Guesstimating fuel consumption

[BobH]

The engine curves show 2.6 gph at 1500 RPM. With the transmission in neutral the fuel consumption would obviously be much less. So the question is, for the given 2.6 gph, is that at full throttle with enough load on the engine to hold at 1500 RPM?

Bob

 

[ksanders]

The engine curves show 2.6 gph at 1500 RPM. With the transmission in neutral the fuel consumption would obviously be much less. So the question is, for the given 2.6 gph, is that at full throttle with enough load on the engine to hold at 1500 RPM?

Bob

If the manufacturers curve shows 2.6 GPH thats at full rated load.

If in a particular boat under way, the actual consumption is less then you are pulling less than full rated HP for that RPM.

If the fuel usage is more, then you are pulling more than full rated HP for that RPM.

Using the mfg fuel curve and comparing it to actual verified fuel burn is a good way to determine a particular boats loading of the engine.

 

[Brooksie]

Usually, on a marine engine, the consumption curve is based on the theoretical propeller load. That is, with a propeller and gear ratio which will allow the engine turn up to it's full rated RPM underway at WOT. Then consumption anywhere else on the curve can be read off for that same propeller & gear.
This curve is made by calculation not tests and real results vary from vessel to vessel; but it is a close starting point.

 

[Nomad Willy]

The way to determine actual engine load is to know what the engine will burn at rated rpm and WOT. Then observe your fuel consumption at .. For example cruise rpm. The 120 Lehman is about 6gph at max so if you're burning 3gph you're at 50% load. It's that simple but getting accurate burn rate numbers is somewhat difficult.

 

[LaBomba]

On my old boat I had a Northstar Chartplotter and one of the inputs was from a fuel flow meter. The meter merely was clamped in the fuel line (had to be vertical to keep air bubbles out) and read out constantly the actual fuel flow. Works good on planing boats for adjusting trim tabs etc. to get the best fuel efficiency and surprisingly sometimes increasing speed actually reduced fuel consumption. That theory likely wouldn't work on a displacement hull. These may be available for many chartplotters today with multiple inputs. Just another toy!

 

[Nomad Willy]

LaBomba,
Sounds good to me but I wonder how accurate they are? I suppose if one was propped correctly one could go to WOT and observe the rate of flow indicated and see how close that comes to the manufacturers spec sheet. If it matched then one could have confidence in the instruments accuracy .... but only at WOT and that may be nit picking but most flow meters have a range of accuracy so measuring 1gph on a small boat like mine may be not very accurate at all compared to a boat that flowed 8gph. One should be able to get that information from the manufacturer.

But even if you get an accurate instrument once you've taken readings at speeds where they may be needed or wanted you may as well throw the thing over the side. Of what use would it be after observations up and down the scale have been made. Seems like just another toy to me. How many of us really need to know with great accuracy how much fuel we burn? Some do QI realize but I think it is a VERY small number. I know I sure don't. Willy burns a gallon an hour and she carries 100 gal of fuel.

 

[Pau Hana]

The Northstar/Navman line were very accurate in determining fuel usage- but for gas engines only (no return line).

We were going to go with Floscan, but are now taking a hard look at Maretron and their FF100 system.

 

[ksanders]

I've had the floscans on several boats. with the diesel units we can get to within about 5% on average.

This is over a wide range of rpm settings.

I would suspect they would be more accurate if calibrated for a specific throttle setting.

 

[DavidM]

Bob:

There are two types of horsepower and fuel consumption curves for diesel engines: full load wot hp and prop hp.

Full load hp and fuel consumption are measured by the manufacturer with the engine on a dyno and the throttle held wide open. Enough load is put on the engine to keep the rpm steady at each setting measured.

Prop hp is determined by a formula that uses an exponent somewhere between 2.5 and 3.0 depending on the manufactuer. It recognizes that except at wot rpm the prop hp will be less than the full load hp. The manufacturer then applies fuel consumption data to this hp to get prop fuel consumption.

So if you were looking at the prop fuel consumption curve, then it is a reasonable approximation of real world fuel consumption at 1,500 rpm. If you were looking at full load data, then it is way too high at prop loadings.

David

 

[Bendit]

The power/prop calculator at boatdiesel.com is surprisingly accurate. It can give you fuel consumption estimates at the full range of engine revs.

 

[psneeld]

You can also rig a small fuel tank accurately marked and do trial runs of an hour or so at different RPMs. Very accurate...

But in reality accurate fuel consumption is only a fleeting concept for most cruisers...too many variables from quality of fuel to temperature to bottom growth, routing, idling, loading, bridges, etc..etc...

I find for long cruise planing, it all works out to be a rough estimate no matter how careful you want to be.