How materials have affected boat design
QUESTION: Have new materials affected the actual design of sailboats, particularly multihulls, and if so, how?
ANSWER: Most definitely yes… but there are now some new
issues to face.
But let's step a little into history first. Early boats had no
effective means of sealing any part of their structure to
provide emergency buoyancy, so materials had to basically float.
This may have started with woven reed or the remarkable bamboo plant, but solid wood trees soon became the mainstay of all ship and boat building. But after a few centuries of that, the more durable woods like teak and mahogany became scarce and prices soared. Although certain woods were stronger across the grain than others, they were all anywhere from 20 to 60 times stronger with the grain than across it, so when combined with scarcity, ways had to be found to bridge that difficulty. Most structures, including boats, were therefore framed with wood members crossing at about 90 degrees to each other and in a crude and bulky fashion, this compensated for the cross grain weakness. The fastenings were then the main weakness as they caused local cracking and water infiltration and early iron ones, would rust.
Amazingly,
samples of early laminated wood placed at 90 degrees to each
other, have been found in Egypt dating back some 5000 years ago,
but the use of a peeler to slice off thin veneers from a
rotating log to laminate under pressure into plywood, really got
underway in the 19th century and soon revolutionized the design
of structural buildings. Marine grade plywood had to wait the
development of waterproof resins and the two World Wars
accelerated their discovery and use. But as a child who badly
wanted to build a boat while growing up in war torn Britain in
the 40s, plywood of ANY sort was just not available to anyone.
So back to solid lumber for my first one. At least by that time,
we had copper rivets and caulking to solve most of the leaking
issues.
But
how did plywood affect boat design? There is one major thing to
keep in mind with stronger and stronger materials, and that
comes from them being strong enough to be used either thinner or
in larger panels than solid wood ever was. This seriously
affects their rigidity, flexibility and fragility. When boats
were made out of solid timber, there was seldom much concern for
panel stiffness or even overall rigidity. The thicknesses, both
available and required simply for local strength, assured a
pretty solid structure and some overall hull flexibility worked
well to absorb the punishment of wave shocks and rigging loads.
Although the use of plywood solved the wood cracking and leaking
issues, the skin was now too thin to work without a good framing
system to keep the boat in shape and also, it could only be
formed or twisted to either cylindrical or conic shapes, so some
fine early forms ideally suited for certain low-speed vessels,
(such as wine-glass sections), were no longer possible. In
contrast, the flat hard-chine forms required for faster planing
motorboats were a natural for plywood construction, assuming
they were built rigidly enough, and even today this is still a
very viable material for such designs. Ingenious designers then
sought to solve the shape issues by cutting plywood into
narrower strips and creating multichines or using it lapped, but
this makes for extra work and is seldom used for anything over
about 17', other than the odd double-kayak. But the multiple
lengthwise joints does add rigidity and in small sizes, this
often eliminates the need for regular-spaced transverse framing.
Although
Fiberglass was invented in the 30s, it was not used much in
boatbuilding until the 50s 
after various
polymers were discovered that bonded well to it. But then it
exploded on the scene and when machines were invented to chop
and spray short fibers in random pattern along with a spray of
bonding resin, everything seemingly became possible. Boat shops
sprang up out of garages and storage sheds and many who knew
nothing about boats, got into the business. At first, it seemed
that once you had a female mould, that you could turn out a new
boat shell every couple of days, and these boats would neither
rot nor need painting and would apparently 'last for ever'! But
a look back on that period, shows some interesting lessons
learnt.
Although remarkably durable, the material did not last for ever and sunlight in particular, would slowly break it down. The total skin thickness was built up crudely and was often too thick which caused both cracking and crazing, and the boats often proved to be even heavier than the earlier plywood ones. The positive thing is that 'people' (not designers so much), did experiment with all kinds of boat shapes and out of that came motor-boat hulls with triple V shapes and longitudinal steps and many other varieties. New weaves of fiberglass were developed, like woven cloth, random mat, woven roving, bi-axials and C-flex etc and thousands of sailboats were built of solid glass laminates, many of which are still sailing today. Some had too much flex that caused local cracking in stressed areas and others were built too heavy. Problems have also occurred with moisture getting into some laminates after the surface 'gel-coat' has broken down—generally due to sunlight or being too thickly or unevenly applied. But overall, it has proved a successful material that once a good mould is built, allows almost any shape to be recreated without much extra cost… except for return curves (such as a tumblehome) that generally prevent the shell from being released from the mould. In terms of design, the material is more flexible than most wood or plywood structures and after adding the bulkheads and framing required to keep things in shape, often ends up heavier too. In terms of rigidity per pound of weight, wood is still a remarkably good material and there are still a lot of high-performing wood boats on the market. Unfortunately it typically needs far more maintenance than fiberglass and due to the dropping availability of quality timber, its price is climbing faster than that of fiberglass too. However, combinations can still be attractive and often, plywood boats are now given added protection with a sheathing of thin glass (or other woven) cloth with resin. As with all these things, there are good and bad techniques to use and also, the type of plywood to be covered can influence the bond and resulting service life.
So
what next? Well, we have identified that fiberglass can be too
heavy and also too flexible. Building in hat-section stiffeners
and frames can help to stiffen the skin but these are labor
intensive extras. Even aluminum alloy or stainless steel frames
have been tried and while successful technically, are expensive
to incorporate. So here is where the present day leaning towards
the use of composites comes in. By using two thin skins of
resin+fiber, spaced apart through the use of various much
lighter core materials, a panel can be created that is both
light and rigid—potentially the best of both worlds.
These
composites are still constantly under development and we are
seeing a huge variety of both core material as well as for the
outer skins. Core materials presently vary from lightweight wood
(strip-cedar and end-grain balsa) to various foams (both
flexible and rigid) and also to honeycombs made of resin-dipped
paper. Exterior skins not only include glass fiber (the most
common) but also kevlar and graphite that offer extra resistance
or strength at higher cost. They all have their supporters and
all have both advantages and disadvantages and perhaps that's
something for another Q&A. But what they ALL do, is reduce
the need for internal framing other than for bulkheads. Although
such composite sandwich construction is somewhat more limited in
what shapes one can incorporate compared to solid glass, they
are certainly lighter and more rigid—and that can significantly
up boat performance. Negative things to offset are: the
vulnerability of such thin surfaces skins to being pierced or
fractured due to their own brittleness and lack of bulk; the
difficulty of connecting to the composite in areas of high
stress plus the risk of delamination of the sandwich due to
impact, panel flexing, or stresses within or applied to the
surface itself. All these things are requiring a much more
sophisticated knowledge of engineering than plywood ever did and
the world's experts are still learning new things daily. The
higher the strength of the skin the more difficult it becomes to
bond to the much weaker core, so in some areas, the theoretical
skin thickness has to be increased just to prevent it from
deforming or fracturing under the inevitable flexing of a boat
in waves.
For
monohulls, the new ability to build the main hull SO much
lighter than before, has meant that a far higher proportion of
the total weight can now go in the keel. Many have wondered why
boats like the Volvo 60s are so amazingly fast these days,
reaching speeds once only the domain of fast multihulls. Well,
they are now far more like a huge dinghy. But instead of a crew
to keep them upright on their flat planing hulls, they now have
about 80% of their total weight in the keel and once really
moving, also gain stability from the hydrodynamic action on
their hull—now built far flatter and wider than ever before,
being as they now sail far more upright. Of course, with the
huge powerful rigs that these boats now carry, this all this
puts enormous stress on the hull-keel connection and today, we
sometime see photos of monohulls floating upside down with the
keel gone—a by-product of lightweight, composite construction
and the drive for speed.
These
new construction developments now present some interesting
design issues for all composite boats but these are pushed to
their absolute limit and ultimate failure when trying to create
the fastest possible multihull to cross oceans at speeds not
even dreamed off just a few years ago (like averaging 25k for 24
hours with peaks near 40k!).
Briefly, it goes like this.
As
these materials permit a multihull to get lighter (and therefore
faster), their required buoyancy volume drops. This gives
smaller section hulls and amas. When combined with the length
required for the highest speeds, these hulls end up like
pencils. As they also impact waves faster and faster, the loads
on these long pencil hulls increase and yet, their cross
sectional area progressively becomes too small to provide the
required strength and at the limit, they break off at their
weakest point. This point is typically close to the attachment
for the forward aka to the ama and of late, numerous extreme
ocean racing multihulls have fractured close to that connection.
Either the ama itself breaks off just ahead of (or on occasion
just behind) the aka, or the aka breaks off just before it
enters the ama.
Arguably the most successful design (Sodeb'O) cleverly chose to use the larger cross section of the main hull as the longer hull to oppose pitching and pitchpoling and so far, this design has proven strong and unbeatable. The amas are reduced in length to a point that their cross section can take the imposed loads. (see photo left)
Even some
smaller trimarans are vulnerable and the design and construction
of moulded, composite akas has become so critical that noted
designers such as Ian Farrier will not even supply the plans to
build them to anyone, but will only furnish the akas complete as
a kit from a supplier who has proven to meet certain
construction standards. Despite that precaution, even that
cannot totally prevent failure, as noted in the
photo. 
The use of continuous section cross beams as for demountables, actually results in lower stresses, as they generally have waterstays that are attached well out on the aka.
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