Fuel efficiency ?

[ABB]

I've read mention of fuel consumption in different threads, and would like to start a dedicated thread on fuel economy & speed/distance. Not trying to depress fellow motorboaters ... just doing some planning for my cruising next year ...

So for example,
- fuel consuption / rev's / speed combinations
- impact of different weather conditions
- how to interpret the engine performance charts, &
- what some terms mean, e.g. fuel consumption 216g/kwh@ 3600 rpm

To get it started, based on some rough numbers over the last couple of months - in a Merry Fisher 925 - Single Nanni 320....
- c. 18 - 20 litres per hour (includes idle time, cruising mostly at c.70% max revs, 2700)
- 2.2 litres per nm

Looking forward to making more accurate logs over the winter ... topping to full at shorter intervals etc.

 

[Geoffs]

I did some measurements on mine a while back (Larson Cabrio 240 Volvo D4/260). Red curve is fuel consumption in Nm /Litre, other curve is speed in knots. Fuel consumption figures taken from Volvo data. Readings taken in smooth water and slack tide (actually average of two readings in opposite directions).

Overall I average about 1nM per litre in various sea and tide conditions, cruising about 25 knots. Whilst not looking foreward to price increase, I could be worse off.

 

[jointventureII]

We get around 1.15-1.2 litres per nm, 115hp 4-stroke OB on a 23ft orkney.
This tends to be the case from around 13knots (about 3400rpm) to 19knots (about 4200rpm), not changing as much as I would have thought, perhaps due to the hull shape.

 

[Coupe]

also...

impact of dirty bottom

impact of dirty air filters

/forums/images/graemlins/blush.gif

 

[oldgit]

Best economy 4/5 knots @900 rpm 1.4gph
Not so good 10 knots 1900rpm 2.5gph.
Worst 12/13 knots 3000rpm 3.3gph
Fuel burn @ Wot 3400 rpm .....not the faintest idea as something would almost certainly break.
Total fuel burn last year
290 gallons
Total hours 138.
Avg. 2.10 gph.
Avg cost 48p per litre.
2 x 130hp diesels on outdrives.Numbers are for both engines combined.

My (very) iffy maths makes that 1.3 litre per mile in best case ?

 

[colingr]

Similar to whitefin, works out at about 6 gals / hour at 20-25 knots on an 18ft Maxum with Yamaha 115 V4. Rises to 8-9 Gals / hour @ wot.

 

[AdeOlly]

I carried out some performance vs economy trials in the summer and the results were quite interesting. I had been under the impression that operating at about 80% of WOT would give best economy. Wrong. That only gives me about 1.8mpg /forums/images/graemlins/shocked.gif My economy improves all the way up to 95% WOT when it improves to 2.3 mpg at about 28 knots /forums/images/graemlins/grin.gif

For info, boat is 4500kg dry, all up weight for the trials probably nearer 5300kg. Engines are twin AD31P (150) on duoprops.

The reason for my results is that once the engines are up to 3000 rpm the fuel burn is 25l/hr (each) and only increases to 29l/hr at WOT, whereas proportionately boat speed increases a good deal more.

If anyones interested I'll dig out my exact figures of speed vs fuel burn, which are from Volvo's graphs.

 

[ABB]

Great responses, thank you.

Amazingthe amount of variation, and something I need to monitor closely to get comfortable with my range, economy etc.

Interested in more views, and yes if more data avaialble would love to see the shape of those performance curves and the trade off between revs, speed, economy etc.


ANy views on downside of not operating within engine guidelines? My Nanni handbook syggests 70% time at 70% revs, 20% time at 80/90%, and 10% time at 100% ( .... or should I ask!)