Originally Posted by Nauti Buoy
Is there a formula to work out how many litres of fuel an engine will burn at a given speed...... don't know if that makes sense but was told there could be a way of working out a max fuel flow.....
This is taken from RYA Navigation Handbook, I take no credit nor blame for accuracy;
Power = (K x engine speed^4) / boat speed
^ means 'raised to the power of'
eg A RIB has a 120hp 4-stroke engine capable of 6000rpm and 40knots. Cruising speed is 30knots at 4200rpm.
1. Find 'K' by putting real numbers into the formula:
120 = (K x 6^4) / 40
120 = (K x 1296) / 40
so, K = 3.7
2. Use K to find out how much power is being developed at crusing rpm:
Power = (3.7 x 4.2^4) / 30
Power = (3.7 x 311) / 30
Power = 38hp
3. A 4-stroke petrol engine developes about 12 - 14hp.hr per gallon, so 38hp divided by 12 gives about 3 gallons per hour. For reference, a 2-stroke petrol engine developes about 8 - 10hp.hr per gallon, diesels produce about 20hp.hr per gallon.