- 35 percent is given up to the atmosphere in heat
- 25 percent is given up in heat and vibration absorbed by surrounding water
- 10 percent is given up to overcome wave resistance
- 6 percent to overcome wave formation and prop wash against the hull
- 7 percent to overcome skin friction
- 2 percent is wasted in friction at the propeller shaft
- 1 percent to overcome air resistance
How much do you need? How much fuel you need to carry depends on the design of your boat and the intended use. In a sailing vessel, for instance, a 50 gallon tank of diesel feeding a 25 hp auxiliary engine cursing at 80 percent of top speed could propel a 33 foot sailboat 300 miles at 7 knots in calm weather. That same amount of fuel would take a twin engine 40 foot sportfisher only about 33 miles at 40 knots. This boat would need 450 gallons to cover 300 miles at that speed. Planing hulls need much more power than displacement hulls and use more fuel per mile. However their ability to carry fuel is limited because their planing ability is affected by weight. Many naval architects build in as little tankage as possible taking into consideration the design of the boat, reasonable usage and the distance between refueling stops in and around its normal area of operation. How much does it weigh? Gasoline weighs about 6.1 pounds per gallon. Diesel weights about 7.1 pounds per gallon. Both gasoline and diesel engines use about 0.6 pound of fuel per horsepower per hour. On average, diesel fuel contains about 140,000 BTUs per gallon or 10 percent more energy than the same volume of gasoline. Where does it go when it is consumed? Your engine uses the fuel you purchase in several ways.