Flexible mounts is a whole different subject. The point I was making has to
do with transferring the load to the skin. It needs to be a smooth
transition. Otherwise the skin flexes at the "step" and eventually breaks.
Note that a flexible joint doesn't really cure this since I'm talking about
the stresses at the perimeter of the backing plate. Eventually the joint
will reach its limit and ask the skin to pick up the load. I believe you
can see this in the crazing of the fiberglass hulls around the chain plates
and winch mounts on some of the less well designed production boats.

Bulkheads are good. They transfer the load to the skin around the whole
perimeter of the pontoon rather than just to a spot on top, but do fillet
them generously to the skin around the whole perimeter. I'm sure there's a
practical limit (diminishing returns), but I don't know what it is. My gut
feel is that a 2" radius would be good. (??????) Maybe with a 6" belt of
epoxy/fiberglass applied to the inside first.

As for having the pontoons move independently of the deck (and of each
other), it sounds good, but I'd look for someone with first hand experience.
Could you get the same effect by having a flexible deck? What kind of sea
states are you expecting to handle?

Roger

http://home.earthlink.net/~derbyrm

"MRusson" wrote in message
...
On Sat, 27 Aug 2005 13:02:55 GMT, "Roger Derby"
wrote:

Just be careful when creating "hard points" that you spread the load
smoothly out to the skins. Any abrupt change in loading such as occurs at
the edge of a metal backing plate will lead to failure in that area.

Prime areas for concern include deck attachment points and bulkhead to
skin
joins.

Would these need to be a fixed hard point on the pontoon top? Could
these be made flexible somehow? Rubber mount points with steel pins
perhaps? I can see the pontoon top flexing and cracking with a small
hard point with a sub-standard backing plate. Could bulkheads in the
pontoons be inceased in size or more placed in the pontoon to help
overcome the flexing? I would like to attach the deck directly to
hardpoints at the bulkheads of each pontoon. The idea was to have a
bulkhead in the pontoon at every other deck mount point. If the deck
is 18 feet long, i could space the bulkheads at 24" apart inside each
pontoon. I could space the deck supports at 12" on center and bolt
through every other one to the pontoon tops. I think that 24" on
center at the deck supports is too far apart and would make for a
spongy deck.


Big epoxy fillets are good; as are stacked backing plates of decreasing
size
with the largest and limpest next to the skin.

The larger the better?


Thanks Roger.....

M Russon