Example of Level Flotation Calculations

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

Formula:

Where:

Clev = C (maximum weight capacity) minus weight in column 6, Table 4 for the maximum horsepower rating of the boat, (See Appendix A)

B = Buoyancy of flotation material in pounds per cubic foot.

Discussion:
This may look confusing. The reason for the regulation requiring the manufacturer to support only part of the passenger load is that this weight (in testing) is added dry, and the assumption is that the actual passengers will be partially submerged thus enhancing buoyancy. The last entry [.25(Clev – persons capacity)] is the gear weight allowed or the difference between the values for maximum weight capacity and persons capacity exhibited on the boat’s capacity label.

NOTES:

1. If Clev is negative, use zero.
2. Use the persons capacity and maximum weight capacity that will be shown on the boat’s capacity plate.

In order to achieve a level attitude when conducting the flotation tests, it will be necessary to distribute the flotation material symmetrically on both sides and fore and aft of the passenger carrying area mid-point at the hull sides as close to the sheer line as possible. It should be located outside a vertical plane that is parallel to the keel, and within 6 inches of the hull sides at the widest point on the floor line. This will assure the necessary righting moments to pass the stability test. The flotation material must be as far out and as high in the gunwales as possible.

Step 4: Determine the total flotation material needed (F)

Formula:

F = Fb + Fp+ Fc

The total flotation material required is the sum of the results in steps 1, 2, and 3 above.

FIGURE 5.5 - Flotation Material Placement

FIGURE 5.6 - Flotation Material Placement

FIGURE 5.7 - Placement of Passenger Support Flotation Material
 

Example of Level Flotation Calculations

Assume an outboard engine-powered runabout with the following specifications:

From the Applicability section, we determine that this boat must comply with the Level Flotation requirements. It is an outboard powered, more than 2 HP, mono-hull boat under 20 feet in length, and the requirements include a Level Flotation system and some tests to determine its compliance.

In Level Flotation we must establish a swamped waterline, or the position in which the boat will float after preconditioning and swamped for 18 hours (See regulation). We will assume in this example that this swamped line is at the hull sheer or deck-to-hull joint; therefore, the hull will be considered swamped and its component’s weight converted to submerged weight, while the deck will be considered as dry weight since it will be out of the water.

We will now run through the calculation steps to determine the amount of flotation material necessary, and where to install it.

Step 1: Flotation material needed to support the swamped boat

Formula:

Fb = [(Wh x K) + Wd] ÷ B

Let’s identify the boat’s components:

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)

Substituting in the formula above:

Step 1: Flotation needed to support the submerged boat

Fb = (169.8 + 528) ÷ 60.3 = 11.57

Fb =11.6 cubic feet of foam

This foam should be installed under the floors, symmetrically distributed about the boat’sbalance point.

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

Formula:

Fp = S ÷ B

Where:

S = The swamped weight of the maximum horsepower capacity engine for which the boat is rated on the capacity label, plus the submerged weight of the battery.

B = The buoyancy of the flotation material in pounds per cubic foot.

The boat has been rated for a 140 HP outboard engine. When the boat is swamped, this engine will be partially submerged to the power head or the cowling - approximately. Table 4 (in Appendix A) gives us the weights needed here. Look at the Table. The 140 HP engine falls in the 80.1-145.0 range and, while the dry weight of the engine may be more, the number to use is the swamped (Column 2) weight. To this we must add the weight of the battery, and since it is going to be submerged, we use the submerged weight.

Therefore, S = 352 + 25 = 377 lb.

And, B = 60.3 as calculated before

Substituting in the formula above:

Fp = 377 ÷ 60.3 = 6.3 cubic feet of foam

This is the portion of the total foam that must be carefully located inside the volume formed by the portion of the boat forward of the top of the transom, where the engine is mounted.

NOTE:
The outboard engine weights in 33 CFR Subpart H (see Appendix A Table 4 of this Guideline), are outdated and much lighter than modern 4 stroke outboard engines. The reader is encouraged to refer to ABYC Standard S-30, Outboard Engines and Related Equipment Weights, for the latest outboard engine weight table. It is strongly recommended here that manufacturers use this new table of weights and be conservative on this important point.

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

Formula:

Where:

Clev = C (maximum weight capacity) minus weight in column 6, Table 4 for the maximum horsepower rating of the boat. (Appendix A)

B = Buoyancy of flotation material in pounds per cubic foot.

Be careful here. Let’s substitute in the formula one step at a time to avoid confusion. Let’s add the weight components first and then divide by B.

Look at the specifications. The boat has been rated for:

• Persons capacity in pounds = 1,100 lb.
• Maximum weight capacity = 1,400 lb.

This foam should be distributed along the hull sides and under the deck gunnels in the passenger carrying areas.

Step 4: Determine the total flotation material needed for Level Flotation

Formula:

F = Fb + Fp + Fc

F = 11.6 + 6.3 + 5.7 = 23.6 cubic ft. of foam flotation

 

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