Low RPM = More Fuel?

bcurling · #1 10-11-2017, 05:15 PM

I was told that running a larger motor (15hp at half throttle to get 7kts which is the hull speed for a particular boat), will use more fuel than the intented design motor of 9.9hp at full throttle. I always thought that using less throttle to achieve a certain speed would consume less fuel than a smaller motor at full throttle. Am I off base on this? Someone please set me straight on this.

phatdaddy · #2 10-11-2017, 05:58 PM

A lot of variables in that equation, but if your talking old school carbed 2 strokes. Rule of thumb, 9.9 will burn 1 gph wot and a 15 will burn 1.5 gph wot. So if the 15 will get the job done at half throttle, should be less fuel per hour.

That's my take, but some one smarter will probably be along later

bcurling · #3 10-11-2017, 06:11 PM

Thanks PhatDaddy that helps me out. These motors would be carbed 4 stroke. I should have mentioned that in the original post.

phatdaddy · #4 10-11-2017, 08:46 PM

gas use aside, i;d rather run something at half throttle than full .

THEFERMANATOR · #5 10-12-2017, 12:47 AM

For that size engine, you won't see much difference. It takes X amount of fuel to make Y amount of HP, and Y amount of HP to go Z in speed. It's not like you're comparing a small 2 cylinder kicker to a 4 cylinder primary pusher engine. I doubt you would see a huge difference if both kickers were propped the same. And I would say prop choice plays a bigger role in fuel consumed than the difference between a 9.9 and 15.

bcurling · #6 10-12-2017, 06:28 AM

I agree with you on propping correctly. I was not so much concerned about the cost of the fuel consumption as I was about the comment the fellow made to me regarding the two different sized motors on the same boat targeting the same speed. It just simply did not seem correct to my thinking. Just because one would be using a little larger motor and lugging it (according to him), about half throttle to achieve the same speed as the smaller motor means you would burn more fuel. That is the reason I reached out the forum in order to get some solid impute and confirm I have not been asleep for the past 68 years.
Still looking for my V20!
Thanks guys.

scook · #7 10-12-2017, 10:14 AM

I went with a 15 for my kicker because it will troll down plenty slow but if I need it for auxiliary power in the event of a main engine failure it should push the V at hull speed against a stronger head wind.

Destroyer · #8 10-12-2017, 10:16 AM

"Lugging" an engine means that you are driving at full throttle with the engine at a low RPM and the transmission is in too high a gear. Since an outboard engine does not have the ability to shift gears like an auto does it's really impossible to "lug" an outboard engine in the classic sense.

Manufacturers of outboard engines take that into account when they design an engine and will specify that an engine should achieve a specified RPM at wide open throttle (WOT) IE: 6000RPM@WOT. As long as you can hit that specified number the chances are pretty good that your engine is propped correctly and you will not be harming the engine if you run it at a lower RPM*.

*Nota Bene All engines (both 2 and 4 stroke) need to be run at higher rpms every now and then in order to burn off carbon build up on the rings and the heads. Constantly running your engine at a slow speed will cause carbon build up and will, over time, foul your spark plugs and cause other problems, including increased fuel consumption. That *might* be what your friend was talking about when he said that you would use more fuel.