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Re: Does SJ 23 Sink when holed or flooded?




> To float, there should be enough volume
>of foam to compensate for the high density keel.

Paul Kamens, a nautical architect and serious club racer, posts often in
Usenet in the rec.boats.* hierarchy.  He tossed out that same rule of thumb
once, for wooden core boats.  The reasoning is that the net displacement of
the hull materials is about the same as its mass; if we add enough
flotation to counteract the lead ballast we'll be about neutrally buoyant.
On his boat (a Merit 25 which he has singlehanded between Hawaii and San
Francisco) much of the flotation comes from closed-cell foam berth cushions.

Some quick calculations (I am _not_ a nautical architect):

SJ 23 has 700 pounds of ballast.  Let's shoot for adding 1000 pounds of
buoyancy.  At 8 pounds per gallon, that's 125 gallons--that is, adding an
empty 125 gallon tank would meet the goal.  Let me shift to metric:

1000 pounds = 454 kg, round to 500 kg, or 500 liters
1 liter of water is a cube 10 cm on a side, we want 50 cm x 100 cm x 100
cm.  At 15 cm thickness (about 6 inches), that's 33333 sq cm of cushions.
Figuring 6 foot/183 cm long cushions, I get
183 cm long x 15 cm thick x 182 cm wide = 500000 cubic cm

Or back to English units, roughly 6 feet by 6 feet of 6 inch thick closed
cell foam would counteract the weight of the ballast.  In more concrete
terms (hmmm, a bad choice of words if we're trying to stay afloat), replace
settee and v-berth cushions with 6" thick closed cell foam and we're pretty
close to neutrally buoyant.  This assumes, though, that the cushions are
floating up against the cabin ceiling.  Would want to add foam in the
forward locker (under the v-berth) and some of the tough-to-access lockers
aft, perhaps under the starboard pipe berth or under the cockpit floor.

Hal Mueller            http://www.peak.org/~hal/
Corvallis, Oregon      http://www.peak.org/~hal/ccd/ (dance)
Home:  (541) 752-4776  hal@peak.org
Work:  (541) 754-4490  halm@mail.cor.epa.gov


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