Cool New Gimmicks...? A Cautionary Note
Copyright 2000 -2006 Michael Kasten
Looking around among the many various "trawler yacht" designs currently being offered,
one will observe that many of these designs are genuinely innovative, while others may perhaps be overly quirky.
That said, I am not intending to pick on any one design or designer in particular.
Instead, it is the intent of this brief article to substantiate the following cautionary statement:
It is not always the best idea to latch onto off-beat concepts "just because they are different."
Builders
and designers of "trawler yachts" have recently experienced quite a lot
of activity. As a result, they are all competing for the
attention of a relatively small client base who have the means to
purchase a new power yacht. Among the various heavily
advertised "trawler yachts" we can see examples of the
over-hyping of several offbeat concepts, many of which are, in my view,
highly irresponsible.
In order to reveal
the most troubling of these, we need only look at all the noise
currently being made in the media about double bottom construction;
hydraulic primary propulsion; Z-drives; propeller nozzles; maintenance
strakes; bow bulbs; and engines located far aft for the sake of the
accommodation spaces.
The following is a brief explanation of each of these "features" and what we find to be troubling among them.
Engine Room Location
It
is well known among sailors and among competent designers that weights
located in the ends of a vessel will virtually always create trouble in
terms of pitching moment. In other words, the vessel will be slow to
react to the sea. With a large weight such as an engine located far
aft, the vessel will be readily pooped. That same vessel will
also tend to bury her bow due to the mass of trimming ballast which
will have to be located far forward in order to correct for the
location of the engine. The same applies if the engine or a large
tank is located too far forward; there will have to be ballast located
at the other end of the ship to compensate.
We cannot responsibly encourage the placement of large masses such as engines, tanks, batteries, etc.
right in the ends of a vessel, regardless of the possible benefits to the interior accommodation....
Bow Bulbs
Several
well known trawler designers seem to favor bow bulbs on nearly
everything that moves. In my view, placing bow bulbs on
relatively small vessels cannot be encouraged for any vessel under
around 65 feet or so, due to potentially dangerous handling
characteristics in following seas. This is particularly so for
short and fat boats which will already exhibit excessive yaw in
following seas. What is favorable in the towing tank is not
necessarily a benefit in the ocean, where wave-size-to-boat-size
quickly becomes an overriding factor.
Bow
bulbs may often pandered on the basis of pitch reduction. The
benefits in this regard are measurable. It is important to
realize though that the benefit of using a bow bulb simply to attenuate
pitch on a well designed hull form is controversial at best. For
example, we have observed that on smaller craft a bow bulb can increase
the likelihood of slamming in short steep seas due to its relatively
flat bottom shape. For more information on the possible hazards
of bow bulbs on small craft please see my article on "Running vs. Heading Up."
Propeller Nozzles
Propeller nozzles
at least are not dangerous...! Nozzles are known to provide very
substantial advantages to slow speed vessels doing towing or trawling.
On
free moving vessels there is not the same degree of benefit, even at
the relatively slower passage making speeds. Even if a nozzle
were able to provide a small percent advantage at a certain free
running speed, but requires a 300% increase in the cost of the system,
it is at best a questionable "feature." Still, a nozzle can at
times be justified on a free running vessel where there may be a
requirement for extreme shoal draft, or where there may be the danger
of propeller tip damage due to debris, ice, or shoals.
Hydraulic Drive
Hydraulic propulsion
is most often promoted in order to enable placement of the main
propulsion engine in a location other than amidships, in other words to
make use of an unfavorable location in terms of trim and pitching
moment. Although ordinarily highly reliable, any hydraulic
propulsion system must be engineered correctly or it will be very
unsatisfactory. Even if engineered and installed correctly, the
use of hydraulics for primary propulsion is expensive to install,
highly complex, ordinarily very noisy, and incredibly inefficient when
compared to a simple direct shaft drive. This can hardly be viewed as a
"feature."
On the other
hand, one might very rationally consider a modest hydraulic drive
system for a get-home installation, primarily to save weight and to
provide flexibility of machinery location. A PTO on the generator
might for example be used as the motive force. In this
case, the hydraulic propulsion equipment will still be somewhat
expensive, noisy, and inefficient, but it will not be asked to operate
continuously, nor at full vessel speed.
Z-Drive
A Z-Drive
may be justifiable in certain extreme cases, for example aboard a
working tug that needs all-around maneuverability. A Z-Drive as
primary propulsion on a trawler yacht however falls into the category
of being both extremely expensive and unnecessarily
complex. For use with the main propulsion engine aboard a
yacht there is the same caveat regarding weights located toward the
ends of a vessel... it cannot be recommended due to the
likelihood of adverse pitching behavior.
Still, on a yacht one might effectively employ a Z-Drive or Sail Drive
with a "get-home" engine, perhaps even using hydraulic power -
whether from the generator or the main engine.
Twin Keels
Twin Keels
are defined as being in the form of a pair of bilge fins, usually foil
shaped, located more or less amidships. By contrast, Bilge Keels are
usually defined as being relatively long and shoal, ordinarily made of flat plate. Both forms, if implemented correctly,
can be very effective at reducing roll, so can be quite justified on that basis.
If they are not sized and located correctly, there can be rather severe
handling anomalies.
For more information
about the design and use of Twin Keels on passagemakers, please check out the article on Roll Attenuation at this site.
Similarly,
we have observed much hype put forth in favor of using a tri-keel
arrangement; in favor of unconventional LCB locations; and in favor of
propeller nozzles for free moving vessels. The "tri-keel" keel
arrangement in this case refers to the vessel having a deep fore-body
and twin "skeg keels" aft. This concept was originally developed
by the US Coast Guard primarily for use on fast patrol vessels intended
for shoal water, where the propellers needed good protection and the
vessel must take the ground upright. These were not created as
"trawlers" but as high speed chase boats, and the shapes were optimized
for planing performance rather than being intended for displacement
speeds.
Among
these "features" the use of twin props with twin keels aft is an
arrangement that can be very favorable, however not for the
"efficiency" reasons espoused. This arrangement will mainly be
used for the sake of engine and drive line redundancy, and for
achieving shoal draft while maintaining good maneuverability.
Double Bottom
Among all the "features" mentioned, the double bottom
may have a certain amount of merit, most particularly on larger vessels
having a very adequate bilge capacity. One can certainly consider
this option, provided that the double bottom does not result in overly
wide or overly long tanks that may seriously compromise the stability
of the boat due to the free surface effect of the liquids within.
In other words, the bottom tanks must be subdivided in order to reduce
the stability-degrading effects of the free surface of the tank
contents.
A
double bottom can be a good thing as long as it does not compromise the
vessel's handling characteristics or limit one's access to the hull.
Quoted Stability Range
To
make a vessel "survivable" in a 360 degree rollover is the ultimate
goal for any true blue water boat. For any vessel, survivability
requires water tight integrity of all openings such as windows and
hatches, etc. Despite this fairly obvious requirement it is quite
common for trawler yachts to have a relatively large windows for the
sake of achieving a good view. True survivability can hardly be
claimed for boats with big picture windows and lightly built
superstructures...!
On
the other hand, a 180 degree range of positive stability for power
vessels that do have strong and rugged superstructures and with small
and robustly constructed window and door openings will not ordinarily
be an excessive claim. If structure is approached
conservatively, if the openings are designed with an adequate support
structure, and if the glazing is of the right materials and adequate
thickness, this degree of survivability amounts to only a slight
penalty in terms of weight and cost.
For additional
reading on the calculations required in order to support various
stability claims, and on what information one should expect to have
readily available with any new design, please review our web article, Essential Design Data.
Compromise...
Inevitably, all boats involve compromise!
Many of the above 'gimmicks' are pandered in order to get the attention of potential buyers, or worse,
in order to justify a compromise that has been made necessary by another compromise...!
Such
gimmicks can in fact become a real obstacle to good design. Many
who are fairly new to boating will be attracted to these offbeat
"features." There is the tendency for the novice to become
fixated on some unusual feature or other which he may have recently
read about, often being unwilling to abandon it until much effort (and
consequent engineering expense) has been expended in pursuit of that
feature, and if the notion is bizarre enough it may ultimately need to
be corrected on the finished boat.
These various gimmicks should be seen for what they are: simply marketing
tricks put upon an inexperienced and unsuspecting public. For every
claimed advantage, one must relentlessly ask what the possible
disadvantage may be.
Innovation
This
article does not mean to imply that we should blindly follow the
"status quo." Far from it! Innovation is very
much to be encouraged. It is the only way we can progress.
The
salient point is that yes, we should of course look at all favorable
options when creating a new vessel design, but we should always
approach the new and unusual with a healthy skepticism. These are not
mere idle warnings. I have seen the unfortunate results many
times...
Please take the above as it is intended: A simple red flag in terms of
encouraging good "boat sense" when considering the various
'features' being pandered for a new vessel.
Having
exercised good judgment regarding what makes a proper boat for
voyaging, one can enjoy many miles of safe and efficient boating...!
Michael Kasten
Metal Boat Quarterly #21 - Winter 2000
Editorial - Updated 2003 & 2006
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