How much fuel does my boat use?
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It is always good to know how much fuel we are going to use... either to plan a long passage, to keep on top of our boat running costs, and, of course, to avoid that embarrassing tow back! For planing boats like our favourite RIBs there is obviously quite a complicated relationship between power, speed and fuel consumption - just slowing down may not help if we are dropping into that inefficient ''hump speed' region. There is, however, a formula that provides a reasonable approximation to the power v fuel flow conundrum:
Where K is a constant for a particular engine/prop combination and Power is measured in Horsepower(hp), Engine speed in 1000s of R.P.M. and Boat Speed in Knots
For one of our RIBs, which has a 175hp Engine, we get a top speed of about 36 knots at 5200rpm. It cruises at 20kts at 3800 rpm. What is the fuel consumption at Cruising Speed?
Step 1. Find K using the top speed numbers above:
Step 2. Use K to calculate what horsepower is developed at cruising rpm:
= 64 horsepower at cruising speed
A petrol engine develops about 12-14hp hrs per gallon so 64 hp, our RIB cruising at 20kts requires about 5 gallons per hour (about 22 litres/hour).
We don't have to do these sums every time! It is a relatively simple process to stick the numbers into a spreadsheet and come up with a useful table, or graphs, like these below that we produced for our RIB Vesper (K=11) specific to your boat
