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- Posted by Chief (

In Reply to: looking for weight capacity posted by dann6 on January 29, 2004 at 14:57:57:

Try to figgure the cubic feet displacment. IF you have trouble take the deminsions to someone along with a picture that can do math very well. Figgure water wieght at 62.4 pounds per cubic foot. So If your boat were 4 feet wide by 8 feet long with 12 inch sides that would be 32 cubic feet of air in the boat. In other words you are removing 32 cubic feet of water since the boat don't leak. Now using a caluclator multiply 32 by 62.4 and you get 1996.8 That is approximately how much the water would wiegh IF the boat were filled. NOW the wieght of the boat must be subtracted from this. SO IF the boat wieghs 100 pounds we now have 1896.8. That is approximately how much wieght it will take to sink the boat up to the top of the rails. NOW divide that by 2. You get 948.4. SO that is approximately how much wieght it will take to make the boat sit in the water 6 inches deep. Now divide 948.4 by 2 and you should get 474.2 That is approximately how much weight it will take to set the boat 3 inches deep in fresh still water. Everything above this line is called Freeboard. SO add a few inches to the top before you do the math. Now if your sides were actually 16 inches deep instead of 12 you have 16 minus 3 for a total of 13inches of free board. This is ONLY an example but you can take it from there. Also the Cost Guard has info on this. All small boats must have enough floation to stay afloat with the max load in the boat and the boat filled with water. So If you ended up with an expected pay load of 474 pounds your boat should be tested in a shallow calm pond or lake. So shallow that you can stand up in it without being in danger. YOU can simply add water inside the boat until it sinks down up to the rails. Keep count of the gallons. The wieght of a gallon of fresh water is approximately 8 pounds. Check that figgure out I may be off a bit. By multiplying that by how many gallons you used you have a good idea of how much is a safe load and how much wieght you boat displaces total. This is not guaranteed but is a good example. There are computer programs that will do this for you. One is called hulls and can be found on one of the boat groups.