Whitelighter
Well-Known Member
Problem is you are looking at fuel consumption from a car perspective, which is fair enough as that is your relevant experience.
Thing is unlike a car the 'road' a boat uses is also moving in one direction or another. Add to that relatively low friction and wind also starts to play its part.
A litres or gallons per mile calculation isn't really the thing to look at, and you certainly won't get that out of the engine manufacturer. What you should be considering is litres per hour at given RPM and then work out your boat speed in neutral water at those rpm. Once you know that you can look at the tidal flows along your route and either add or deduct the tidal influence (often will be a vector as it's unlikely to be dead ahead or astern unless going up a river) from/to your neutral speed.
Once you know all that you then have wind and seas state which can either slow you down or speed you up. It for this last reason that a 20% reserve on your fuel really should be the minimum as that only really allows a variance of a few knots either way! which isn't much.
If your boat is comfortable at 24knts for a cruise for example, your trip is going to take you just under 7 hours. I reckon on your boat this sort of speed is going to be around 54l/hour (taken from some us figures I found). So with no adverse affects or assistance you are looking at a fuel burn of 360l (total time is 6hr 36min).
That gives you a 10% reserve.
You could drop the speed to 20-21knts and 4000rpm and maybe use 38lph but your travel time increases to 8 hours. Fuel usage would be a more practical 305l, which offers you a sensible reserve. Though it's still tight if you lost just 2 knots due to tide or wind or both you burn an extra 30 litres. You can see now why a 10% reserve is too tight.
In my opinion it's doable, but you need to get accurate figures for your boat and it's definitely not a mornings blast. At WOT your engine is sucking 100l/hour and there is no way your boat will be doing in excess of 40knts to avoid running dry very short
Thing is unlike a car the 'road' a boat uses is also moving in one direction or another. Add to that relatively low friction and wind also starts to play its part.
A litres or gallons per mile calculation isn't really the thing to look at, and you certainly won't get that out of the engine manufacturer. What you should be considering is litres per hour at given RPM and then work out your boat speed in neutral water at those rpm. Once you know that you can look at the tidal flows along your route and either add or deduct the tidal influence (often will be a vector as it's unlikely to be dead ahead or astern unless going up a river) from/to your neutral speed.
Once you know all that you then have wind and seas state which can either slow you down or speed you up. It for this last reason that a 20% reserve on your fuel really should be the minimum as that only really allows a variance of a few knots either way! which isn't much.
If your boat is comfortable at 24knts for a cruise for example, your trip is going to take you just under 7 hours. I reckon on your boat this sort of speed is going to be around 54l/hour (taken from some us figures I found). So with no adverse affects or assistance you are looking at a fuel burn of 360l (total time is 6hr 36min).
That gives you a 10% reserve.
You could drop the speed to 20-21knts and 4000rpm and maybe use 38lph but your travel time increases to 8 hours. Fuel usage would be a more practical 305l, which offers you a sensible reserve. Though it's still tight if you lost just 2 knots due to tide or wind or both you burn an extra 30 litres. You can see now why a 10% reserve is too tight.
In my opinion it's doable, but you need to get accurate figures for your boat and it's definitely not a mornings blast. At WOT your engine is sucking 100l/hour and there is no way your boat will be doing in excess of 40knts to avoid running dry very short
