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It depends on how much water is in the hull. 21 gallons which is about 79 liters of salt water weighs somewhere around 180lbs. That's like an extra passenger in the boat which I don't think would really use too much fuel. Have you changed the drain tube? A deteriorated brass drain tube will allow water into the foam and replacing it will show if the plywood and foam inside the hull is dry. At the top of this page find "main" and use the search feature in there to learn about changing the brass drain tube. When you read about it it will sound like it's hard but it really isn't. Anyhow, like I said, if the boat floats right I don't think you have a water in the hull problem.