The shorter the nap, the smoother the finish, and the further the paint goes. I can paint my 36' boat with a gallon of paint. Foam rollers won't work with some bottom paints because the solvent softens the foam and they swell and fall apart.Try using a foam roller when painting your bottom. I can paint my 33' boat with one gallon. I paint the entire hull about every four years and just touch up bare spots in between.