On Tue, 5 Aug 2008 20:50:00 -0700, "Bob La Londe"
wrote:

Ok, I'm in the middle of a rebuild/repair on an aluminum boat. I just
jerked the plywood sole out of it in order to replace it with aluminum
decking, and I found what looks like plant arranging foam between the ribs
for flotation. It soure doesn't look or feel like a sealed cell foam. Its
also quite a bit narrower than the space available. I was thinking about
replacing it with foam that fits snug between the ribs to increase the
flotation slightly, but I'ld like to use the best thing possible. Would
styrofoam be more suitable? I know styro hardly absorbs any water at all.
It lasts indefinitely as the flotation under docks as long as its not
exposed to sunlight. .

How about a poured foam? Are there any that form sealed cells as it
hardens? Would it work here, or would it be too hard to get it level with
the ribs so I can put in the new sole? I'm not sure I want poured foam in
here anyway, as any water that comes in the forward deck hatches etc drain
through the strength creases in the hull to the back of the boat where it
can get kicked out by the bilge pump.

I'm thinking if I can styro that its just the right thickness it will be the
best foam replacement, but there is a lot more experience in this group than
I have.

Bob La Londe
www.YumaBassMan.com


I have a "bolt together 3 part dinghy" that has styrofoam blocks glued
under the seat as flotation and have seen encapsulated styrofoam used
on other commercially made boats. Sometimes it is wrapped in plastic
sheet and heat sealed. Remember that the foam is not what keeps you
afloat it is a volume that weighs less then the water so it can be
anything that doesn't absorb water.

Re your other post about the transom. You have two forces acting on
the transom. One, a bending moment due to the weight and thrust of the
motor and secondly, a crushing force due to the pressure of the engine
mount clamps or bolts.

The first can be handled by designing a series of formers so that the
former and the inner and outer skin act as a beam - obviously a number
of formers. The second could be taken care of by either heavier - say
double or triple skinned sections separated by a heavy former or even
solid bar stock, or a heavy wall "box" section in line with the engine
mounts. You would need to "taper" the support out so that you don't
have a stress concentration which would cause cracking.

Have a look at some Home built airplane sites to see how they handle
forces imposed on aluminum structures.


Bruce-in-Bangkok
(correct Address is bpaige125atgmaildotcom)