How Capacities Are Calculated

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jimh
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How Capacities Are Calculated

Postby jimh » Fri Jun 17, 2022 12:34 pm

Regarding the method of calculating capacity values that boatbuilders must use and affix to the hull of their boats, ]I thought that to know and understand the actual methods of calculating the capacities might possibly be interesting. Among the required capacities to be shown on a capacity plate is the boat's maximum horsepower. The method of calculating maximum horsepower is the topic of a much earlier article in the website's Boston Whaler Reference section, available at

https://continuouswave.com/whaler/refer ... tedHP.html

The method to calculate weight capacity for an outboard boat is given in federal regulations at 33 CFR § 183.35 - Maximum weight capacity: Outboard boats. See

https://www.law.cornell.edu/cfr/text/33/183.35

To calculate the maximum weight capacity, two different weights must be known:
  • Boat Weight, which is the sum of
    1. Hull weight;
    2. Deck and superstructure weight;
    3. Weight of permanent appurtenances; and
    4. Weight of full permanent fuel tanks.
  • Maximum Displacement, which is the weight of the volume of water displaced by the boat at its maximum level immersion in calm water without water coming aboard except for water coming through one opening in the motor well with its greatest dimension not over three inches for outboard motor controls or fuel lines.
The Maximum Capacity weigh must NOT be greater than one-fifth of the difference between Maximum Displacement and Boat Weight.

The regulations also provide a method to calculate when the boat is level.

The practical way to know the weight of water displaced at maximum level immersion is to record the amount of weight added to the boat to decrease the freeboard to the specified level, “without water coming aboard,” and add this to the boat weight. The difference is then the amount of weight added. The regulation then limit the rated capacity to only one-fifth of the difference. This means that the hull remains far above its maximum possible immersion without taking on water when loaded to its specified capacity. The “bureaucrats” that developed the regulation were probably senior officers in the U.S. Coast Guard. To know when a boat is loaded to its rated capacity there remains at least four-fifths of its reserve buoyancy seems quite safe to me.

Again, these capacities are a result of the federal Boat Safety Act of 1971. The purpose of the act was to promote safe boating.