One of the basic calculations in boat or ship design, whether it's a one-person skiff or an aircraft carrier is its displacement. Archimedes' principle states that a body floating in water will displace an amount of water equal to the weight of the object. In other words, a 10 pound weight, if it floats or is suspended, will displace ten pounds of water. This fact makes calculating the underwater volume of the boat a simple matter of estimating the weight of the boat.

Calculate the weight of the ship without crew, cargo, provisions and other items that are not part of the basic structure of the ship or boat. Include engines and a full load of fuel. You can calculate this using dry dock measurements or engineering approximation.

Add the weight of crew, passengers and cargo. You can estimate 200 lb. per person if you do not have an exact number. The weight of provisions will depend on what you choose to pack and the length of the cruise you intend to take. Include extra equipment, such as fishing gear, safety equipment and ammunition, if it is a military ship.

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Divide the total weight of the vessel by the specific gravity for the water in which you intend to sail. Salt water has a specific gravity of 64 lb/ft^3, while fresh water has a specific gravity of 62.4 lb/ft^3

Example: Displacement =15,000 lb Saltwater underwater volume = 15,000 / 64 = 234.375 ft^3 Fresh water underwater volume = 15,000 / 62.4 = 240.385 ft^3