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Old 18-01-2014, 05:44   #61
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Join Date: Jun 2012
Location: here and there
Boat: P30
Posts: 199
Re: Where to Get Foam to Make Boat Unsinkable

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....

S/V Voyager
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