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HOW TO CALCULATE FLOTATION NEEDS
How to calculate how much flotation your boats need.
The following are simplified methods to calculate the amount of flotation needed for a boat. The notation used, such as Boat Weight = Wb , 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 everybody 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. S-30 also includes both 2 cycle and 4 cycle engines. This results in heavier engine weights than the USCG table. So you will come out with slightly higher amounts of flotation.
Additionally there is
a different formula to calculate flotation for persons weight for boats
with over 550 pounds of persons capacity. You will see this in the
Method of Calculation with example:
OUTBOARD MOTORBOATS OVER 2 HP:
14 Foot outboard
Factors for the Specific Gravity of
Calculate the amount of
Flotation required, Ft, for The boat using:
Boat (Hull) weight(Wb) = 800 lb
TOTAL Amount of 2 lb Foam Flotation (in cubic feet)
Recalculate total Boat Weight
The following is the method for Modified Level flotation:
OUTBOARD MOTORBOATS LESS THAN 2 HP & MANUAL PROPULSION:
See Example Calculating Capacities and Making Labels
Note: The requirement for flotation for persons weight is 2/15 of the persons weight or 0.1333 times the persons weight. But in the Guidelines the USCG uses 1/4 or 0.25 times the persons weight to provide a safety factor. This provides a safety factor of 2 or twice the amount of flotation for persons weight. If you use 2/15 you will have a lower amount of flotation.
Flotation for persons Weight (Fp): over 550 lb persons capacity
The following method is for Basic Flotation:
Factors for the Specific Gravity of
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