Fuel Consumption Confusion

[sailorboy1]

I would bet the "sweet" spot for all Yanmar modern engines is about the same rpm regardless of the HP or boat size (assuming the HP was matched correctly to the boat to start with). That doesn't mean they all will have the same fuel usage as the larger the boat & engine the more it will use, just that at that rpm is the best comprise between speed and fuel efficiency.

At least that is what I have found on the 2 boats I have have, 1 a 44HP and the other a 57HP.

[goboatingnow]

Oops sorry yes I was referring to the TE model 75hp

Dave

[chala]

Originally Posted by Don Lucas

I can motor at 6.5 knots and burn 1.2 gph to do it. The engine is efficient at this speed. That's 5.42 km/gal.

Or I can motor at 5 knots and burn 0.7 gph to do it. The engine is not as efficient if I do this. That's 7.14 km/gal which is 31% better running at that engine efficient speed ,

What is more efficient 5.42 km/gal or 7.14 km/gal?
6.5 knots = 12.038 km/h / 1.2gph = 10.0317 km/gal
5 knots = 9.26 km/h / 0.7gph = 13.229 km/gal
Fuel required to cover the same distance (12.038 / 9.26) * 0.7gal = 0.91gal
Fuel saving ((1.2-0.91) /1.2)*100 = 24.167%

[chala]

Originally Posted by rigormortis

The only way I can think of figuring this out is to use a metered amount of fuel in a jerry can with the fuel line and fuel return line both inserted in the jerry can and do some test runs for 30 minutes at different RPM's and in calm waters, no currents and make notes.

This will not tell you au much fuel you may use in bad weather. Like Barkaniel say:

Originally Posted by barnakiel

If the tanks are deep enough

See if you can use a dipstick or a sight glass. I have a sight glass, which is in effect a transparent vertical length of hosepipe that I calibrated gradually by pouring 20 l at time and marking the level. Like SvBEBE I keep a log where I record the sea and weather condition. I can use between 2.8 and 5 l an hours. Depending of the region, I may plan for the max of 5 l an hour or 1 l a Nm if I do not us as much it is the bonus.
By experience if I am faced by a 5 knots current with a 35 knots wind on the nose and accompanying sea at 5 l an hours I will make ½ knots SOG this give me an indication if it is worth continuing toward a sheltered anchorage or falling back into the previous one.

[deckofficer]

Looks like I might learn something on this thread. All of my stationary and over the road use of diesels, best fuel economies vs work done always seemed to occur at the engine's torque peak rpm. For medium speed diesels that was around 900 rpm, for high speed diesels, depending on size, from 1200~1800 rpm. For small diesels such as the turbo Yanmar in the 100 hp range, the torque peak occurs at 2000 rpm. So help me here, has my rule of thumb been basically correct, operate in the rpm range that your engine is the most volumetric efficient, i.e @ rpm of peak torque?

[David M]

The sweet spot where you can find the least amount of fuel burned per horsepower produced can be found in the specific fuel consumption curve provided by the manufacturer. One does not have to estimate to get this information.

[David M]

Originally Posted by Honey Ryder

remember that adding a knot generally doubles fuel consumption.

And doubling the speed cubes the horsepower required to reach that speed for a displacement hull.

[deckofficer]

David M,

I've respected your answers all over the forum. Was my basic rule of thumb close? After all these years I hope so, otherwise I've wasted some dead dinosaur juice.

[Ziggy]

Originally Posted by deckofficer

All of my stationary and over the road use of diesels, best fuel economies vs work done always seemed to occur at the engine's torque peak rpm. For medium speed diesels that was around 900 rpm, for high speed diesels, depending on size, from 1200~1800 rpm. For small diesels such as the turbo Yanmar in the 100 hp range, the torque peak occurs at 2000 rpm. So help me here, has my rule of thumb been basically correct, operate in the rpm range that your engine is the most volumetric efficient, i.e @ rpm of peak torque?

This is my experience also. Peak torque rpm seems to be the sweet spot for most diesels.

[deckofficer]

Thanks Ziggy, I needed that confirmation. I have kept my carbon footprint in check then over the years.

[Dockhead]

Peak torque on the 4JH3 HTE Yanmar intercooled turbo diesel is about 2900 RPM.

http://www.simplicity-marine.com/pdf/Yanmar/JH.pdf

That's a lot faster than I cruise it. It could be that your rule applies better to naturally aspiiated engines. When turbo engines start to produce a lot of boost, the torque goes up (as well as HP).

[Ziggy]

Originally Posted by Dockhead

Peak torque on the 4JH3 HTE Yanmar intercooled turbo diesel is about 2900 RPM.

http://www.simplicity-marine.com/pdf/Yanmar/JH.pdf

That's a lot faster than I cruise it. It could be that your rule applies better to naturally aspiiated engines. When turbo engines start to produce a lot of boost, the torque goes up (as well as HP).

Dockhead,
You may very well be right--I have no experience with turbos. The naturally aspirated 4JH3E torque curve peaks at around 2000 rpm.

[LakeSuperior]

Originally Posted by deckofficer

Looks like I might learn something on this thread. All of my stationary and over the road use of diesels, best fuel economies vs work done always seemed to occur at the engine's torque peak rpm. For medium speed diesels that was around 900 rpm, for high speed diesels, depending on size, from 1200~1800 rpm. For small diesels such as the turbo Yanmar in the 100 hp range, the torque peak occurs at 2000 rpm. So help me here, has my rule of thumb been basically correct, operate in the rpm range that your engine is the most volumetric efficient, i.e @ rpm of peak torque?

Operating at 70 to 80 percent of the peak HP that can be developed at that rpm is appproximately where the peak of the BSFC curve is located.
This is where the maximum amount of energy is developed per unit of fuel. In other words, the highest fraction of mechanical energy to thermal energy is developed here.

[Cheechako]

To me the sweet spot is where the engine smooths out, runs like a well lubricated machine making near hull speed and Puuuuuuuurrrrrrrrrrrrrrrrs! I really dont care if it's burning .7 or .8 gal per hour..... for you Powerboaters... other things would be a concern though!

[deckofficer]

At least with these engines you have a wide rpm range. The engines I spent most my time with had an rpm range of 30~108 with numerous rpms to be avoided for prolonged use. You had to pass through those rpms that for balance reasons "the book" instructed you to not stay at 3 different rpm ranges. Not to mention for astern propulsion, no gear box, you shut down the engine, changed over the valve timing, and re-started in reverse rotation.