Safe Loading - Subpart C

Category Two: Outboard Boats 

Persons Capacity

A Special Discussion of 33 CFR 183.41, Subpart C – Safe Loading

Paragraph (a) of part 183.41 above states that the persons capacity of an outboard boat must not exceed the lesser of two options:

  1. The maximum weight capacity minus weight in column 6 of Table 4; or
  2. If this number is less than 550 pounds, you must test the boat and compute its
    persons capacity.

Now assume that the calculation in (1) results in 550 lb. persons capacity. This is the number to be used as persons capacity in pounds, while the computation for number of persons is:

32+550 = 4.12, or 4 persons.

However, assume the calculation in (1) results in 547 lb. Since this is less than 550 lb. you must conduct the dry stability test on this boat. The dry stability test gives us a total of 545 lb. The calculation for persons capacity in pounds [see 183.41 (a) (2) (iv) above] will be:

Persons capacity (lb.) = A/.60 where A is the weight added during test
Persons capacity (lb.) = 545 /.60 = 908 lb.

Choose the lesser of the two options: 547 lb. or 908 lb., which is obviously 547 lb.

Then the computation for persons capacity in number will be:

Persons capacity (No.) = 32+547 = 4.10 or 4 persons.

The problem here occurs if, in the example above, only 328 lb. were added before the boat’s inclination allowed water to come in. In this case, the computation for persons capacity in pounds is = 328 / .60 = 546 lb.

Choose the lesser of the two methods: 547 lb. or 546 lb. Obviously you will pick 546 lb.

Then persons in numbers is 32 + 546 = 4.09 or 4 persons.

The intent here is to show that whenever you start finding very low values of capacities, the decision on how to certify the boat becomes difficult, and the recommendation is to discuss it with the Coast Guard. Look at the figures above again.

In one instance, the boat tested during the dry stability test withstood 545 lb. before taking on water, while in the other instance the boat withstood only 328 lb. Yet both certify for the same number of persons (four), and both are very close in persons capacity measured in pounds.

Example of Calculations for Maximum Weight Capacity and Persons Capacity

In section 4.2 we have an example of the calculations for maximum displacement using a form for the Simpson’s Rule formula. In that example we arbitrarily assumed it was an outboard boat, simply because Figure 3 in section 4.0 pictures an outboard boat. The only difference from the sterndrive boat noted in section 5.3 as an example for the capacity calculations is that the outboard boat has a small volume of the engine well, below the float-plane and forward of the transom, which must be deducted when figuring cubic capacity; hence the displacement for the sterndrive boat was 11,731.2 lb. For the present example of an outboard boat we will use the displacement shown in Figure 5, or 11,668.8 lb.

Following are the specifications for our hypothetical outboard boat:

Length overall: 16.5 ft.
Calculation length: 16 ft., 2 in.
Beam: 6.75 ft.
Calculation beam: 6 ft., 6 in.
Boat weight: 1353 lb.
Displacement: 11,668.8 lb. (from Fig. 5, Sec. 4.2)
Outboard engine: 100 HP
Engine weight: (from Table 4, USCG)
Fuel: Portable fuel tank

We measured the hull, applying Simpson’s Rule, and we came up with the displacement (shown in Figure 5, section 4.2) of 11,668.8 lb. Now we will follow the steps of the regulation’s formulae.

Step 1: Compute maximum displacement
Done in Figure 5 = 11,668.8 lb.

Step 2: Compute maximum weight capacity

W = maximum displacement – boat weight
W = 11,668.8 lb. – 1353 lb.

W = 2,063.16 lb., rounded down to 2,063 lb.

This is the figure that may be used on the capacity label to certify the boat:
2,063 pounds, persons, motor, gear


Step 3: Compute persons capacity in pounds:

Following the regulation above, we note that the persons capacity in pounds is found by subtracting engine and controls weight, dry battery weight, and weight of portable fuel tank from the maximum weight capacity calculated above. We now go to Table 4, where column 6 shows 550 lb. as the sum of the weights listed above.

Persons capacity = maximum weight capacity - value in Column 6 Table 4
Persons capacity = 2,063 lb. – 550 lb.
*Persons capacity = 1,513 lb.

Step 4: Compute persons capacity in whole numbers:

Follow the regulation formula

Persons capacity (no.) = 32 + persons capacity in lb.
Persons capacity (no.) = 32 + 1,513

Persons capacity (no.) = 10.9, rounded up to 11 persons

*The regulation also states that if the figure above (persons capacity in pounds) computes to less than 550 lb., a dry stability test must be performed, and the figure to use for certification will be the lesser of the two results: the value in Step 3 above or the computation below.

For boats with a maximum persons capacity of less than 550 pounds, the persons capacity in pounds will be determined by the following dry stability test. Proceed as follows:

  1. Float the boat in calm water with all its permanent appurtenances.
  2. Add, in normal operating positions, the dry motor and controls weight, battery weight, and full portable fuel tank weight, if any, for the maximum horsepower capacity marked on the boat. Permanently installed fuel tanks shall be full of fuel.
  3. Gradually add weights along one outboard extremity of each passenger area, at the height of the seat nearest the center of that area, but no higher than the height of the gunwale, and distributed equally forward and aft of that center in a plane parallel to the floorboards until the boat assumes the maximum list or trim, or both, without water coming aboard. Then repeat this test on the other side of the boat.
  4. Compute the persons capacity in pounds using the following formula: Persons capacity = A/0.6 where A is the weight added in paragraph 3 above to obtain maximum listing without water coming in. (If there is a difference between one side and the other, use the greater of the two weight figures obtained when the test was performed.)

Let’s assume that the maximum persons capacity above in Step 3 came to 545 lb. According to the regulation, the boat must be subjected to the dry stability test. Let’s also assume that during the test, and after locating the dry weights of the engine, controls, battery and fuel tank, we added a total of 500 lb. on one side of the boat, and then on the other side, to obtain maximum inclination (or list) before water ingress. You now must compute as per the regulation:

Persons capacity in pounds = 500 / .60
Persons capacity in pounds = 833 lb.

The law states that we must use the lesser of the two values found:

  1. Persons capacity (lb.) in Step 3: (as assumed above) 545 lb. or
  2. Persons capacity (lb.) in dry stability test: 833 lb.

Using 545 lb., because it is the lesser of the two figures, we may then calculate the persons capacity in whole numbers as above:

Persons capacity (no.) = 32 + persons capacity (lb.) in test
Persons capacity (no.) = 32 + 550 lb.

Persons capacity (no.) = 4.12, rounded down to 4 persons.

The manufacturer now has the information to certify the boat, and the label may read:

11 persons or 1,513 lb.
2,063 lb. persons, motor, gear
100 horsepower motor

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