance. As with many traditional types, there is certainly no
aesthetic penalty for using a single chine, as is evidenced by reviewing any
of the above mentioned sailing craft.
Assuming that by design each type has been optimized with
regard to sail area and hull form, it becomes obvious that the typically
pandered differences between the performance of a rounded hull form versus
that of a single chine, unless heavily qualified, are simply
unsubstantiated.
In fact, since costs are significantly less using single chine
construction, one can make an excellent case in terms of better
performance via the use of a simpler hull form....!
How is this possible...?
With metal boats, labor is by far the largest factor in hull
construction, and as we have observed greater complexity pushes the hours
and the cost of labor up exponentially. Therefore dollar for dollar, a
single chine vessel can be made longer within the same budget.
In other words, in terms of the vessel's "performance per dollar" the single
chine vessel can actually offer better performance (i.e. greater speed) than a similar type of rounded hull form...!
By comparison, a multiple chine hull form offers practically no advantage
whatever. A multiple chine hull will require nearly as much labor as a
radius chine hull. The only savings will be eliminating the cost
of rolling the plates for the actual radius. In my view, multiple-chine
shapes are very problematic visually, and they are much more difficult to
"line off" nicely. There will be just as much welding as with a radius
chine shape, and in general a multiple-chine hull will be considerably less
easy to keep fair during construction.
If you look at the designs on this web site, you'll soon discover
that there are no
examples of multiple-chine vessels among my designs, whether power or
sail....
Why...?
Basically, multiple chine shapes cost more to build, and at least in
my view multiple chine shapes are not as visually appealing. As a
result the preference has always been to consider the available budget
and to make a graceful single chine boat longer for the same cost, and realize some
real speed, comfort and accommodation
benefits...!
In the end what ultimately defines a good boat is not whether she is one
type or another, but whether the boat has been well designed and whether the
resulting vessel has satisfied the wishes of those who had her designed and
built.
Keel Configuration
The keel of any vessel, sail or power, will be asked to serve many
functions. The keel creates a structural backbone for the hull, it
provides a platform for grounding, and it will contain the ballast.
In a metal boat, the keel is not just "along for the ride." In a metal
vessel the keel can contain much of the tankage including a meaningful sea
water sump, and the keel can serve as the coolant tank for the engine
essentially acting as the "radiator." It is usually convenient to allow at
least one generous tank in the keel as a holding tank.
A metal hull can take advantage of twin or bilge keels without any
trouble. It is an easy matter to provide the required structural
support within the framing. Often, bilge keels can be integrated with
the tanks, allowing excellent structural support.
An added advantage with both sail and power boats is that the bilge keels
can be used as ballast compartments. Having spread the ballast
laterally becomes a big advantage in terms of the vessel's roll radius,
providing an inertial dampening to the vessel's roll behavior.
Bilge keels can also be designed to permit a good degree of sailing
performance to a power vessel which has been set up with a "get-home"
sailing rig. Aboard a power vessel, when faced with the choices
involved with having an extra diesel engine as a "get-home" device in the
event of failure of the main engine, I would very seriously consider the
combination of bilge keels and a modest sailing rig.
Bilge keels will usually make use of a NACA foil section optimized for
high lift / low drag / low stall. With metal, this is easily
accomplished.
Integral fuel and water tanks are always to be preferred on a
metal boat. Integral tanks provide a much more efficient use of space.
Integral tanks provide added reinforcement for the hull and ease of access
to the inside of the hull.
Integral tanks are very simple to arrange for during the design of the
vessel. If the tank covers are planned correctly there will be
excellent access during construction as well as in the future for
maintenance.
The one exception to this generality is that polyethylene tanks may be
preferred for black or grey water storage, since they can be readily
cleaned. This is especially so in aluminum vessels, due mainly to the
extremely corrosive nature of sewage. In steel vessels, when properly
painted there will always be an adequate barrier, and integral black and
grey water tanks again become viable. For aluminum construction, if
integral holding tanks are desired the tanks must be protected on the inside
as though they were made of mild steel... and the coatings must not be
breached...!
Hull size, materials of construction, and the location of the specific
region of the structure in question will each have a bearing on the results
of the scantling calcs. The method of calculating the hull structural
scantlings is usually processed as follows, assuming first that the vessel
data is already given (hull length, beam, depth, freeboard, weight, etc.).
- Select plate material according to owner preference, available
budget, and desired strength or other material properties
- Select preferred plate thickness according to availability,
suited to vessel size and displacement
- Calculate local longitudinal spacing to adequately support the
plate
- Select frame spacing to satisfy the locations of interior
bulkheads or other layout considerations
- Calculate scantlings required for longitudinal stringers to
satisfy their spacing and the span between frames
- Calculate scantlings required for transverse frames according to
the depth of long'l stringers and the local span of the frames.
Per item 3, when considering an alternate material it is possible that
due to a difference in plate yield strength as compared to the original
design material (say steel), that the long'ls will be placed slightly more
closely (say for the same thickness of plate, but a plate of lesser
strength).
Generally, since the long's support the plate, they are the primary
variable when plate thickness, or strength, or location is changed. It
is no big deal to the structure, to the overall weight, or to ease of the
building of the vessel (as compared to say steel) to have a tighter long'l
spacing. This is the proper strategy to accommodate plate of different
strength or thickness.
Once the plate is adequately supported, then scantlings of items 5 and 6
can be calculated according to their spans and the material strengths for
the chosen framing materials.
It becomes obvious from the above that it is an advantage (in terms of
weight) to select a relatively lesser thickness of plating, and a relatively
more frequent interval for internal framing. On the other hand, it is
usually an advantage in terms of building labor to select plate of a
slightly greater thickness and a less frequent framing interval (so simpler
internal structure).
There is quite a lot of misleading and incorrect information associated
with the implied promise of "frameless" metal boats, a notion that is
pandered by several offbeat designers and builders. The concept of
"frameless" metal boats is attractive, but flawed.
If one applies well proven engineering principles to the problem of hull
design as detailed above, one quickly discovers that for the sake of
stiffness and lightness, frames are simply a requirement. For example, in
order to achieve the required strength in a metal vessel without using
transverse framing will require an enormous increase in plate thickness.
Even with light weight materials such as aluminum alloy this would
automatically result in a substantial weight penalty..
With light weight materials such as aluminum, one can certainly gain some
advantage by the use of greater plate thickness, primarily in terms of
maintaining fairness during fabrication, and in terms of ruggedness in use.
Still, as strong as metal is, even with light weight materials there is
definitely a need to support the plating and to reinforce and stiffen the
structure as a whole using frames and stringers.
In general, the most suitable arrangement for internal structure is a
combination of transverse frames and longitudinal stiffeners. Framing
may sometimes be provided in the form of devious strategies... For
example framing may be in the form of bulkheads or other interior and
exterior structural features, placed in order to achieve the required plate
reinforcement. Many so-called "frameless" boats do indeed make extensive use
of longitudinals in combination with bulkheads or other internal structure
to reduce the span of the longitudinal stiffeners.
While it is true that many metal boats are successfully plated,
and their plating then welded up without the aid of metal internal framing
during weld-up, in order to provide adequate strength in the finished
vessel, frames must then be added before the hull can be considered
finished. Even on a hull that will eventually have substantial
internal framing this construction sequence can provide a big advantage when
trying to maintain fairness during weld-up.
Experienced metal boat builders and designers have often come to
recognize the potential benefits of building a metal boat over molds which
do not hold the boat so rigidly as to make trouble during the weld-up.
However, the competent among them also know that to leave the boat without
internal framing is quite an irresponsible act.
Framing Systems
Framing systems are several, but can roughly be categorized into
- Transverse Frames Only
- Transverse Frames with Longitudinal Stringers
- Web Frames with Longitudinal Stringers.
Among those, the Transverse Frames Only system is fairly common in
Europe. In the US, the most commonly system used is the second system,
where transverses are used in combination with longitudinal stringers.
In terms of scantlings, typically, long'ls will be half the depth, but
approximately the same thickness as the transverse frames. It is an
ABS requirement that transverse frames be twice the depth of the cut-out for
the long'l.
Among some light weight racing yachts, a system of Webs with fairly beefy
Long'l Stringers is the preferred approach, or alternately Webs with smaller
Intermediate Transverse Frames, in combination with Long'l Stringers..
A somewhat generalized walkthrough of the usual design sequence is as
follows:
- For any given vessel size, plating will need to be a certain
minimum thickness suited to that vessel size.
- For that given minimum plating thickness (for that particular
boat) the long'l stringers will need to be a certain distance apart
in order to adequately support the plate.
- The dimensions of the Long'l Stringers are determined by the
vessel size, the spacing of the long's and the span of the long's
between transverse frames.
- Finally, the dimensions of the Transverse Frames are determined
according to the vessel size, the frame spacing, the span of the
frames between supports, and by the requirement that the frames be
no less in height than twice the height of the long's.
In other words, by this engineering approach the transverse frames are
considered to be the primary support system for the long'l stringers, and
the long'l stringers are considered to be the primary support system for the
plating.
When a long'l member becomes the "dominant" member of the structure
(usually locally only), it ceases to be referred to as a long'l stringer,
and becomes instead a long'l "girder" (an engine girder for example).
If long'l stringers are not used, then the frames are the only means of
support for the plating. They must therefore be more closely spaced in
order to satisfy the needs of the plating for adequate support. In
general though, long'l stringers are to be considered highly desirable,
primarily because they contribute considerably to the global longitudinal
strength of the yacht.
When calculating the strength of any beam, there is a benefit when the
beam gains depth (height). Beams of greater height have a higher
section modulus. Just as with beams of greater height, when calculating a
vessel's global longitudinal strength it is the height of the vessel
that makes the greatest contribution. Small and medium sized power and
sailing yachts usually have very adequate height, so long'l strength
calculations are less critical. For larger yachts or for yachts which
have a low height to beam ratio, there it is necessary to consider
long'l strength very closely. Witness the catastrophic failures of
several recent America's Cup vessels....!
As a general guide to the boundary of acceptability, the ABS rules
consider that a vessel must be no more than twice as wide as it is high
(deck edge to rabbet line), and no greater than 15 times its height in
overall length. Beyond these limits, a strictly engineering "proof"
must be employed rather than the prescriptive ABS Section Modulus and Moment
of Inertia requirements for calculating the strength of the global hull
"girder."
The ABS Motor Pleasure Yachts Rule, 2000, is a very suitable scantling
rule for boats of any material. Originally created for "self propelled
vessels up to 200 feet, the scope of the Motor Pleasure Yachts Rule has been
subsequently restricted to vessels between 79 and 200 feet. In that
size range, the ABS Rules for Steel Vessels Under 200 Feet, and the ABS
Rules for Aluminum Vessels may also be applied, in particular to
commercially used vessels. For sailing craft of all materials, the ABS Rules
for Offshore Racing Yachts is applicable to sailing vessels up to 79 feet.
The most appropriate means of assessing the adequacy of structure is to
assure that a vessel's scantlings comply with the applicable ABS rule, or
alternately the applicable rule published by Lloyd's Register (England),
German Lloyds (Germany), Det Norske Veritas (Norway), Bureau Veritas
(France), etc.
As we can see from the above, framing is highly desirable for any metal
yacht. Without framing, plate thickness would become extreme, and
consequently so would the weight of the structure..
The labor involved in fabricating a metal hull can be reduced by a
substantial amount via NC cutting. What is NC...? It simply
means "Numerically Controlled." Builders who are sufficiently
experienced with building NC cut hull structures estimate that they can save
between 35% and 55% on the hull fabrication labor via computer cutting.
As an example, a fairly simple vessel of around 45 feet may take around
2,500 hours to fabricate by hand, complete with tanks, engine beds, deck
fittings, etc. ready for painting. If one can save, say 40% of those
hours, or some 1000, then at typical shop rates the savings can be
dramatic. By comparison, the number of design hours one must spend at the
computer to detail the NC cut files for such a vessel may amount to some
three to four man-weeks, or perhaps some 160 hours.
With this kind of savings, the labor expended to develop the NC cut files
will be paid for many times over. In fact, the savings are sufficient
that NC cutting has the potential to "earn back" a fair portion of the cost
of having developed a custom boat design...! Where there may be any
doubt, please review our web articles on NC cutting.
Anymore, it is inconceivable to build a commercial vessel of any size
without taking advantage of NC cutting. While this technology has been
slow to penetrate among yacht builders, these days it is plain that builders
and designers who ignore the benefits offered by computer modeling and NC
cut hull structures simply have their heads in the sand.
Small metal boats, unlike tankers and container ships, are not designed
with an appreciable corrosion allowance. They must therefore be prepared and
painted in the best way possible in order to assure a long life.
Current technology for protecting steel and aluminum boats is plain and
simple: Epoxy paint.
When painting metal, a thorough degreasing is always the first step, to
clean off the oils from the milling process, as well as any other
contaminants, like the smut from welding, which have been introduced while
fabricating.
The next important step is a very thorough abrasive grit blasting on a
steel boat, or a somewhat less aggressive "brush blast" on an aluminum boat.
The process of sand blasting a metal boat is expensive and can in no way be
looked at with pleasure, except in the sense of satisfaction and well being
provided by a job well done.
While there is no substitute for grit blasting, there are ways to limit
the cost of the operation. When ordering steel, it is very much to a
builder's advantage to have it "wheel abraded" and primed. Wheel
abrading is a process of throwing very small shot at the surface at high
speed to remove the mill scale and clean the surface. Primer is then
applied. Having been wheeled and primed, the surfaces will be much
easier to blast when the time comes.
In terms of the paint system, aluminum boats are dealt with more easily
than steel boats. Aluminum must be painted any place a crevice might
be formed where things are mounted, and should also be painted below the
waterline, if left in the water year-round. The marine aluminum alloys
do not otherwise require painting at all.
On an aluminum boat, any areas which will be painted should receive the
same aggressive preparation regimen used on steel: thorough cleaning,
sand blasting, and epoxy paint. Aluminum is less hard than steel, so
sand blasting aluminum is relatively fast compared to steel. The blast
nozzle must be held at a greater distance and the blast covers the area more
quickly.
On a Copper Nickel or Monel vessel, there would simply be no need for
paint anywhere.
Many schemes are used to insulate metal boats. Blown-in foam is an
excellent insulator, and offers considerable sound deadening. Blown-in
foam does offer additional protection for the interior metal work, but only
if it is adhered well to the surface.
Sprayed in foam, while popular, does have drawbacks which are often
overlooked. Urethane foam is not a completely closed cell type of
foam. With time, urethane foam will absorb odors which become
difficult or impossible to get rid of. This is especially a problem when
there are smokers aboard.
Nearly all urethane foam will burn fiercely, and the fumes are quite
toxic. Blown in foam should be coated with a flame retarding paint.
An alternative to blown in foam is a good quality flexible closed cell
cut-sheet foam to fit between the framing. Some sheet foams are fire
retarding by composition, but if not, they should be painted just like the
urethane foams.
The best choice among the foams for cut-sheet foam installation are
Ensolite and Neoprene. There are several different varieties of each.
The choice of insulation foam should be made on the basis of it being
fireproof, mildew proof, easily glued, easy to work with, resilient, and if
exposed, friendly to look at. Ensolite satisfies all these criteria.
Ensolite is both better and more expensive than Neoprene.
Styrofoam or any other styrene type of foam should be strictly avoided.
Purchase a piece at the lumber yard and throw it onto a camp fire....
You will be immediately convinced.
Sprayed in polyurethane foam is the best in terms of insulating value,
since it nearly completely prevents condensation by sealing off the air from
the metal hull surface. If the insulating value of the system is the
paramount criteria, then sprayed in poly foam will be the preferred choice.
A fire retarding formulation should always be used.
Zincs are essential on any metal hull. In the best of all possible
worlds, there would be no stray currents in our harbors, but that is not a
reality. Regardless of the bottom paint used, zincs must be used to
control stray current corrosion, to which we can become victim with a metal
boat, even without an electrical system!
The quantity of zinc and the surface area are somewhat determined by
trial and error. As an example, on a metal hull or around 35 feet, the
best scheme is to start with two zincs forward, two aft, and one on each
side of the rudder. With a larger boat of say 45' an additional pair
of zincs amidships would be appropriate. As a vessel gets larger the
zincs will become more numerous.
Since zincs will be effective for a distance of only around 12 to 15
feet, it is not adequate to use one single large zinc anode. The zincs
will ideally be located near the rudder fittings, and near the propeller.
The zincs forward are a requirement, even though there may be no nearby hull
fitting, in order to prevent the possibility of stray current corrosion,
should the paint system be breached.
Using the above scheme, after the first few months the zincs should be
inspected. If the zincs appear to be active, but there is plenty left,
they are doing their job correctly. If they are seriously wasted, the
area of zinc should be increased, rather than the weight of zinc.
During each season, and to adjust for different marinas, the sizes of the
zincs can be adjusted as needed.
Good electrical connection between the zinc and the hull must be assured.
For maximum corrosion protection, metal boats should ideally not
be bonded. This of course is contrary to the advice of the ABYC.
However one must keep in mind that the ABYC rules are primarily aimed at
satisfying the requirements of GRP vessels. Little by little, we are
seeing the ABYC create special case recommendations for aluminum and
steel boats. Great progress...!!
Electrical System
Considerations
Aboard a metal vessel it is best to make use of a completely floating
ground system. In other words, the negative side of the DC power in
this case will not permitted to be in contact with the hull or any hull
fittings, anywhere.
This is contrary to the way nearly all engines are wired.
Typically, engines make use of the engine block as a mutual ground for all
engine wiring. Also, the starter will typically be grounded to the
engine, as will the alternator.
With a floating ground system, a special type of alternator is used which
does not make use of its case as the ground, but instead has a dedicated
negative terminal.
Needless to say, for the sake of preventing corrosion, there should
not be a connection between the AC shore power and the hull. This
includes that insidious little green grounding wire. Of course this is also
contrary to the ABYC recommendations, which are primarily concerned with
prevention of shock, rather than the protection of the hull itself.
All AC power coming aboard a metal boat should be passed through a marine
quality isolation transformer. Other "black box" devices should be
strictly avoided, including things like zinc savers, impressed current
systems, etc.
I know there are plenty of people out there who will disagree with the
above brief statements about electrical systems. Whether you agree or
disagree, please don't come all unglued over these matters and instead, for
much more complete information on these topics, please see the resources
mentioned just below...
We can see that metal can make considerable sense as a hull building
material. On the basis of strength, ruggedness, ease of construction,
first cost, and ease of maintenance, there is plenty of justification for
building a metal hull, whether steel, aluminum, Copper Nickel, or Monel.
Steel wins the ruggedness contest. Aluminum wins the lightness contest.
Copper Nickel and Monel win the longevity and freedom from maintenance
contest.
Part of the equation for any vessel is also resale. In this realm,
aluminum does very well, albeit in this country not as well as composite
construction. This is mainly a matter of market faith here in the US
where we are relatively less educated about metal vessels. As for
resale, a vessel built of Copper Nickel will fare extremely well.
After all, the Copper Nickel or Monel vessel will have essentially been
built out of money...!
Metal is an excellent structural material, being both strong and easily
fabricated using readily available technology. In terms of impact,
metal can be shown via basic engineering principles and real world evidence
to be better than any form of composite. If designed well, a metal boat will
be beautiful, will perform well, will be very comfortable, and will provide
the peace of mind achieved only via the knowledge that you are aboard the
safest, strongest, most rugged type of vessel possible.
It is said among dedicated blue water cruisers in the South Pacific that,
"50% of the boats are metal; the rest of them are from the United
States....!" Although this statement may seem so at times, it is
fortunately not 100% true!!
It is my hope that the above essay will be of some value when considering
the choice of hull materials. If you are intending to make use of
metal as a hull material you may wish to review the article "Aluminum for Boats" that first appeared in Cruising
World magazine, and the article "Aluminum vs. Steel" comparing the relative merits
of both materials. Also, in defense of steel as a very practical boat
building medium check out the article on "Steel
Yachts."
In addition, there are two excellent booklets available on our
Articles and Other Links page. The first of them, the "Marine
Metals Reference" is a brief guide to the appropriate metals for marine
use, where they will be most appropriately used. It also contains
welding information and a complete list of the physical properties of marine
metals. The second booklet, "Corrosion, Zincs & Bonding"
offers a complete discussion of electrical systems, corrosion, zincs,
and bonding.
For considerably more information on the question of hull materials, please see
our web articles on the following:
Aluminum for Boats |
Aluminum vs. Steel |
Steel Boats |
Composites for Boats |
The Evolution of a Wooden Sailing Type
Copyright 2007 Michael Kasten
