Fuel consumtion half throttle

I have a tohatsu sailpro 6hp powering my 1000 lb catamaran. At half throttle it does 6 knots in flat water, and 7 knots at full throttle. The tohatsu spec says that fuel consumption is 0.5 gallons per hour.

I have read on several forums that small outboard fuel consumption is rated at full throttle, and that consumption does not vary much at lower throttle settings. Is this true? I often cover long distances at quarter throttle because I think that I'm getting better gas mileage.....

 

Have you ever measured your fuel consumption?
It doesn't sound like it.
Why not?

 

If you look at a website like "Yamaha Performance Bulletins", you will get a feel for what you can expect from outboards, it is pretty well a rule of thumb that the fuel consumption will double between 3/4 of maximum revs, and maximum revs. You won't get much benefit from backing off further than 3/4 revs. Apart from the extra wear and tear, full throttle is generally not advisable. You will find that the most fuel efficient revs is quite low, but if you want to make more than walking pace, not much use to you, and the fuel burn with a 6hp motor is not going to hurt the pocket regardless what speed you run it.

 

You need to read the graphs carefully. The consumption rates may be HP/gal, in which case it will usually show the lowest close to the maximum torque RPMs. However, fuel consumption is directly related to power output. You will probably get the lowest fuel consumption at hull speed.

 

The outboard data in the "performance bulletins" is really interesting but is for planing hulls. You can see that the curves are affected by when the boat is on plane.
I had guessed as Gonzo says that consumption is going to be affected by hull speed and torque curve of the motor. I have searched high and low but cannot find any torque curves for these small engines.
If they are most economical at low revs, and if it is never a good idea to run them flat out, it makes me wonder if fitting a large prop to make it faster at low revs would yield longer range?

 

I don't know there is much use worrying about a 6hp motors fuel consumption, it won't use much at any speed. You can just about be certain that the most efficient speed would be at idle, in terms of actual utility though, that would be way too slow. You don't have any way of knowing what RPM you are at, so fine tuning is largely guesswork. The point is that if the engine can drive the boat up to 6 knots with still more throttle available, it is good enough. But of course when headwinds and choppy water are involved, you may need full throttle. If you think it is labouring, go to a lower pitch prop.

 

You can attach a tachometer to it and get accurate RPM readings.

 

I have a similar catamaran powered by a similar motor. I run most often between 1/2 and 2/3rds throttle. I have never accurately measured fuel consumption, but have gone 70 miles on one 3 gallon tank (it holds almost four). I do find a marked increase in consumption when running over my usual cruising speed.