For every 10' of waterline use one fender, with a minimum of three fenders. A 19' boat would need 3 fenders, a 50' boat would need 5 fenders. Placing one fender near the maximum width (beam), others located at the bow and stern. For every 4 to 5 feet of boat length, the diameter of the fender should be 1 inch. A 20 to 25ft boat should use at least 6-inch fenders; a 25 to 35ft boat should 8- inch fenders.