Outboard engine fuel consumption

[Rivers & creeks]

I've googled this to death but can't find it anywhere. Can anyone point me toward the fuel consumption figures for a four stroke Mariner 9.9hp, (standard gearbox but does have a high thrust prop) - the engine is three years old. I have a very rough idea but we're planning a long trip that may require quite a few hours running at about 75% throttle.

Thanks for your input. Simon.

 

[Fire99]

No doubt there are many variables but my Honda 10hp 4-stroke pushing a 23ft 1.5 tonne yacht runs at something like 2 litres per hour on 3/4 cruise.

 

[Rivers & creeks]

Thanks that gets me started. Here's a question - does the engine use the same fuel at 3/4 revs whether it's pushing a 1.5 tonne or a five tonne yacht? I have it in mind that the throttle on a petrol engine is just a way of delivering more or less fuel????

 

[Fire99]

Yes. Load would play a factor. If the boat, for whatever reason, is causing more load on the engine, your 3/4 throttle will give less speed and hence you'll use more fuel for the same journey.

However, boat engines seem to be affected by lots of things. i.e. Prop pitch, Hull Shape etc.

That said, even with a larger boat 1/2 gallon pr hour on a clean-hulled 5 tonne yacht would seem about right on a 3/4 throttle cruise.

 

[William_H]

Fuel consumption

Yes. Load would play a factor. If the boat, for whatever reason, is causing more load on the engine, your 3/4 throttle will give less speed and hence you'll use more fuel for the same journey.

However, boat engines seem to be affected by lots of things. i.e. Prop pitch, Hull Shape etc.

That said, even with a larger boat 1/2 gallon pr hour on a clean-hulled 5 tonne yacht would seem about right on a 3/4 throttle cruise.

3/4 throttle is a very variable quantity. If you had a manifold pressure guage and tachometer you might be able to get a more reliable setting on which to base you throttle setting.
An engine will use more fuel for a given throttle setting if it is running at higher RPM. So a heavier boat might be said to produce less fuel consumption but of course that means less hull speed. Similarly the high thrust prop will give more RPM so more fuel consumption.

I think your best bet is to use GPS to monitor speed. Hopefully water speed is the same, or use log if you have one. Presumably you are using the motor because wind is too light. In which case set up for a water speed of something like square root of hull water line length in feet. (5 knots) Go for say an hour and monitor fuel consumption. ie run fuel out of a 1 litre bottle. This will give you a good basis for fuel consumption.
Lower speed will give much less consumption. Higher speed or head wind will give much higher consumption.
As for your voyage. Keep a track of fuel used, but keep some ie 2 litres for emergency. 2 litres should take you out of trouble for perhaps an hour at gentle throttle setting. good luck olewill

 

[Rivers & creeks]

Thanks, that makes sense. We have a tacho and that helps take the guess out of the RPM. It's frustrating that the mariner site is missing the consumption figures for the 9.9hp engine

 

[AngusMcDoon]

I've googled this to death but can't find it anywhere. Can anyone point me toward the fuel consumption figures for a four stroke Mariner 9.9hp, (standard gearbox but does have a high thrust prop) - the engine is three years old. I have a very rough idea but we're planning a long trip that may require quite a few hours running at about 75% throttle.

Thanks for your input. Simon.

With a 1.9 tonne 31' trimeringue in flat calm weather at 5.5 knots using a 10 year old Yamaha 4 stroke 9.9 with high thrust prop I get 22 naughtycal miles per imperial gallon.

Consumption increases significantly if going into much wind.

 

[oldharry]

Consumption increases significantly if going into much wind.

... which is why manufacturers fight shy of giving consumption figures for their engines. The best way to find out is by monitoring how much fuel you use on average over a period of time. Fill the tank, run it for 2 - 3 hours at normal cruising speed in average conditions, then measure how much fuel it takes to bring the tank up to the same level as before. Then you can work out how long your total fuel supply will last, in hours. Allow at least 25% extra over what you might need. Running out a mile from home is probably more dangerous than half way across - you are too close to the shore to have time to get a tow!

 

[VicS]

It's frustrating that the mariner site is missing the consumption figures for the 9.9hp engine

Do they give consumption figures for other engines?
Do other manufacturers give fuel consumption figures for their 9,9s that might at least give you a rough starting figure.

Do any outboard manufacturers give fuel consumption figures for any of their engines?

Mariner do state that the fuel consumption of the 4 stroke is 40% less than the 2 stroke. Take it from there!

Always carry a good reserve of fuel .. its not like an aeroplane where the weight of extra fuel becomes a major consideration.

 

[Rivers & creeks]

Well, after some serious ringing round today I got hold of the service engineer for the UK distributors. 4l per hour at full revs but no figures for other rev speeds. At least this trip will give me some reliable empirical evidence. I am aware of the benefits of carrying spare fuel, what I was trying to calculate was how much spare to take. At least now I know the best and worst case scenarios for motoring the whole way. Thanks for the replies. Simon

 

[Fire99]

Well, after some serious ringing round today I got hold of the service engineer for the UK distributors. 4l per hour at full revs but no figures for other rev speeds. At least this trip will give me some reliable empirical evidence. I am aware of the benefits of carrying spare fuel, what I was trying to calculate was how much spare to take. At least now I know the best and worst case scenarios for motoring the whole way. Thanks for the replies. Simon

Well 4L p/hour is a good starting measure. You can also almost guarantee that consumption will reduce by atleast (if not more) a third when you drop to 3/4 throttle.