slow speed or cruising speed

[busumark]

what is the correct speed for best fuel consumption slow speed or cruising speed. some say that it is better with slow speed but in this chart it shows that best fuel consumption is at 3500 rpm doing 19.4 knots. maybe someone has done saome tests and tell us his results.

[skip]

The ideal rpm varies from boat to boat, engine to engine and depending on the prevailing sea conditions. In my opinion without a fuel flow gauge built in to most of the modern engines and outboards or available as an add-on you will never be able to correctly adjust your rpm depending on the conditions, even down to the amount of fuel and load on the boat.

The key indicator is nmpl or nmpg (nautical miles per litre or nautical miles per gallon) depending which you prefer to use being the relationship between the amount of fuel used and the distance travelled effectively giving you your efficiency indicator. The further you can go on 1 litre or 1 gallon of fuel the better.....

So if we look at Busu's chart, the most efficient RPM on that boat on that day was 600rpm giving you 2.1 knots and 6.95nmpg.....however I don't think anybody wants to travel at 2.1 knots so let's elminate that figure.

3500rpm at 19.4knots gave 6.05nmpg even though the boat was using 3.2 gallons per hour....you could slow down and use less fuel per hour but at a slower speed it's going ot take you longer to get there so overall you would use more fuel. So 3500rpm on that boat on that day with that loadout was the most efficient speed, the ideal cruising speed and also the most efficient across the entire rev range.

In terms of trolling anything above 2000rpm 5.6knots and that engine really start to use fuel, probably because you're hitting the boats max hull speed and then the engine it trying to push the boat up on the plane.

I would say things are very different with a diesel engine where slow speed cruising is probably alot more efficient.

[busumark]

but how can you know or calculate your best rpm for everytime you go out ?

[skip]

You need a fuel flow sensor that provides you with this information. Usually they connect via NMEA protocols to your GPS unit so it reads your speed and shows you your consumption thereby giving you your nmpg/nmpl usage. From that you can then fine tune your rpm and trim to get the optimal burn.

[Destination Sea]

its a bit difficult to calculate the best rpm. Air temperature effects volumetric efficiency thus power. So  day i ex 2000rpm is the best but day 2 2200rpm is required for the same power to be generated.

[skip]

I can adjust my best rpm each day and adjust every few minutes as I need to, even turning round a point and the current changing makes a difference as does the boat getting lighter when the fuel is used up.

The Lowrance 527c I have together with the fuel flow sensor is all that's needed, Benri has something similar for his Suzuki

[Destination Sea]

with just a rev counter i had in mind

[skip]

With just a rev counter you will never know your actual fuel burn unless it's one of the new intelligent gauges that interface with the engine, the best you can get would be to compare the speed @ x rpm with that of the day before.