How to Calculate Draft

by Jordan Whitehouse ; Updated October 18, 2017

A boat draft calculation will only take you a few minutes.

Tay Jnr/Digital Vision/Getty Images

In nautical terms, draft refers to the distance between the bottom of a boat's hull and the waterline. Draft is an important figure to take into consideration when you're choosing the appropriate water depth to take your boat into. If your boat has a draft close to the depth of the water, you should avoid those water areas to prevent damage to the bottom of your vessel. Before you embark on your voyage, calculate your boat's draft.

Measure the distance between the bottom of a boat's hull and the waterline when the boat is empty. As an example, this distance is 3 feet.

Measure the length and width of your boat's deck with the measuring tape. Record these numbers. As an example, a boat has a length of 20 feet and a width of 6 feet.

Multiply the length and width measurements to calculate the deck's area. In the example, multiply 20 by 6. The area of the deck is 120 feet.

Determine the unit weight of the water you'll be travelling in. Salt water has a unit weight of 64 pounds per cubic foot, while fresh water has a unit weight of 62.4 pounds per cubic foot.

Divide the weight of each object or person that will be on the boat by the unit weight of the water. For example, two people will be travelling on the boat that is going through salt water, weighing 150 lbs. and 200 lbs respectively. When you divide 150 by 64 you get 2.34. When you divide 200 by 64 you get 3.13.

Add together each of the figures obtained in Step 4 to determine the total volume of water displaced by the boat. In the example you would add together 2.34 and 3.13 and get 5.47.

Divide the total volume of water displaced by the surface area of the boat's deck. In the example you would divide 5.47 by 120 (the surface area of the deck) and get 0.046 feet.

Add together the result from Step 1 and the result from Step 7 to obtain your boat's draft. In the example, you would add 3 to 0.046. The draft of your boat travelling in salt water with two people on it weighing 150 lbs. and 200 lbs. is 3.046 feet.

Photo Credits

  • Tay Jnr/Digital Vision/Getty Images

About the Author

Based in Halifax, Nova Scotia, Jordan Whitehouse has been writing on food and drink, small business, and community development since 2004. His work has appeared in a wide range of online and print publications across Canada, including Atlantic Business Magazine, The Grid and Halifax Magazine. Whitehouse studied English literature and psychology at Queen's University, and book and magazine publishing at Centennial College.