Flotation - Level Flotation


183.202 - Flotation and certification requirements

Each boat to which this subpart applies must be manufactured, constructed, or assembled to pass the stability and flotation tests prescribed in Secs. 183.225 (a), 183.230 (a), and 183.235 (a).

The regulation is complex and difficult to read. The objective of this Guideline is to help the manufacturer calculate first how much foam will be needed to support each of the components (the boat, the machinery, and a portion of the passengers) and then indicate where that flotation material must be installed to pass the required tests. The methods were developed over many years, and the Guideline uses numbers and factors that result in flotation that meets or exceeds the requirements of the regulation. Following these steps carefully will result in successfully floating the boat in the manner prescribed. Some experimentation in testing may be necessary.

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

Flotation material located at the sides, as far aft and as high as possible, will help make boats with machinery located aft float level when swamped. Some boats may require the keel area inside the boat to be void of flotation material so that the space can flood from either end to provide proper balance in the swamped condition.

Integral air chambers cannot be counted toward meeting the Level Flotation requirements. If non-integral air chambers are used for flotation, the Level Flotation requirements must be met excluding the two largest air chambers.

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

a - the swamped boat;
b - the swamped outboard engine and submerged battery;
c - a portion of the persons capacity;

The total flotation material needed is the sum of a + b + c. The boat must float level and pass a stability test.

Calculations: To determine the total flotation material needed to support the boat and keep it level, we use the following formula:

F = Fb + Fp + Fc


F = Total flotation material
Fb = Flotation for the swamped boat
Fp = Flotation for the submerged propulsion machinery
Fc = Flotation for the passengers (a portion of)

The idea here is that the boat, without its propulsion equipment and passengers, will have to be swamped and supported with a certain amount of flotation material distributed symmetrically so as to keep it relatively level. When you add the propulsion systems, the boat will be disproportionately heavier aft and consequently the flotation material needed to support the engine and drive will have to be located far aft to counter this unbalance. Finally, through experience we know that the flotation material needed to support the portion of the passengers (persons capacity) must be located way out on the boat’s sides and as high as possible. This is so that the boat may pass the stability test’s maximum permitted heel angle.

Symmetrical location criteria for the flotation system are established for each of the three quantities above. For example, one cubic foot of flotation material three feet forward of the boat’s balance point (when out of the water) can be balanced by three cubic feet of flotation material one foot aft of the boat's balance point. The symmetry may, and should, be varied to account for equipment (such as batteries) if located off-center.


Example of Level Flotation Calculations

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


Fb = [(Wh x K) + Wd] ÷ B


Wh = dry weight of the hull
(Everything below the swamped waterline)

Wd = dry weight of deck (everything above the swamped waterline, including factory supplied windshield, hardware and accessories)

K = conversion factors for materials used, from Table 4.1

B = buoyancy of flotation materials used, in pounds per cubic foot

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.

Since the hull will be submerged (as in Basic Flotation), Wh is the sum of the weight of the different components of the hull (fiberglass, wood, steel, etc.) multiplied by the conversion factor in Table 4.1. Wd, however, is the sum of the dry weights of the deck components since the deck will remain above water. Buoyancy of the flotation material shall include allowances for some water absorption.

This flotation material shall be located symmetrically about the balance point of the boat.

Figures 5.1, 5.2, and 5.3 following show the principle of symmetry about the balance point of the boat. Look carefully at how the boat’s balance point (longitudinally) may be found on the factory floor by balancing the boat over a piece of pipe. Use the information contained here to save time in the test tank. You may distribute the flotation material needed for each step prior to testing.

FIGURE 5.1 - Placement of Flotation Material

FIGURE 5.2 - Placement of Flotation Material

FIGURE 5.3 - Placement of Flotation Material


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


Fp = S ÷ B


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. All are found in Table 4, columns 2 and 4 (See Appendix A)

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

The Coast Guard has been given authority to randomly check boats for compliance with the flotation requirements. When this check is conducted, simulated outboard engine weights are used rather than an actual outboard engine. The weight used during this check is obtained from the outboard engine weight table included in Subpart H (Flotation) of the CFR (see Table 4). Due to changing technology, outboard engines heavier than those listed in Table 4 are available. This is particularly true in the case of 4 stroke 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.

Distribute this material symmetrically within 36 inches (30 inches for boats of less than 15 feet in length) of the outside of the transom top at the motor-mounting area or around the propulsion system if engines are not stern mounted.

In small boats, sometimes it is difficult to pack the flotation material in the compartments prescribed here. Use the space on the deck aft, but stay within the 36-inch area (30 for boats under 15 ft.).

FIGURE 5.4 - Flotation Material for Propulsion System

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