Pat, Your answer "at 16 knots the heavier boat would require more horsepower and burn more fuel" is a statement, not an explanation. I don't deny that it's right. Experience tells us so. But why? Your previous, fuller answer, was mainly comparing cats to monohulls. That is not the issue. Gary

>Your answer "at 16 knots the heavier boat would require more >horsepower and burn more fuel" is a statement, not an explanation. I >don't deny that it's right. Experience tells us so. But why? Gary, sounds like you're after the kind of answer only a physics professor could provide. Perhaps these definitions of weight, mass, velocity et al might help: http://www.av8n.com/physics/weight.htm http://whatis.techtarget.com/definition/0,,sid9_gci552571,00.html --Georgs