This is the BWE
Don't Over-Think It But If You're Going To Do It At Least Do It Right version.
- Use a scale to weigh all your gear that doesn't float. My gear is 50 pounds.
- Add the weight of your kayaks rigging. My PA14 has about 40 pounds of rigging.
- Don't worry about your raw hull, HDPE has a slightly positive buoyancy.
- Don't worry about specific densities and wet weights, remember this is the BWE version.
- Calculate the buoyancy you need by dividing your gear and rigging weight by the weight of saltwater.
How much foam would Harry need?
90 lbs / 64 lbs per c.f. =
1.41 cubic feet of foam.
How many milk jugs of foam would Jorge need?
1 gallon = .134 c.f., so 1.41 / .134 =
10.52 milk jugs.
How many ping pong balls would Myth Busters need?
1 ping pong ball = .0012 c.f., so 1.41 / .0012 =
1175 ping pong balls.
How many pool noodles would I need?
Mine are 2-1/2" round x 55" long with a 7/8" hole down the middle.
1 noodle = .137 c.f., so 1.41 / .137 =
10.29 noodles.
Each of my noodles provide 8.768 lbs of buoyancy in saltwater (64 * .137).
These numbers are for
neutral buoyancy, i.e. it won't float to the surface (ignoring
Archimedes' principle). Add more flotation so you have positive buoyancy. A foam mfg. guide I read recommended 2:1 for a safety margin. I would use 10 of my noodles and rely on the factory installed foam blocks in my PA as my extra flotation.
Place your flotation in your hull high and to the sides for submerged stability. Place 50% of your flotation fore and 50% aft of your rigs center of gravity so that it will float level while submerged. Level floating allows you to access your safety gear, allows you to get on top of your submerged rig, and at least gives you a shot at bailing water out. You don't want to have to swim away from the Titanic.
If you want to over-think it you could start with the Flotation Requirements sections of the USCG Boatbuilder's Handbook.