I'm going to assume you are looking at cold molded  design for trawler hulls,
not some hi tech racing boat.

I don't think you can beat steel for rugged strength: if you could beat it
easily, then many trawlers,draggers, and other offshore commercial vessels
would be cold molded. Even in Europe and the far east. So steel wins on
strength, durability  and ease of repairs. Maintenance is likely higher than
fiberglass but modern coatings have helped a lot. If you will be boating in
artic or Antarctic environs, for example, steel may be the only way to go for
passages through ice. I have the impression older steel vessels may not have
the resale value of fiberglass, but that is a very subjective observation.

Aluminum is next in strength with some weight savings, but that's not
significant for most recreational boats. In fact, the strength of steel or
aluminum isn't even necessary for most recreational trawlers of moderate size.
Seems like the USCG likes aluminum for their mid sized patrol and rescue boats
like non-sinkables.

Fiberglass in various types wins for a combination of low maintenance,
strength and weight. But it is by no means strongest. I seriously doubt
another 5,000 to 10,000 lbs at typical hull speeds of 6 to 9 knots, depending
on length,  is much of a fuel efficiency  factor for most recreational
trawlers. And if it is, due to say 5,000 hours of annual time underway, then
likely the additional weight and strength of steel is desirable as a dampening
factor in open ocean seas and storms. Fuel economy at trawler speeds has a lot
more to do with design than hull material. At 500 hundred annual running
hours, weight is likely not much of a factor at slow trawler speeds. Proper
propeller selection and engine size for a particular hull design is likely a
lot more crticial .

I'm sure any marine architect could provide a ballpark figure of additional
fuel costs for a typical trawler due to some weight variation. In planing
hulls for example, its another story entirely : check Huckins fairform flyers
with aluminum hardware, curtains instead of doors, etc, for weight savings.
I'd ask a marine architect something like: "In a typical 50 foot trawler,
what's the additional fuel usage per thousand hours underway for another
10,000 lbs of hull weight?

In addition, high strength materials like kevlar together with hi strength
fiberglass resins and fabrication techniques likely rivals cold molding in
strength, weight and durability.  As an example, Hinckley sailboats use such
materials.

As always, form follows function, so picking steel vs aluminum vs cold molding
vs traditional fiberglass is mostly a matter of (in order) (a) how the boat
will be used and (b) what the owners personal preferences may be, (c) budget,
(d) maintenance considerations.

High end racers are still being cold molded in the US: I saw a 175 foot
sailboat, hull inverted, in East Boothbay Maine two or three years ago.... at
Hodgkin Bros boat builders, I believe. What a sight!!!!

Absent any other information, I'd personally go with traditional fiberglass
construction using  epoxy or vinylester resin instead of polyester and a high
strength fabric like Kevlar as the near ideal hull material. And I'd like a
closed  cell cored hull above the waterline for sound and heat/cold
insulation.

Rob Brueckner
Hatteras YF

I would not so quickly jump to the conclusion that steel beats aluminum on
the issue of strength.  It depends:

http://kastenmarine.com/aluminum.htm :

"It takes over 60,000 pounds per square inch (psi) to tear apart a chunk of
mild steel, and 30,000 psi to deform the same piece, to make it yield. With
aluminum, around 45,000 psi will tear it apart, and around 35,000 psi will
deform it. Yes, you read that correctly: size for size, aluminum has a
higher yield strength. In these facts lie the extreme benefits of metal for
hull construction: The "plastic range" of either metal is quite high, so the
material can take a terrific beating without failure."

"An aluminum bare hull, built to the same strength standard, will weigh
roughly 45% less than the same hull in steel.  As a result, if high strength
is of the highest priority, the aluminum boat can be built to the same
structural weight as the steel vessel, and then be considerably stronger."

http://kastenmarine.com/alumVSsteel.htm

"Again, for the sake of an easy to follow comparison, we might say that "one
inch" of steel plate will yield beyond its ability to recover its original
shape at approximately 36k psi, and will fail at approximately 60k psi.
A "strength-equivalent" aluminum structure, having used deflection
(stiffness) as the design criteria, will have been built using roughly 50%
greater plate thickness. We might then say that this strength-equivalent
one and a half inch" thick aluminum plate will yield at around 51k per
square inch of surface area (around 29% greater yield strength than the
equivalent" region of steel plate), and will fail at around 67.5k psi
(around 12.5% greater ultimate strength than the "equivalent" region of
steel plate)."
Both of the above are pretty good explanations of the steel vs aluminum
issue.

Alan Wagner
Tampa, Florida
Building (Kasten designed) "Passage of Time"  in aluminum
http://kastenmarine.com/valdemar52.htm
http://yachtsmiths.com/valdemar_52_gallery_01.htm

-------Original Message-------

From: Robin
Date: 4/22/2007 5:01:33 PM

I don't think you can beat steel for rugged strength: if you could beat it
easily, then many trawlers,draggers, and other offshore commercial vessels
would be cold molded. Even in Europe and the far east. So steel wins on
strength, durability  and ease of repairs.
.      .      .

Aluminum is next in strength with some weight savings, but that's not
significant for most recreational boats. In fact, the strength of steel or
aluminum isn't even necessary for most recreational trawlers of moderate
size..

                            .      .      .

Rob Brueckner
Hatteras YF

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