It’s
stronger, pound for pound, than fiberglass. It’s stiffer, pound for pound than
steel, aluminum, or fiberglass. It floats, it accepts fastenings well (you can
actually attach things with nails or screws), it’s plentiful, and it’s easily
repaired with simple tools. On top of all this, it’s biodegradable and it’s
warm and appealing to the human soul.
Unfortunately,
a wooden boat cannot be mass-produced as simply and cheaply from a standard
mold as can a fiberglass craft. And
unless you make special arrangements, a wooden hull won’t last as long as a
fiberglass one because, besides being biodegradable, it’s also easily digested
by a variety of hungry microbes, borers, and sea worms.
For a
one-off hull, however, there’s still nothing to beat wood. The tendency these
days is often to seal the wood during construction with several coats of epoxy
resin. This is supposed to make it resistant to rot and give it a life that
should last as long as fiberglass. The coating of epoxy is efficient at
blocking the passage of water and so will keep the wood drier than most bugs
can stand, but no epoxy coating will block the passage of water vapor totally.
That’s why I don’t think it’s a good idea to coat thick timbers with epoxy.
They only have to swell a little and the epoxy will split, allowing water to
enter and effectively become trapped there.
I believe a
better approach is to laminate thinner pieces of timber so that each individual
piece is isolated and encapsulated, but I still have doubts about totally
sealing wood with plastic resins.
In my
limited practical experience in these matters, I have always felt it safer to
paint one surface of the timber with ordinary oil-based paint that can
“breathe” and allow trapped moisture to escape. My gut feeling is that no
matter what you do, water will find its way in sooner or later, and you’d do
better to provide a path for it to get out again.
Today’s
Thought
Ingrained in
most of us is a creative spirit, and nowhere can this find a better reward than
in building a boat.— Edwin Monk.
Tailpiece
“Waiter, there’s a fly in my soup.”(8)“That’s funny, madam, most people find cockroaches.”
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)
2 comments:
Doesn't it depend what you mean by strength? Glass fibre excels in tension - e.g. according to Wolfram Alpha, teak has a tensile strength around 120 MPa, where glass fiber averages around 560 MPa (but can go to 6000). Another nice thing about glass fibre is how easy it is to orient the strands in the direction from which the loads will come.
It’s stronger, pound for pound, than fiberglass. It’s stiffer, pound for pound than steel, aluminum, or fiberglass. It floats, it accepts fastenings well (you can actually attach things with nails or screws), it’s plentiful, and it’s easily repaired with simple tools. On top of all this, it’s biodegradable and it’s warm and appealing to the human soul.
While the latter part of this is fine, the former has problems. Give a pound of aluminum, steel or fiberglass and you can make a stiffer structure than you could with the same pound of wood. A reference for Alaskan Sitka Spruce on matweb (this is about as good of a high performance wood as you can hope to find) has a Young's modulus (ie, stiffness) of 11 GPa on the high end, and 430kg/m^3 for density, for a specific modulus of about 25 MPa/kg/m^3. A low carbon steel has a modulus of about 200GPa and a density of 7820kg/m^3 for a specific modulus of about 25. An aluminum (6061) ends up with a modulus of about 70GPa and a density of 2700kg/m^3, giving a specific modulus of about 25 again.
So what, they all seem equal right? Those numbers would only be comparable for an identical geometry, once you can form the material to create a more efficient moment of inertia (say, a beam or tube) the metals will really take off in terms of specific stiffness, and composites even more so since the fibers can be preferentially laid out only where they are doing the most good. That is also using the best wood available, something else with knots in it or that is a heavier species will suffer all the more. Maybe you can make similar structures out of the wood, but it will be vastly more labor intensive and defeat a lot of the good points of working in wood. Apart from stiffness, the metals also have much greater strengths, particularly where point loads are encountered that would crush the grain in the wood, creating a local weak point.
Wood is great for lots of reasons, but being a high performance material just isn't one of them.
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