In article , Roger Long
wrote:

"Peter Wiley" wrote

No it wouldn't and saying that demonstrates that you don't know the
characteristics of materials. Steel is more ductile than aluminium -
it
will deform more before reaching its plastic limit and tearing. It
is
also less susceptible to work hardening and notch sensitivity, and
welds can be 100% the strength of the parent material, which is not
the
case with al. Steel is also far more resistant to abrasion.

True, (except for the first line), but we are talking about the
behavior of an entire structure not a test specimen. The aluminum
hull behaves sort of like a rubber inflatable boat and the steel hull
like a fiberglass one. The glass hull is "stronger" but may be more
likely to puncture in many situations.

How does this work? The aluminium has a lower elastic limit (amount of
flex before permanent deformation), lower plastic limit (deformation
before rupture) and lower resistance to abrasion than steel does. It is
structurally an inferior material. It compensates somewhat due to its
lighter weight enabling thicker sections to be used, regaining some of
the difference, and in shipbuilding due to its better corrosion
resistance.

The steel hull would have been in pieces not because it is weaker but
because it would have flooded and been rolled around on the rocks full
of water instead of retaining enough buoyancy to get up above the surf
line.

Oh, come ON. That boat (SG) is ballasted to a displacement of 14400
lbs. The steel version is ballasted to a displacement of 14400 lbs.
Both versions have all inside ballast. The keel shoes are 1" thick
plate in both cases. The chine bars are 3/4" thick rod. The frames are
2 1/2" by 1/4" flat bar. Can you explain to me how this ballast
mysteriously changes between a steel hull and an aluminium hull? How
the aluminium hull managed to keep water out that a steel hull would
have allowed in?

Why would a 4mm thick steel hull plate be ruptured, allowing water
ingress, when a 6mm aluminium plate hull kept the water out?

This argument has no credibility.

BTW, I have a full set of blueprints for a Colvin Witch hull so I do
know how they're built.

Obviously, anything can happen in any accident but I've seen enough
damaged boats to have great respect for aluminum.

I have enough understanding of the materials characteristics to design
boats like this one:

http://home.maine.rr.com/rlma/WHOIrv.htm

Nice boat. I have 2 30', 6 tonne alum boats, 4 jet barges and a number
of other small craft. However I also have steel vessels up to 6500
tonnes. Any impact that an aluminium boat will withstand, a steel boat
will also withstand, and likely with considerably less structural
deformation.

PDW

Oh, I see why we are having this conversation.

I wasn't talking about Sara Gamp. There doesn't appear to have been
much pounding in that case. A steel boat would have done fine as well
and probably would have experience less deformation and damage. If
you want to experience minimum deformation and damage from typical
impacts, steel is stronger. However, if you really beat on a boat in
extremis, the energy absorption and yielding of an aluminum structure
may leave you with something deformed and battered but mostly
watertight in a situation where the steel hull would have ruptured.
The strength and resistance of the steel concentrates the impact.
It's the rubber boat vs fiberglass dinghy thing except that the
aluminum doesn't spring back.

I'll admit that it's difficult to justify in typical engineering terms
and, yeah, I'm talking about examples I'm familiar with that are
anecdotal and without the controls of having a nearly identical steel
hull go ashore at the same time. Professionally, it's not defensible
to claim that an aluminum hull is more survivable but, hey, this is a
newsgroup. The point is that aluminum is certainly an excellent
choice for a long distance cruiser that is careful about the
electrical system (and those pennies). If you were faced with getting
a really badly damaged hull, patched, jury rigged, and back to safety,
you might be glad that it was aluminum. If you make a habit of
banging into things though, your boat will end up carrying more
permanent dents if it is aluminum than if it is steel.

I've seen enough steel hulls broken to be pretty sure that the boat I
was referring to was more watertight than a steel hull of similar size
and weight would have been after that treatment. When I saw it, it
was about 150 feet back from the normal high tide line on a rocky
island.

--

Roger Long



"Peter Wiley" wrote

Oh, come ON. That boat (SG) is ballasted to a displacement of 14400
lbs. The steel version is ballasted to a displacement of 14400 lbs.
Both versions have all inside ballast. The keel shoes are 1" thick
plate in both cases. The chine bars are 3/4" thick rod. The frames
are
2 1/2" by 1/4" flat bar. Can you explain to me how this ballast
mysteriously changes between a steel hull and an aluminium hull? How
the aluminium hull managed to keep water out that a steel hull would
have allowed in?

Why would a 4mm thick steel hull plate be ruptured, allowing water
ingress, when a 6mm aluminium plate hull kept the water out?

"Peter Wiley" wrote

Nice boat.

Thanks.

I have 2 30', 6 tonne alum boats, 4 jet barges and a number
of other small craft. However I also have steel vessels up to 6500
tonnes.

How did you end up with all those boats?

--

Roger Long