Anyone dare to do the math? 
INBOARD MOTORBOATS
Fiberglass Wood
Boat weight (Wb) = ( ____ X K 0.33 ) + ( ____ X K -0.81 ) =_______
Deck weight (Wd) (if any) = ________+ _________ =_______
Maximum Persons weight (Wp) =_______
Swamped motor, drive unit + Battery Wt (Wm) = _______
(includes motor, drive unit, propeller, shaft, battery & transmission)
Dead Weight = Max Wt Capacity - total dry weight Wdw = ______
(If zero or negative ignore)
Flotation for Boat Weight (Fb) = (Wb + Wd) / 60.4
(cubic feet of 2 lb density closed cell foam)
=_______ X _____ /60.4 = ______
Factor: K =
Fiberglass = 0.33
Aluminum = 0.63
ABS Plastic = 0.11
Oak, white = -0.18
Oak = -0.56
Mahogany = -0.72
Plywood = -0.81
Flotation for persons Weight (Fp) over 550 lb persons capacity
Fp = (0.5 X 550)+0.125(Wp-550)+.25(Wdw) / 60.4
= 275 + ________ + _______ / 60.4
Flotation for persons Weight (Fp) Fp = _______
boats with less than 550 lb persons capacity
Fp =(0.5 X Wp)+.25 (Wdw)/60.4 = ______ + ______ / 60.4 = Fp = _______
Flotation for swamped machinery (Fm) = (Wm) / 60.4 = _______ / 60.4=
Fm = _______
Amount of Flotation (in cubic feet) = Fb + Fp + Fm = Ft
= ______ +_______ + ______ + _______ = _______
Weight of Flotation in pounds (Fw)
= 2 lb/cu ft x Ft = 2 X ____ =_______
Total Boat weight = Weight of Flotation + Boat weight = Fw + Wb =
= ___________ + ___________ = ______
Keep track of total boat weight over a production run. If total boat
weight increases significantly (say more than 50 lbs) you need to add
flotation.
"calculating floatation
Boats float because they weigh less than the water they displace. If you look at the volume of a boat when it's floating, it displaces MORE water than it would if the boat was swamped.
To make things simple, let's say your boat is a rectangle - 10 feet long, 4 feet wide, and 1 foot high. It's made of aluminum. The most your boat could displace is 40 cubic feet of water, and that would be with the gunwhales right at the water line.
A cubic foot of water weighs around 60 lbs. It actually wieghs a little more, but I'm trying to keep this easy for those mathematically challenged like myself. If you can displace up to 40 cubic feet of water, and each cubic foot wieghs 60 lbs, then your boat has a capacity up to 2400 lbs. Anything mor than that would push the gunwhales under water and you would sink. In reality, you would never load it that heavy, since any little wave spilling over could easily caus you to take on water.
Now let's say after reading THT you decide that you don't want to sink if you take a big wave over the bow - how much flotation do you need? 2500lbs? No. Basically, you know that your aluminum hull weighs 500 lbs. Does that mean you need 500 lbs of buoyancy to keep it floating? No. You need to know what the volume of that aluminum is, so that when it's swamped, you know how much water the aluminum itself displaces. If a cubic foot of aluminum weighs 500 lbs, and your bare hull weighs 500 lbs, then they used one cubic foot of aluminum to make your lovely boat.
If they used one cubic foot of aluminum to make your boat, then when you are swamped, your boat is only displacing one cubic foot of water (you don't count the water inside the boat). Since water weighs 60 lbs per foot, and your aluminum is 500 lbs per foot, then you would need 440 lbs of buoyancy to keep your boat from going to Davy Jones locker.
If you add an engine to your boat, or people or anything else, you would need additional floatation - not equal to the weight of the engine, but equal to the difference in the weight of the water that the engine would displace, and the weight of the engine.
So you wrap your engine in a big plastic bag and dunk it in a gigantic water filled glass cylinder with markings on the side. The water goes up when you dunk the engine in, so you figure out that your engine has a volume of 2 cubic feet. Let's say it weighs 200 pounds. So if it displaces 2 cubic feet, then the amount of buoyancy required for that 200 lb engine would be the difference between the weight of 2 cubic feet of water and your engine. 2 cubic feet of water would be 120 lbs, and your engine weighs 200 lbs, so you need an additional 80 lbs of buoyancy to keep the enigne afloat.
I made up the number here, but my intent was to explain the principles behind displacement vs. weight when talking about floatation."
2 pound foam weighs 2 pounds per cubic foot meaning each 2 gallon kit produces 480 pounds of bouyancy. A 2 gallon kit (1 gallon of part A and 1 gallon of part B) will yield up to 8 cubic feet of cured foam. 6 gallons would make most any boat any of us use unsinkable with 1440 pounds of bouyancy
