10 January 2007
This page contains a few brief notes about our
work with composites, as well as how we view the comparison between composite
construction (GRP, fiberglass, etc.) to metal boat construction.
What's Our Experience With Fiberglass Boats...?
Here is a little background to address that question...
Despite the bulk of my design work being focused on ocean cruising
yachts in
metal I have also designed some large traditional wooden craft.
Not as well known is that my design work has also included several
years' work with boats built of reinforced plastics, i.e. composites -
so I'm no stranger to composite yachts!
Two of the various composite vessels
on which I've done significant design work are the
160' MY Evviva and the 100' MY Lady Dianne. They have both been
best-of-show award winners at Monaco - very high quality yachts.
You can see the Lady Dianne under way here.
With the Evviva, cost was not an issue - in other words there was no
budget. A dream customer you might think... but he was definitely not an
easy one to please! Evviva's structure was designed for minimum
weight, but without
going to the extreme of carbon fiber - which was at that time in relatively limited use. We
therefore used a cored hull, vinyl ester resins and predominately Kevlar
bi-axial laminates. There was also liberal use
of uni-directional laminates of various types in order to maximize the section modulus of
the framing. Cores were air-bagged to the outer skin in order to
reduce the weight of core-bond used. We used some CF but only for stiffness
locally, as well as to build the passerelle.
Pre-pregs (cloth laminates pre-impregnated with resin) were not
at that time readily available outside the aircraft industry, however for some small parts we
did use
resin infusion technology. Today, pre-pregs are much more
available and resin infusion techniques are becoming more and more
common. Both are still relatively expensive, but they are very
effective at weight control - since the amount of resin used is precise.
An additional assignment on the Evviva - and possibly the most challenging one - was that there be no secondary
laminates
anywhere on the exterior. Secondary lams were only allowed on the interior and where necessary to join
deck and hull, etc. Therefore all the various 'outages' for hull fittings, plumbing, hatches,
joints, etc. became 'bosses' on the
mould, so all of them had to be planned in advance. This was no small feat
considering that construction was more or less concurrent with the design work
being done.
The requirement for there to be no secondary lams was carried to extremes.
For example, each deck and its adjacent house sides were made as one single piece, with the
interior decks as a drop-in while the house structure was still upside down in the mould. Once
house and deck were fully assembled, each successive assembly was turned upright and craned onto the hull in one
piece -
no small task at 160'...!
On other vessels our construction strategy has of course been rather different,
and has always adapted to the customer's
preferences. In most cases, we find that form follows budget... so the
majority of other composite yachts
are much less extreme!
However, given the will and the means to do so, there are much more exotic
technologies available and weight savings can be dramatic.
Fiberglass vs. Aluminum & Steel...
Since we can work equally well with metal, composite or wood
structure, and since we find that there are
distinct advantages to each material, we do not offer any prejudice
- except possibly with regard to ferro cement which does have merit but definitely not within a
high labor cost venue...!
Composites are an absolutely excellent engineering
material and offer a considerable range of choices with regard to economy, strength,
lightness, ruggedness, construction methods, etc. Since there are
literally
hundreds of fabric choices
and a number of different resin systems, a strict comparison between
a highly variable composite structure and an entirely homogeneous structure
such as metal is not so simple!
It is perhaps less difficult to make a comparison between steel
and composite construction. Under around 150 feet, steel will
nearly always be heavier. Even that is subject to debate, since
above that size one can still make use of high strength fabrics
to effect a lighter structure than steel could possibly achieve - but at
a far greater cost.
Many writers have made comparisons between
aluminum and 'fiberglass' construction, however I find this to be very
problematic. Here is why...
On the basis of cost, weight and overall strength, for a
composite structure we must
first ask, "What's the available budget..? Will it be a cored
structure, or a single skin laminate..? What resin system and what
cloth will be specified..?"
It is not at all difficult to design a fiberglass structure to
be as strong as or stronger than an aluminum structure - and to also be
as light - however since we ordinarily must work within a given budget
there will always be definite limits on what will be possible with
either type of structure. We also
have to ask, "What do we mean by strength..?" Are we talking about impact
strength, or stiffness, or fatigue strength, or tensile strength, or yield strength..? We
then should consider questions such as insulation, fire resistance,
acoustic performance, corrosion vs. boat-pox, long term maintenance, eventual re-sale...
It is obvious that each of the above limitations, questions or
considerations will reveal a different winner in the contest.
Thus, the question of whether to favor composite or metal construction
can really only be answered on a case by case basis after fully
considering the owner's requirements and preferences, the available
budget, and the
intended use of the boat.
Can we say that one or the other choice is better...? Not
really. We can only say whether the choices made properly address
the wishes of the owner. Of course if the original request and owner's preference
leans strongly in favor of composites - so be it and there is no need to
question that choice. At that point it simply becomes a matter of
appropriate design, and the resulting yacht must meet the same structural standards
regardless of the choice of materials.
In our work we design very conservatively to the ABS
or the ISO
rules for each material type. With metal, our approach is to
specify the simplest structure that will do the job. That turns out to
be the most friendly for everyone involved
- from ease of
construction to simple long term maintenance.
Similarly with composites, except that
far more variation becomes possible with regard to cost, construction
technique, choice of materials, etc. Each
specific fabric and resin combination having its peculiar advantages.
For example, if it is within the budget to make use of resin infusion
techniques, pre-pregs, or high strength fabrics, then weight can be held to
an absolute minimum.
Building Efficiently...
Once the basic design has been created, there is still much that the design
office
can do to augment the construction process, i.e. to make it faster, more
efficient, more accurate, and ultimately to save money.
For metal, we
advocate NC cutting for the entire structure. It is very accurate and the NC cut
parts are quick to
assemble. For composites, we advocate 5-axis router cutting to create the moulds
for hull and superstructure. This dramatically shortens the amount of time the builders' shop
floor is occupied. In other words, because it is possible to completely eliminate lofting and
the mould making is out-sourced, the builder's in-shop overhead costs are
reduced.
Designing For The Long Term...
Metal and composites are both highly suited to the construction of sailing
yachts as well as LRC types and expeditionary motor yachts. These vessel
types have
actually become our specialty, i.e. ocean worthy voyaging yachts intended for long term
ownership.
Our work with metal boats has primarily been to elevate that concept from the
usual perception that metal boats are only for back yard builders - to that of
metal being a viable
choice for professional boat builders and for long term yacht ownership.
From my side, the design work is not nearly as great a challenge as finding the
right builder. In the case of metal boats and if the build venue is within North
America, qualified metal boat builders can indeed be difficult to find.
In the case of composite construction, they are everywhere...! Still, one
must choose wisely to be certain the builder is not only capable of
building the structure itself, but also able to install the equipment correctly and
to complete
the interior to the desired level of finish.
As always, I am pleased to discuss new design
projects in whatever materials are preferred - and without introducing a bias to
the equation. For a snapshot of our approach to
creating a new design please see our
Introduction and
Custom Design web pages, or
please feel free to
inquire for more information.
For considerably more information on the question of hull materials, please see
our web articles on the following:
Metal Boats for Blue Water
| Aluminum for Boats |
Aluminum vs. Steel |
Steel Boats |
Composites for Boats |
The Evolution of a Wooden Sailing Type
Copyright 1997
- 2007 Michael Kasten
