How to Calculate Buoyancy for a Small Boat

by Will Charpentier
Every boater should understand buoyancy and buoyant force.

Every boater should understand buoyancy and buoyant force.

How buoyant your boat is depends, in part, on how large it is. Buoyancy is a balancing act between buoyant force, which pushes your boat out of the water, and weight, which sinks your boat into the water. The lighter your boat -- just you and a fishing pole -- the lower the buoyant force required to keep your boat above the water and the more buoyant the boat. As you add things --- a cooler full of food and drink, a stereo, tackle boxes and fishing buddies --- the boat becomes heavier, rides lower in the water and is less buoyant.

Measure the length and width of the boat with a tape measure. Measure the depth of the boat from the bottom of the boat to the top of its side. Multiply the length by the width and the result by the depth. If your boat's sides are 18 inches, or 1.5 feet, high and the boat is 9 feet long and 4 feet wide, multiply 1.5 by 9 by 4. The result is 54, the volume of your boat in cubic feet.

Decide how much of the boat, also called the amount of positive buoyancy, you want your boat to have. If you want to have 50 percent of your boat's sides out of the water, use 0.50. If you want 75 percent of your boat's sides out of the water, use 0.75.

Multiply your boat's volume, 54, by the amount of the boat you want underwater --- for example, 50 percent, or 0.50. Multiply the result, 27, by 62.4, the weight of 1 cubic foot of fresh water. The result, 1,684.8 is the number of pounds of buoyant force keeping half of your boat out of the water.


  • Multiply the volume of your boat, 54 cubic feet, by 62.4. The result, 3,369.6 pounds, tells you the weight your boat when it's full of water. This is the tipping point, called neutral buoyancy, where 0 percent of the boat would be above water, To ensure you keep your boat above water, read the U.S. Coast Guard placard on the inside of your boat's transom, that lists the maximum allowable load of both passengers and gear for your boat.

About the Author

Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.

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