The following are simplified methods to calculate the amount of flotation needed for a boat. The notation used, such as Boat Weight = W_b , is the same as used by ABYC, the Coast Guard and NMMA, so if you have problems you can call them and they will recognize what you're talking about. If everbody is speaking the same language it makes life a little easier.

The first method is for outboard motor boats over 2 hp. The procedure is basically the same for all boats. You need to find three things;

The boat weight and how much flotation you need to support it.

The machinery(engine) weight and how much flotation to support it.

The persons weight and how much flotation to support it.

I have simplified the calculations but in real life there is much more that goes into a boat than fiberglass and wood. Things such as screws, fittings, glue, paint, windshields, consoles, seats, etc. all add weight. There are several ways to determine the weight of each of these. The simplest way is to weight each item before you install it and add up the weights. But you need to know that different materials have different weights under water. That's what the table is for. It provides factors for determining what something weighs submerged. For instance;

Fiberglass; weighs about 1/3 under water what it weighs dry. So the factor is 0.33.

You need to determine where everything will be when the boat is swamped.

If it will be dry, use the full weight; if wet the submerged weight.

The engine will not be fully submerged on an outboard so you need to use a swamped weight. That is what it would weigh if it were only partially submerged. For that you need to take the swamped weights from the table.

Use Table 4 for outboard motor weights. You can also use Table 4-2 at the bottom of the table page for metric weights, or use ABYC Standard S-30,Outboard Engine and Related Equipment Weights Available from ABYC. If using Metric the value for water is 1000 kg/cubic meter

If you use S-30, the engine weights are heavier (because they are more up to date) than in the USCG table. So you will come out with slightly higher amounts of flotation.

Additionally There is a different calculation for boats with over 550 pounds of persons capacity. Watch for that in the example.

Method of Calculation with example:

OUTBOARD MOTORBOATS OVER 2 HP:

14 Foot outboard
40 Horsepower motor
Maximum Weight Capacity = MC = 1400 lb
Maximum Persons Capacity = PC = 900 lb
Boat Weight = 1000 lb
Use 62.4 lb per cubic foot for specific gravity of fresh water.
Use 2 lb density Foam For flotation.
Use 60.4 lbs per cubic foot  for dry weight supported by 2 lb foam.

Factors for the Specific Gravity of materials:
Factor: K
Fiberglass     =   0.33
Auminum        =   0.63  
ABS Plastic    =   0.11
Oak, white     =  -0.18
Oak            =  -0.56
Mahogany       =  -0.72
Plywood        =  -0.81

Calculate the amount of Flotation required, Ft, for The boat using:
Fb = Flotation for boat
Fp = Flotation for people
Fm = Flotation for the outboard engine

 
Boat (Hull) weight(Wb) = 800 lb
Hull is submerged: use submerged weights (dry weight X factor)
Fiberglass = 600 lb
Wood = 200 lb                  

                               Fiberglass            Wood
Boat Weight submerged       (600 X K 0.33 )+( 200 X K 0.81)     Wb = 360
 
Deck is not submerged: use dry weight.
Deck weight (Wd) = Fiberglas 100 lb + Wood 100 lb = 100 +  100  Wd = 200

Maximum Persons weight (Wp)                                     Wp = 900
 
Swamped motor and controls + Battery Wt =  (Wm)
 
(from column 2 and column 4, Table 4, 33 CFR 183.)  = 352 + 25  Wm = 377

Flotation for Boat Weight (Fb) = (Wb + Wd) / 60.4
(cu ft of 2 lb density closed cell foam)
                                = 360 + 200 /60.4 = 560/60.4 Fb   = 9.27

Dead Weight =
  Max Wt Capacity - Persons Capacity - Motor Controls Battery Wt  = Wdw
               Motor Controls Battery Wt = From Column 6 Table 4) = 550
Dead Weight =    1400 - 900 - 550 = Wdw =(If zero or neg Wdw = 0) = 0
 
Flotation for persons Weight (Fp) over 550 lb persons capacity
Fp = (1/2 the first 550 lbs Persons) + (.125 remainder) + .25 dead weight
Fp = (0.5 X 550)+0.125(Wp-550)+.25(Wdw) / 60.4 = 275 + 350 + 0 /60.4
 
Flotation for persons Weight (Fp) Fp 10.35
 
For boats with less than 550 lb persons capacity
Fp = (0.5 X Wp)+.25 (Wdw)/ 60.4= _______ + ______    / 60.4  Fp  = _____
 
Flotation for swamped motor (Fm)= (Wm) / 60.4
 
(Wm from Table 4), 33 CFR 183 column 2) =   352 / 60.4 = Fm      = 5.8
 
 
TOTAL Amount of 2 lb Foam Flotation (in cubic feet)
= Fb + Fp + Fm = Ft =  9.27 + 10.35 + 5.8 = 25.42 Ft = 26
 
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.

Recalculate total Boat Weight
 
Weight of Flotation in pounds (Fw)=2 lb/cu ft X Cu Ft=2X 26 = Fw = 52
 
Total Boat weight = Weight of Flotation + Boat weight=
                                              Fw+ Wb = 52 + 800 = 852