Safe Loading — Subpart C

Calculation of the Maximum Displacement

It is important to note that all three categories of boats in 33 CFR Subpart C covered by this regulation require that the maximum displacement be found (by any of the methods described here). Once the maximum displacement is determined, the maximum weight capacity and the persons capacity are found by simple calculations unique to each category of boat.

The large majority of boats under 20 feet in length covered by this regulation will calculate to more than sufficient displacement for the maximum weight capacity generally desired. It is possible that a designer or manufacturer might arbitrarily “de-rate” the maximum weight capacity to a level considered a more reasonable, lower figure. This weight capacity affects the amount of costly foam buoyancy required under the flotation regulation, so there is no need to rate a boat for excessive capacities. This is not true for all boats, and particularly for boats with low freeboard; it is merely something to keep in mind.

Consider a 19-ft. cuddy cabin sterndrive with a deck that rises to 20 inches above the deck-to-hull junction. This portion (the superstructure above the hull sheer) will certainly add considerable displacement and thus maximum weight capacity to the boat, because the static float-plane will be high above the hull sheer. However, if the manufacturer does not have a need to rate this vessel for 12 or 14 passengers and a great deal of gear, the easiest way to calculate the maximum displacement is to ignore the deck and measure the hull mold below the hull sheer and the static float-plane, as described above. On the other hand, consider a low-freeboard open fishing boat such as a Jon boat. In this case, it will be important to carefully locate the static float-plane at its maximum possible height, in order to use all of the displacement allowed.

Calculating the maximum displacement may be done by several methods. Selection of a method depends on what is available to the person working out the calculations. There are commercial enterprises that offer this service, but we recommend that the manufacturer be familiar with these methods so that decisions unique to each manufacturer may be made with a good understanding of the calculations.

First and foremost, we must understand displacement. Archimedes (a Greek scientist of 287-212 BC) discovered that, “any body completely or partially submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body.” Consequently, if we push a boat down into the water to the point before water enters the boat while the bow and the stern are at the same distance from the water’s surface, the weight of the volume of water displaced by the boat will represent the force keeping the boat buoyant and afloat. The weight of this water represents the “displacement” of this boat to the particular level it was “pushed” down to the static float-plane, because the magnitude of the buoyant force always equals the weight of the fluid displaced by the object—in this case a boat. There are other considerations, such as mass and density, which will determine if the object will sink or float. However, the importance of what we learn from this principle may be visualized by the example below.

Consider a cigar box with these dimensions:

Height = 3 in.
Length = 6 in.
Width = 4 in.
Its volume will be: V= 3 X 6 X 4 = 72 cubic inches.

Now place the box in the water and slowly add known weights until the water is about to enter the box. If you could measure the water displaced, it would be 72 cubic inches, and its weight will be the displacement of the cigar box, in that particular configuration.

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