Originally Posted by Mollers
How exactly are you calculating these efficiency figures?
Sorry chaps, didn't plan to get you all reaching for your calculators!
2 questions have come up here: how to estimate the efficiency of an engine and how to estimate the speed of a boat. They are completely different things, so I'll try to clarify.
My slightly glib statement that petrol engines are about 25% efficient and diesels are 40% efficient is just a rule of thumb based on experience. Its not specific to boats - car engines are about the same. Here's an example of how you can work it out though. The figures are from an Iveco 8140.43S turbo diesel on a project boat.
Data from engine manufacturer says the engine burns 207 grams of fuel per kW-hour at 2000 rpm. (Its putting out 55 kW or 74 bhp at that rpm, but we don't need that figure.) The engine is using 207/3600 grams per second, per kilowatt out (3600 seconds in an hour). Diesel fuel is 44,800 kJ/kg, i.e. 44.8 kJ per gram. So the engine is using:-
207 x 44.8 / 3600 kJ per second, per kilowatt, i.e. 2.58 kJ/s per KW
kJ per second is the same as kilowatt, so this tells us the engine burns 2.58 kW of fuel for every kW useful output. Engine efficiency is useful power out divided by the power in the fuel, so efficiency is
1/2.58 kW/kW, i.e. 0.39, or 39%
Happily, this real example supports my sweeping statement that diesels are about 40% efficient! Good modern turbo-diesels should be somewhere around this figure over quite a broad range of rpm around their rated output. At 3,600 rpm, this Iveco kicks out 92 kW (123 bhp) with an efficiency of 32%.
It seems to be quite difficult to get fuel burn per kilowatt for the petrol outboards, so I can't easily do a comparison.
How fast the boat goes is another matter altogether. For a planing boat, this is mainly about power to weight ratio, but propeller efficiency and hull shape are also important. The graph on our website shows speed versus power for different boat weights. This is just a graph of "Crouch's planing speed formula" from Dave Gerr's "Propeller Handbook". The formula says:
kts = C / square root of (LB/SHP)
- where kts is the boat speed in knots, LB is boat all up weight in pounds (its an American book), SHP is the shaft horse power at the propeller.
This formula is based on a propeller between 50 and 60% efficiency and could be tweaked for fast RIBs, which can be higher - but its a reasonable assumption for most RIBs at cruising speed. "C" is a magic number which takes account of the hull shape and efficiency. Dave Gerr gives these values for C:
150 - average runabouts, cruisers
190 - high speed runabouts, light cruisers
210 - race boats
220, 230 - hydroplanes, racing catamarans, sea sleds etc
The chart on our website uses C=190 as a reasonable guess for most RIBs.