I've had quite a bit of experience working on boats, some for my main
occupation which is as a welder/fabricator and some just on my own or
friends boats. Steel hulls can be good and lets face it; welding steel
is not a difficult skill with a little practice. Im not telling you to
weld up your own hull and go sail the world, but most people with a 3
month part time course or a couple of weeks very intensive training can
learn almost all they'd need to know to operate an oxy torch, a mig
welder (or stick if you dont mind rolling the hull over) and some basic
forming skills. I've worked on steel hulls from 25' to 10,000 tonnes
with hull thicknesses from 1/4" to 1". With the right tools and enough
time you can do as good a job as any manufacturer. Make sure you've
got it right before you trust your life to it though.

Aluminum is a bit trickier, but its mostly more of a tool game. Dents
can be fixed with something as simple as two hammers. Forming is not
hard but you'll need a couple of dollys and to learn how to anneal.
Most aluminum work hardens. Cracks/splits are another story, and leaks
can often be mended with some silicone. If you've got lots of time and
little money and are somewhere close to america, buy an oxy/acetylene
rig (under $300) and then get one of these miracle gas welding torches
like the meco midget, see here;

http://www.tinmantech.com/html/meco_midget_torch.html

these are used for super high tech fabrication of airplanes, F1 race
cars, high end motorcycles and bicycles. The guys at the above webpage
have excellent demo days, materials and videos. Expect to spend 6
months+ learning this very valuable and highly sought after skill. You
will be very popular if you can master it.

If you have lots of money and little time then MIG aluminum is more for
you. I've done plenty of aluminum work with boats, and at the harder
end the latest high tech tip trailers for big trucks. Dont settle for
anything less than a push/pull system (small motor pulling the wire
that is contained in the gun). Expect to pay somewhere in the vicinity
of $20,000 for the latest fronius or kempi welders that will have you
welding aluminum to survey standard within an afternoon. Anything less
(like even $5,000) just will not cut the mustard. I've tried most of
the stuff on the market. Forget anything that costs less than $1000,
or any welder that claimes to be adabtable to aluminum; you're wasting
your time and money. If you dont have the money to get into this game
then call around to find out who is making aluminum truck bodies and
turn up any day of the week except Friday near their closing time with
a case of beer and your boat. They will have _plenty_ of scrap in the
bin.

My personal experience with fibreglass seems to be the same as others
who have already posted. In the early days, folk who were making boats
out of the stuff seemed hesitant, and they used the same thickness of
solid fibreglass as they had been using for wood before then! None of
these boats were speed demons, but they were _solid_ and many are still
doing circumnavigations. After these first boats didnt sink they got
comfortable and started experimenting with different ideas.... these
middle boats are hit and miss and you really need to dig, talk to
owners of the same boat, find out the companies history, talk to the
guys running the company if its still in operation. They are very
helpful if they're still around and have memories that will amaze you.
With newer boats they seem to have got it right with hull thickness and
composites/sandwhiches.

Cant speak for wood except to say whenever i see a wooden boat i want
one but im scared at the same time. There are some very handsome
wooden boats out there, and it strikes me as more a craft than
fibreglass though i cant say why. I cant help but rub my hands over
the hull, and then knock on it. This is something i've seen people who
have never sailed in their life do, so it must be in our basic D.N.A or
something ;-) The sound tells a lot.

Shaun