Low RPM cruise

[a64pilot]

Originally Posted by jkleins

This is a sister boat of ours. John Deere 154 hp 50ft motorsailer 22 tons. This is with sails down with fuel flow meter and feathering prop. I do a little better by my fuel logs but I have a lot of very low idling around anchorages and stuff where I don’t keep as accurate track of rpms so I am probably underestimating my actual usage. I usually just putt along at 1500.

RPM SPEED GPH Range MPG

2600 9.70 7.80 311 1.24
2500 9.40 6.90 341 1.36
2400 9.30 6.10 381 1.52
2300 9.20 5.30 434 1.74
2200 9.15 4.60 497 1.99
2100 9.00 4.10 549 2.20
2000 8.75 3.60 608 2.43
1900 8.60 3.10 694 2.77
1800 8.40 2.60 808 3.23
1700 8.10 2.20 920 3.68
1600 7.70 1.90 1013 4.05
1500 7.30 1.60 1141 4.56
1400 6.85 1.30 1317 5.27
1300 6.30 1.10 1432 5.73
1200 5.70 0.90 1583 6.33
1100 5.40 0.75 1800 7.20
1000 5.10 0.60 2125 8.50

You guys don’t want to believe actual data, but here it’s posted.
BTW Sailorboys motor isn’t a Turbo so much for that.
It follows exactly along with automobiles, the faster you go, the more fuel you burn per distance travelled. This graph is meant to show difference in MPG for tire pressure, but it also shows exact fuel mileage per speed, and it’s a US government EPA study, not some kids who post on YouTube. As you can see, there is no sweet spot for cars, even though you ask the average guy and they are convinced there is.

Physics isn’t just a good idea, it’s the law

[a64pilot]

It is simple physics, and its true for airplanes, cars, boats and I’m sure trains.
If you want to make something move, you have to expend energy, if you want it to move faster, You expend more energy than you get in a relative increase in speed.
It’s funny though, in every thing there is people who want to think their is a sweet spot, in airplanes many are convinced if they climb to a higher altitude and then slowly descend to assigned altitude speeding up as they descend that they will end up at a higher stabilized airspeed, an enormous number of people believe that, it’s even often taught.
It’s a complete myth though, no truth to it at all.

It’s like the stopped prop thing, many if not most are sure that a stopped prop is less drag than a freewheeling one. I did, until proven different. It’s funny though that many, even when they see the testing, still won’t believe it.

[CatNewBee]

Originally Posted by a64pilot

It is simple physics, and its true for airplanes, cars, boats and I’m sure trains.
If you want to make something move, you have to expend energy, if you want it to move faster, You expend more energy than you get in a relative increase in speed.
It’s funny though, in every thing there is people want to think their is a sweet spot, in airplanes many are convinced if the climb to a higher altitude and then slowly descend to assigned altitude speeding up as they descend that they will end up at a higher stabilized airspeed, an enormous number of people believe that, it’s even often taught.
It’s a complete myth though, no truth to it at all

Indeed there is a sweet spot, you have a non-linear curve regarding torque in combustion engines. You need some rpm to become efficient, but it is not at maximum power / maximum rpm.

Also the resistance of air and water are non-linear to the speed, it rises exponential and goes to infinity at speed of light even in vacuum.

[Dockhead]

Originally Posted by Kenomac

I’m not giving up just because I’ve been officially declared wrong.

In order to understand how a displacement hull with a large diesel can be more efficient at a slightly faster speed, you have to first put yourself in the 800 pound gorilla’s bike shoes while he pedals the bike at 5-6mph (see above). He covers significantly more ground but expends little to no additional effort/energy at the slightly faster speed. That’s how a modern turbo diesel works, the turbo kicks in and efficiency increases.

It is definitely true, that diesel engines tend to be more efficient around their torque peaks, than they are at lower or higher revs.


But the difference is very small -- on the order of a few percent. This is my engine:









You can't make up the big increase in hull resistance, with this kind of small increase in engine efficiency.



Did you read the paper I linked to?


This is a good discussion -- these are interesting engineering questions.

[Kenomac]

Originally Posted by a64pilot

You guys don’t want to believe actual data, but here it’s posted.
BTW Sailorboys motor isn’t a Turbo so much for that.
It follows exactly along with automobiles, the faster you go, the more fuel you burn per distance travelled. This graph is meant to show difference in MPG for tire pressure, but it also shows exact fuel mileage per speed, and it’s a US government EPA study, not some kids who post on YouTube. As you can see, there is no sweet spot for cars, even though you ask the average guy and they are convinced there is.Attachment 178421

Physics isn’t just a good idea, it’s the law

Please splain what a Toyota Corolla normally aspired high rpm gasoline engine (your graph) has in common with a 35 ton monohull with a Perkins low rpm turbo diesel?

This otta be good....

[Dockhead]

Originally Posted by a64pilot

. .
Physics isn’t just a good idea, it’s the law

I love it!

[Kenomac]

Originally Posted by Dockhead

It is definitely true, that diesel engines tend to be more efficient around their torque peaks, than they are at lower or higher revs.


But the difference is very small -- on the order of a few percent. This is my engine:


Attachment 178422

Attachment 178423




You can't make up the big increase in hull resistance, with this kind of small increase in engine efficiency.



Did you read the paper I linked to?


This is a good discussion -- these are interesting engineering questions.

Oh... so now I’m less wrong by 5%.

[jkleins]

Originally Posted by Kenomac

Lets please compare apples to apples. First, is the John Deere a modern turbo diesel? Second, a motorsailer can hardly be compared to a modern sailboat hull design. BTW, I looked up your boat... nice boat!

My JD is a turbo and I suspect my hull form is more “modern” then a lot of the folks here. Maybe Yanmar has something that other engines don’t but if you talk to the long range trawler folks they always just go slower when they need to go further. It is not argued on their forums since most of them quantify it pretty accurately and can tell you within a few dozen miles how far they can go.
Thank you, we almost bought an Oyster but my wife twisted her ankle going down the ladder when we were looking and she has had a grudge ever since. [emoji3]

[Dockhead]

Originally Posted by Kenomac

Please splain what a Toyota Corolla normally aspired high rpm gasoline engine (your graph) has in common with a 35 tone monohull with a Perkins low rpm turbo diesel?

This otta be good....

The relevance is this: your Toyota Corolla, just like any car, just like any airplane, and just like any displacement hull vessel, is subject to the same basic principle that resistance through air or water goes up exponentially with speed, requiring more power disproportionate to the increased distance covered at a higher speed.


"More power" = more fuel. That means fuel mileage falls with speed. There are very few and very narrow exceptions, some of which have been mentioned already.

[Kenomac]

Originally Posted by Dockhead

The relevance is this: your Toyota Corolla, just like any car, just like any airplane, and just like any displacement hull vessel, is subject to the same basic principle that resistance through air or water goes up exponentially with speed, requiring more power disproportionate to the increased distance covered at a higher speed.

The last time I checked under the hood of a Corolla, I didn’t see any superheated air being injected back into the fuel mixture to increase efficiency... as in a turbo.

[Dockhead]

Originally Posted by Kenomac

Oh... so now I’m less wrong by 5%.

Yes, exactly!

[Kenomac]

Originally Posted by jkleins

My JD is a turbo and I suspect my hull form is more “modern” then a lot of the folks here. Maybe Yanmar has something that other engines don’t but if you talk to the long range trawler folks they always just go slower when they need to go further. It is not argued on their forums since most of them quantify it pretty accurately and can tell you within a few dozen miles how far they can go.
Thank you, we almost bought an Oyster but my wife twisted her ankle going down the ladder when we were looking and she has had a grudge ever since. [emoji3]

The very first thing we tell everyone upon entering our boats.... “you need to hold the rails and decend ladder style. And no socks without shoes allowed, or you will fall down.”

[Dockhead]

Originally Posted by Kenomac

The last time I checked under the hood of a Corolla, I didn’t see any superheated air being injected back into the fuel mixture to increase efficiency... as in a turbo.

To see how big an effect that has on efficiency, just pull up the specific fuel consumption chart for your engine.


Then compare any improvement in efficiency from 5 to 7 knots (or whatever), to the difference in hull resistance at those speeds. If the efficiency gain is more than the difference in resistance, then you are right. However . ..



I put up my own specific fuel consumption curve above. My engine is a small displacement, high pressure turbo with intercooler -- the kind where you would expect the most efficiency gain from the turbo. Maximum fuel efficiency, 167g/hp/hr, occurs at 2400 RPM. This is less than 1% better than the efficiency at 2000 RPM. After 2400 RPM, the fuel efficiency gets worse again.



Did you read the paper I linked? This will give a great deal of insight into all of these issues.

[Kenomac]

Originally Posted by Dockhead

Yes, exactly!

Or maybe you’re more wrong by 5%.

[hpeer]

FWIW. Official Yanmar mahal screen shots. It COULD be interpreted to define Low Speed” as under 1,000RPM. They allow both methods of blowing her out. I frequently run below 2,200 for extended periods with no notable negative effect. I do try to blow her out occasionally but have never noticed any difference.