Fuel Use Versus Speed

[Grimey]

Are the figures below typical for carburated outboards driving a planing hull.

The important figures are the Litres per n mile. At around 8 knots (control speed on our river)its 3.1 and at cruising its 2.1. Is this difference normal?

If you need them in USGPH divide by 3.8

RPM__SPEED___L/NM___L/H
1000__3.7_____2.7 ____9.3
1500__5.2 _____2.7 ____14
2000__6.6 _____3.1_____20.7
2500__ 9_______3.1_____29
3000__11.5_____2.9_____33.2
3500__16.5_____2.4_____40.4
4000__22_______2.1_____48.7__ CRUISING SPEED
4500__25_______2.2_____54.9
5500__30_______2.5_____75.6___WOT

Figs taken from a calibrated Northstar fuel system (GPS/ Fuel Transducer)

 

[samo_ott]

Re: Fuel Use Versus Speed

Interesting. One would use less litres to go the same distance at full speed.

 

[steelespike]

Re: Fuel Use Versus Speed

Generally WOT uses signifigantly more fuel than cruising speed.
The BRP etec sight has many many performance tests covering many types and hps.These are injected motors but will still give you an idea of use related to speed.
I would think your specs would be the exception.What boat/motor combination is this from.

 

[fdmsiv]

Re: Fuel Use Versus Speed

Is this difference normal?

Yes, for a planing hull these numbers fit, but I don't know if they are correct. It takes quite a bit of power to get the boat on plane, once there you can operate at a much higher efficiency. This is one of the biggest advantages of a planning hull.

 

[ezeke]

Re: Fuel Use Versus Speed

The relationship between RPM and Speed does not graph normally for a properly powered planing hull.

The numbers indicate that you are not on plane at 3500 but are on plane at 4000. The numbers look fairly normal from 4000 through 5500 (for a 15 pitch prop with =/> 10% slip.)

 

[Grimey]

Re: Fuel Use Versus Speed

If you go to search and type in "2003 175 Johnson" you will get the whole story on this motor boat combination and my concerns.
It has a 15x15 prop.

 

[kenmyfam]

Re: Fuel Use Versus Speed

Personally I watch the gas gauge and when it reads around half I fill it up !!!!
You get to know your individual set up after a while and how much it uses. If in doubt then carry some spare fuel with you.
Boating is expensive and if I had to worry about how much fuel I was using at any given time I would end up with a sailboat !!!
Just my 2 cents though.

 

[Grimey]

Re: Fuel Use Versus Speed

I have graphed speed versus litres per hour and it is a straight line with a slight upturn as you approach wide open throttle. This, I understand from other forum comments, is normal.

The big learning is that if you want to cover distance cost effectivly, never do it below planing speed (cruising speed) if there is an option. If you cant avoid lower speeds, be prepared to accept a fuel consumption which is (in my case) up to 50% greater per nautical mile.

Carry a lot more fuel if conditions may not allow you to get the boat on the plane and rather than tow some one in, call up your local sea rescue or both of you may end up in distress.

Its all pretty logical I guess. I never realised the magnitude of the difference prior to installing the fuel monitoring system. I wouldn't be without one now.