Fuel Economy - Engine speed or Boat speed?

[dombuckley]

Taking a Yanmar 4JH4 as an example, the power curve shows 5 litres per hour at 2200rpm, and 16 lph at WOT, though this may be modified by the particulars of your installation. 2200rpm also happens to be peak torque

So twin engine running at 2200rpm is a total of 10 lph for a speed of 8.2kts, giving fuel ecomomy of 0.82 miles per litre.

Single engine running at WOT is 16 lph for a speed of 7knots, giving a figure of 0.45 miles per litre.

Basically, in your scenario, single engine running will use about twice the fuel of twin engine running to get the same speed. Not only do you have to counter the massive drag of a stationary propeller, but there is also drag induced by having to steer to correct for the asymmetric thrust.

 

[James_Calvert]

Besides, 7kts burning 3.5 l/h means that you are in the half a liter per nautical mile ballpark, which for a mobo is pretty much like saying that you are burning nothing.
The next best thing is bound to be a sailboat, I reckon!

That's in the same ballpark or better than a sailboat under engine alone.

Turning their engine off can make for a quieter and more peaceful trip mind...

 

[Irish Rover]

Thanks again guys. No it definitely does not plane. I rarely go full WOT but it's amazing how 14.5 knots feels like you're flying when you've become so used to chugging along at 7.

 

[Irish Rover]

Taking a Yanmar 4JH4 as an example, the power curve shows 5 litres per hour at 2200rpm, and 16 lph at WOT, though this may be modified by the particulars of your installation. 2200rpm also happens to be peak torque

So twin engine running at 2200rpm is a total of 10 lph for a speed of 8.2kts, giving fuel ecomomy of 0.82 miles per litre.

Single engine running at WOT is 16 lph for a speed of 7knots, giving a figure of 0.45 miles per litre.

Basically, in your scenario, single engine running will use about twice the fuel of twin engine running to get the same speed. Not only do you have to counter the massive drag of a stationary propeller, but there is also drag induced by having to steer to correct for the asymmetric thrust.

Sorry if I'm being dumb but I'm not sure I understand where you're getting your figures although I appreciate you're referencing a different engine. In my case I get 7kts with one engine at 2200 and fuel burn of 3.5L which is 0.5 L per mile. With both engines running at 2200 I get 8.2 kts speed so I reckon I'd my fuel burn would have to be less than 2.05 lph on each engine to better what I'm getting on one engine. Does that make sense?

 

[dombuckley]

Ah, I misread and thought you could only achieve 7kts at wide open throttle on one engine: didn't realise you meant the same engine speed. In which case, single engine running will of course be more economical. For exact figures, you should be able to find your engine type on the yanmar website: the fuel consumption curves will be included in the "specifications" tab on the page menu.

 

[Irish Rover]

Ah, I misread and thought you could only achieve 7kts at wide open throttle on one engine: didn't realise you meant the same engine speed. In which case, single engine running will of course be more economical. For exact figures, you should be able to find your engine type on the yanmar website: the fuel consumption curves will be included in the "specifications" tab on the page menu.

Thanks. I found the fuel consumption curve in a manual but it seems to suggest consumption is directly related to RPM whereas [if I understand them correctly] posters on here are suggesting that fuel consumption at a particular rpm [say 2200] will vary considerably based on the workload of the engine. This is probably what I'm finding it difficult to grasp.

 

[Bluemu]

Ah we almost bought a Greenland 34! Nice boat for the price.
We also have a powercat and find that on one motor, it’s more economical than two up to about 7.5kn.
From there to WOT (about 14) two engines are more efficient, and in fact about 8.3kn is WOT on one engine.
By more efficient, I mean L/nm.
There‘s a few interesting points:
- can your gearbox freewheel ok in the off engine? Some can, some can’t
- can you run straight on one engine, or do you need a little rudder? how much can effect efficiency
- the off engine is also doing fewer hours, meaning more time between maintenance which I havent taken into account in my figures
- it’s quieter sleeping in the “off” hull!

 

[Irish Rover]

Ah we almost bought a Greenland 34! Nice boat for the price.
We also have a powercat and find that on one motor, it’s more economical than two up to about 7.5kn.
From there to WOT (about 14) two engines are more efficient, and in fact about 8.3kn is WOT on one engine.
By more efficient, I mean L/nm.
There‘s a few interesting points:
- can your gearbox freewheel ok in the off engine? Some can, some can’t
- can you run straight on one engine, or do you need a little rudder? how much can effect efficiency
- the off engine is also doing fewer hours, meaning more time between maintenance which I havent taken into account in my figures
- it’s quieter sleeping in the “off” hull!

We're very happy with our G34. Lots of space compared to our previous smaller sports cruisers and very economical, straightforward and easy to maneuver. The off engine freewheels OK and if it's not running straight the offset is tiny. Our performance figures look very similar. Which Cat do you have and which engines?