What uses more fuel on plane, or hull speed?

sdilecce · May 24, 2011

Suppose you are low on fuel and were trying to get back to the dock. What would burn more fuel, keeping the boat on plane but going slow enough to keep the rpms down, or putting along at hull speed?

I have heard that displacement hulls, like trawlers are more efficient than planning hulls like motor yatchs. Would this still apply to a planing hull at hull speed?

emoney · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

I'm not sure that you can apply "hull speed" to a planing hull. Hull speed is a measurement of a non-planing hull because of the force of the waves against the water length of the hull, etc. etc. Now, if you look at does the motor burn less fuel when you are at WOT vs. Just "planed" speed, then I would think the latter would burn the least amount of fuel. Or, does the "just planed" speed burn more gas than "idle speed" (just barely out of neutral as if going through a no-wake zone) then I think a lot of that has to do with the motor, but still it would burn less fuel at the idle speed. Then again, I don't know too much so I'd wait for someone a little smarter to come along.

Now, having said all that, if I was low on fuel, I'd probably pick the middle option; just planed speed at the lowest amount of RPM's and start calling friends at the same time.

tommays · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

I have a GPS/fuel flow and the MPG on a 3.0 can vary between 1 MPG and 5 MPG

With 1 MPG most likely trying to get home on a day the wind picked up and your pounding at 10 MPH in to a chop thats knocking out your fillings

The 5 MPG will happen at just the right speed and minor amounts of drive trim have a big effect

Frank Acampora · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

But the real trick is to use the rule of threes: One third out, one third back, one third reserve. Then you don't need to worry about speed unless ---MURPHY strikes

If you have an outboard, I suspect that the most effient operation would occur at plane at the cruise point where timing is fully advanced but carbs are only partway open--usually about 2/3 throttle.

bruceb58 · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

I have a flow meter and my most econonomical speed is at 5 MPH. It is going to vary for each boat/engine combo though.

H20Rat · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

Look at the boattest site for more info... MOST typical bowriders get decent mpg's at idle or no-wake speed. Once you move any faster than that the mpg's take a hit until well into decent cruising speed. If you are in a situation of not enough fuel, also take into account wind/current. If you have a head wind or current, running faster will be more efficient. If either of those are pushing you, idle speed is going to be better.

best mpg's is at 25 mph, 4.65 mpg. no wake speed is 4.47 mpg

http://www.boattest.com/boats/Boat_video.aspx?ID=1199#Test-Result

partskenn · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

smokingcrater said:
Look at the boattest site for more info... MOST typical bowriders get decent mpg's at idle or no-wake speed. Once you move any faster than that the mpg's take a hit until well into decent cruising speed. If you are in a situation of not enough fuel, also take into account wind/current. If you have a head wind or current, running faster will be more efficient. If either of those are pushing you, idle speed is going to be better.

best mpg's is at 25 mph, 4.65 mpg. no wake speed is 4.47 mpg

http://www.boattest.com/boats/Boat_video.aspx?ID=1199#Test-Result

I agree, most boat tests show best fuel economy at somewhere around 1000 rpms.

Ned L · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

Almost every hull regardless of style, shape, or design has a "hull speed" (exceptions being very long narrow hull forms like rowing shells), and for most boats just below that will be the 'most efficient' speed. That being the greatest speed for lowest HP required ratio.
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Smokingcrater -- Very good point that my comment missed.

H20Rat · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

Ned L said:
Almost every hull regardless of style, shape, or design has a "hull speed" (exceptions being very long narrow hull forms like rowing shells), and for most boats just below that will be the 'most efficient' speed. That being the greatest speed for lowest HP required ratio.

The hull is most efficient there, but the engine is horribly inefficient at idle. I looked at a couple other boats on boattest, every single one was just slightly worse at idle than best cruise.

Lyle29464 · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

Wind and current play a big roll. No way to head into them at an idle. If they are pushing you all you need is steerage.

2ndtry · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

Depends on the boat/motor. Check out the data for a 115 optimax (2 stroke):

More typical is for a 4 strokes

Both for 17-18 foot boats, the 2 stroke is best at "hull speed", the 4 stroke at "cruise"

bruceb58 · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

2ndtry said:
Both for 17-18 foot boats, the 2 stroke is best at "hull speed", the 4 stroke at "cruise"

Maybe I am missing something but from the table the 4 stroke is most efficient at 1000 RPM.

That is exactly what I see with my boat.

rbh · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

I wish I had fuel flow meters on our twin 260s.

I think what would be interesting is to get your boat on plane, trim out till the speed picks up and drop your RPMs till the point you almost fall off plane and see what the fuel consumption is like then.

QC · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

smokingcrater said:
The hull is most efficient there, but the engine is horribly inefficient at idle. I looked at a couple other boats on boattest, every single one was just slightly worse at idle than best cruise.

What size boats?

I agree that Otto cycle engines are horribly inefficient at idle, however . . . almost all tests I see show best fuel efficiency of boats is near, or just above, idle. Which, coupled with the poor engine efficiency at the same point, indicates how much more efficient hulls are when they are pushing no water around e.g. at or below hull speed.

bruceb58 · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

rbh said:
I wish I had fuel flow meters on our twin 260s.

I think what would be interesting is to get your boat on plane, trim out till the speed picks up and drop your RPMs till the point you almost fall off plane and see what the fuel consumption is like then.

I always assumed that was my best fuel point. I spent an entire week going around Lake Powell very slow because that is what I thought. When I eventually put a flow meter on, I found that my most efficient cruise was 700 RPM higher!

rbh · May 24, 2011

Re: What uses more fuel on plane, or hull speed?

bruceb58 said:
I always assumed that was my best fuel point. I spent an entire week going around Lake Powell very slow because that is what I thought. When I eventually put a flow meter on, I found that my most efficient cruise was 700 RPM higher!

Well I guess you found that boat has a 700 rpm difference between plow or ski, wondering how mine will do???

What uses more fuel on plane, or hull speed?

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