Where to Get Foam to Make Boat Unsinkable

[smaarch]

AKA: whatever floats your boat

I read thru this 4 year old thread and i have to say i don't get it.
I'm no expert, i don't plan on adding floatation - just a mental exercise here.

If i understand Archimedes' principles of buoyancy correctly: on a monohull the weight if the keel and the shell of the hull form displacement against the water and therefore the boat floats. my assumption is when the water on the outside and inside of the hull equalize (i.e. weight/density), she has essentially lost all of her positive buoyancy and progresses to sink. (also assuming no reserve buoyancy).
Add to this the problem of above waterline openings, the galley sink for example. once the waterline engages the level of these openings, they come into play and the progression to a sinking state increasing more rapidly.

on the OP's Bristol 27 the place the foam would be installed (low, under bunks, etc.) is below the waterline. the buoyancy does not come into play until the cabin is awash and raises to at least the underside of the foam. correct?
this raises a couple of questions: what level does the boat become "re-buoyant"?
in order for the foam to work, it has to be restrained in place (not float freely) and essentially lifting the hull with it. 6000 lbs of boat against 100cf of foam = 60 lbs of upward force/cf (which aligns with the calc the op did for the foam required).
the lower the floatation, the lower the boat sits, the higher this upward force.

hmmmm....two mini coopers of floatation moving skyward against 6000 pounds of boat looking for davie's locker sound like a significant part of the design process....

[rick.frazier]

I'm curious just how far from land you intend on taking this very small boat as to where doing all of this extra work will merit a gain? It's a 22 foot boat.... if you can't swim back to shore if something catastrophic happens, you should probably take up something a bit more relaxed and less strenuous.

[Snowpetrel]

I think he has nearly completed his circumnavigation. I'd be interested to hear how the foam worked out. I always thought an unsinkable boat was a great idea, and I am suprized the etap's and sadlers with this feature are not much more popular.

[MLOI]

I have an Etap 37, which I bought because it is a well built boat that sails well, not because it is unsinkable. We carry a life raft, and sundry other blow up pool toys.
There is definately interest in them, we are currently cruising and regularly get asked for a tour to check out how it is done.
FWIW, we do lose some storage space, but not a lot. Living space is about standard for a 2 cabin 37 ft yacht.

Biggest benefits are actually insulation, and hull stiffness. We spent a day recentlt dropping off backless 3m swells, and nothing flexed or moved. Except my testicles, they were well pounded by unhappy crew...

Mike