In article , DSK
wrote:

Peter Wiley wrote:

Completely & utterly impossible to say without knowing design &
construction details and doing some sort of finite element analysis
which is way beyond my technical competence. That'd only give you a
theoretical measurement anyway.

But it is possible (not that difficult) to measure the hull's deflection
along any
axis in the real world. Not everybody has a dial indicator handy, though.

I agree completely. What I meant by saying you'd only get a theoretical
measurement. As a machinist I keep DTI's that measure to tenth's about,
of course. Hell, I've just been using a cheap laser level to level
formwork for a concrete slab I'm about to pour. When I finished, the
formwork was level to plus/minus 3mm all round, or 0.120". Today it'll
be something else, no doubt. We've used surveying gear to locate 3D GPS
antennae on ships to better accuracies. Isn't rocket science.


Think about it for 5 seconds, Nav. How thick is the plating? How many
longitudinals and frames and what spacing? How many welds and at what
spacing? What sort of keel? Is the deck a different material or not? Is
the shell monococque or not? Etc etc. Steel (I know little about ferro)
boats are typically a monococque construction with steel decks attached
to the hull by welding. Plastic boats may have their decks 'glassed to
the hull but usually it's a handful of self-tappers and a tube of
sikaflex. No rigidity there.

I disagree. Most structures are essentially the same, a box girder. Steel is
surprisingly limp all by itself. That's why they make I-beams instead of just
steel
planks.

True.


You're right that the details of the hull-deck joint are important to the
boats
structure. A lot of mass-produced boats do not have a very good hull deck
joint,
but others do. It is not inherent in the material.

Also true but I thought that's what I said - some plastic boats have
decks glassed to hull and this is damn strong, minimises possible
movement & flex (not to mention leaks.....) Those put together with pop
rivets or self-tappers have a lot more potential to flex.

I do know that some steel boats can be easily lifted by a couple of
eyebolts welded internally without any worries about deformation

So can many fiberglass boats. But this wasn't really an argument about the
relative
merits of steel vs fiberglass, was it?

Nope. Different materials, different strengths/weaknesses. Ditto ferro.
Friend just bought a 38' ferro sloop in really good shape for $30K AUD.
He knows the resale value is going to be the same - crap - but it's a
lot of boat for the money and for him it's a good choice. As he says,
if he gets 10 years out of it that works out to $3K pa even if he can't
sell it at all after that. Still worth it.

Peter Wiley