BOATBUILDER'S HANDBOOK

Flotation - Basic Flotation

FEDERAL LAW

183.105 - Quantity of flotation required

(a) Each boat must have enough flotation to keep any portion of the boat above the surface of the water when the boat has been submerged in calm, fresh water for at least 18 hours and loaded with:

(1) A weight that, when submerged, equals two-fifteenths of the persons capacity marked on the boat;

(2) A weight that, when submerged, equals 25 percent of the dead weight; and

(3) A weight in pounds that, when submerged, equals 62.4 times the volume in cubic feet of the two largest air chambers, if air chambers are used for flotation.

(b) For the purpose of this section, "dead weight" means the maximum weight capacity marked on the boat minus the persons capacity marked on the boat.


In addition to the quantity of flotation required, the regulation then deals with flotation material tests and specifications. See Flotation Materials. The regulation does not deal with the methods to calculate how much foam is necessary, and where to install it. That is the purpose of this Guideline.

Basically, our method is to calculate the flotation material needed to support the following components when the boat is submerged:

a. the submerged boat;
b. the submerged propulsion equipment;
c. a portion of the passenger load;

The total flotation material needed is the sum of the three components above.

Boats requiring Basic Flotation are fitted with buoyant materials or acceptable flotation systems in at least the minimum quantity as determined below. Void compartments or air chambers integral with the hull shall not be included as part of the required flotation materials.

If non-integral air chambers are used for flotation, the requirements for Basic Flotation shall be met excluding the two largest air chambers.

Calculations: To determine the total flotation material needed to support the boat, we will separately determine the flotation needed to support the boat (hull and deck), the propulsion machinery and the passengers (persons capacity).

Formula:

F = Fb + Fp + Fc

Where:

F = Total flotation
Fb = Flotation for the submerged boat
Fp = Flotation for the submerged propulsion machinery
Fc = Flotation for the passengers (persons capacity)

NOTE:
In Basic Flotation we are considering the submerged weights since only a small portion of the boat will be out of the water.

Step 1: Determine the flotation needed to support the submerged boat (Fb).

Formula:

Fb =([Wh x K] + [Wd x K]+ .69We ) ÷ B

Where:

Fb = flotation needed
Wh = dry weight of hull
Wd = dry weight of deck and superstructure
We = dry weight of factory installed equipment, hardware and accessories
K = conversion factor for material used. See Table 4.1 below
B = buoyancy of one cubic foot of flotation material expressed in pounds.

NOTE:
If air is used, B will simply be 62.4 pounds (the weight of fresh water displaced by 1 cu ft). If foam is used, B will equal the weight of 1 cu. ft. of fresh water, minus the weight of one cu. ft. of the foam used, minus the amount of water absorbed by this foam when submerged for 18 hours.

Of course the submerged weight of the hull is the result of adding the weights of each component multiplied by each component factor listed in Table 4.1 below e.g.: fiberglass x 0.33, fir plywood x –0.81 , aluminum tanks x 0.63, etc. There is no factor K as a whole; K is a value (multiplier) used to derive the submerged weight of each component. See example below.

Step 2: Determine the flotation material needed to support the submerged propulsion equipment (Fp).

Formula:

Fp = G ÷ B

Where:

G = 75% of the installed weight of the engine, drive and battery (inboard), or the engine, outdrive and battery (sterndrive) - in pounds to the nearest whole number;

B = Buoyancy of 1 cu.ft. of flotation material used in pounds.

Step 3: Determine the flotation material needed to support the persons capacity (Fc).

Formula:

Fc = .25 (C) ÷ B

Where:

C = Maximum weight capacity.
B = Buoyancy of 1 cu.ft. of flotation material used in pounds.

NOTE:
The regulation calls for 2/15 (0.133) of the persons capacity. This Guideline uses 0.25, resulting in slightly more flotation in the boat. This safety factor will help ensure that the boat meets the minimum requirement in the event of minor weight changes during construction.

Step 4: Determine the total flotation material needed (F) to support the boat. This is the sum of steps 1, 2, and 3 above.

Formula:

F = Fb + Fp + Fc

TABLE 4.1 - Factors (K) for Converting Various Boat Materials from Dry to Submerged Weight

(In Table 4.1 - *1.5 pounds/square foot for 1/4 inch thick linoleum.)

The factors in Table 4.1 are calculated using the following formula:

Factor = Specific gravity – 1
  Specific gravity

This formula may be used to obtain the factor for materials not listed in Table 4.1.

NOTE:
The specific gravity is the ratio of the density of a substance to the density of fresh water at 39º F.

Weight of water - fresh, at 39ºF = 62.4 pounds/cubic foot

Specific gravity of water—fresh, at 39ºF = 1.0
 

 

Example of Basic Flotation Calculations

Assume an Inboard/Outdrive (Sterndrive) runabout with the following specifications:

Length Overall 18' - 6"
Beam 7' - 3"
Propulsion 210 HP Inboard/Outdrive
Machinery weights 1,075 lb.
Maximum weight capacity 1,400 lb.
Maximum persons capacity 1,100 lb. or 8 persons
Dry hull weight 800 lb. (fiberglass 650 lb. + plywood 150 lb.)
Dry deck weight 300 lb. (fiberglass 245 lb. + plywood 55 lb.)
Deck hardware 228 lb. (Mostly aluminum)
Hull hardware 110 lb. (Aluminum 80 lb. + stainless steel 30 lb.)
Total weight 2,513 lb.


From the Applicability section, we determine that this boat will have to comply with the Basic Flotation requirement. This means we need only float the boat with equipment and a certain amount of the passenger load.

In Basic Flotation we consider the entire boat to be submerged. There is no swamped water line, and consequently all the components will be affected by the conversion factors in Table 4.1. We will now run through the calculations to determine how much flotation will be required. We will use Table 4.1 for the weight conversion factors of materials.

Step 1: Flotation needed to support the submerged boat:

Formula:

Fb =([Wh x Kl] + [Wd x K2]+ .69We ) ÷ B

Let’s identify the components and determine their weights when submerged:

    Part Description Weight   Factor (K)
Table 4.1
  Submerged
Weight
Wh = Weight of fiberglass hull 650
x
0.33
=
214.5
   
+
           
    Weight of hull fir plywood 150
x
-0.81
=
- 121.5
    +            
    Weight of hull aluminum hardware 80
x
0.63
=
50.4
    +            
    Weight of hull steel hardware 30
x
0.88
=
26.4
                 
    Wh = Submerged weight of hull 169.8 lb.  
                 
Wd = Weight of fiberglass deck 245
x
0.33
=
80.8  
    +            
    Weight of fir plywood on deck 55
x
- 0.81
=
-44.5  
                 
    Wd = Submerged weight of deck 36.3 lb  
                 
We = Weight of factory installed equipment 228
x
0.63
=
143.6  
                 
    We = Submerged weight of factory installed equipment, etc. 143.6 lb.  
                 


B will be calculated as follows (assuming use of Polyurethane foam of 2.0 lb. density)

B = 62.4 – 2.0 = 60.4; then allow for 5% moisture absorption (2.0 X 0.05 = 0.1)

B = 60.3 lb

Substituting in the formula:

Step 1: Flotation needed to support the swamped boat

Formula:

Fb = ( [169.8] + [36.3] + [143.6] ) ÷ 60.3

Fb = 5.8 cubic feet of foam

Step 2: Flotation needed to support the submerged propulsion equipment

Formula:

Fp = G ÷ B

G = 75% of 1075 lb. (engine, outdrive and battery) = 806.2

B = 60.3 lb.

Fp = 806.2 ÷ 60.3

Fp = 13.4 cubic feet of foam

Step 3: Flotation needed to support the passengers.

Formula:

Fc = 0.25 (C) ÷ B

Fc = 0.25 X 1400 ÷ 60.3

Fc = 5.8 cubic feet of foam

Step 4: Total flotation needed for Basic Flotation

Formula:

F = Fb + Fp + Fc

F = 5.8 + 13.4 + 5.8 = 25

F = 25 cubic feet of foam

NOTE:
The Basic Flotation requirements do not address where the foam is located in the boat.


Summary of Basic Flotation Performance Requirements

The manufacturer must be sure that the boat will float, in any position, when loaded according to the regulation. Figuring the calculations alone may not satisfy the regulation; consequently, the boat should be pre-conditioned, loaded with the weights prescribed, and then swamped.



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