Low RPM cruise

[Dockhead]

Originally Posted by daletournier

Yes Ken, range is range. At 3.5 liters per hour I'm getting approx 800nm. When I motor at an rpm of 1800 I burn approx 2.5l per hour. This gets me up over 1000nm, yes it takes longer but ultimately I can go further.

From here the next fuel stop is 1400nm...

It is well known that what you say here is true:

[Dockhead]

Originally Posted by Kenomac

So Sailorboy, I and anyone else who disagrees with you, have now been officially declared wrong! I can live with that. Seems this is the way many discussions on CF have been going lately.

Most of the coments on this thread have compared gallons per hour, not miles per gallon, or have not specified how fuel economy was calculated.

It is simply not true, that people in this thread have confused gallons per hour with miles per gallon.


It is a simple fact that on any displacement hull vessel, the faster you go, the lower the miles per gallon. I am not stating this to put anyone down. I have greatest respect for both you and Sailorboy, but I know for sure that you are simply mistaken about this. There's no shame in that -- we're all mistaken from time to time, I not less than anyone else. I would not be doing you or Sailorboy a favor, by telling you that maybe your boat can somehow defy the laws of physics.



Install an accurate fuel flow meter, or read any basic hydrodynamics text, and you will understand. Did you read the thing I linked earlier? I recommend it: https://www.awelina.co.uk/hull_drag/...nsumption2.pdf


Cheers.

[dfelsent]

Whoo boy. I set out to write one thing, did some research and came up with an entirely different conclusion.

I found a surprising amount of information aggregated in some United Nations reports. This ties into an interesting article in Ocean Navigator.

Fuel and financial savings <br>for operators of small fishing vessels
Balancing speed with fuel consumption - Ocean Navigator - March/April 2013
http://www.fao.org/docrep/017/i2461e/i2461e.pdf

The summary of my thinking is kind of like this:

Small diesels like ours, especially turbo charged, are most efficient when operated around 80% of RPM.

The drag of the boat increases rapidly as the RPM increases. This means that the combine boat/engine efficiency (miles per gallon) is best at far lower RPM than 80% max.

There is a time cost to operating at low RPM. It takes longer to get to where you are going. This may be important at times, and not important at others.

Most sailboat auxiliaries are larger than needed for flat water operation. If they were a fraction of the power that we like to have, our fuel efficiency would be higher, as we could operate the engine in a more fuel efficient RPM range without incurring a large drag penalty.

Those who have under powered boats are likely more fuel efficient.

Now I am going to have to make some measurements myself before I really believe all this.

[Kenomac]

Originally Posted by Kenomac

It does matter and it’s not “that simple.”

Yes it takes more energy to increase from a stand still up to 5 knots, and to increase from 5 knots to 6 knots during acceleration; BUT after a steady speed of 5 knots or 6 knots has been attained and stabilized, there really isn’t any difference in fuel consumption, so you might as well travel at the faster speed and get where you want to be quicker.

Physics: After the boat is in motion, it doesn’t take much energy for a big diesel to keep it in motion.

Your bike example is not appropriate. Regarding diesels and boats, it would be more akin to an 800 pound gorilla pedaling a bike at 5mph vs 6mph on a flat road at a steady speed, virtually no difference in energy consumption.... until he hits the hills.

Originally Posted by dfelsent

Whoo boy. I set out to write one thing, did some research and came up with an entirely different conclusion.

I found a surprising amount of information aggregated in some United Nations reports. This ties into an interesting article in Ocean Navigator.

Fuel and financial savings <br>for operators of small fishing vessels
Balancing speed with fuel consumption - Ocean Navigator - March/April 2013
http://www.fao.org/docrep/017/i2461e/i2461e.pdf

The summary of my thinking is kind of like this:

Small diesels like ours, especially turbo charged, are most efficient when operated around 80% of RPM.

The drag of the boat increases rapidly as the RPM increases. This means that the combine boat/engine efficiency (miles per gallon) is best at far lower RPM than 80% max.

There is a time cost to operating at low RPM. It takes longer to get to where you are going. This may be important at times, and not important at others.

Most sailboat auxiliaries are larger than needed for flat water operation. If they were a fraction of the power that we like to have, our fuel efficiency would be higher, as we could operate the engine in a more fuel efficient RPM range without incurring a large drag penalty.

Those who have under powered boats are likely more fuel efficient.

Now I am going to have to make some measurements myself before I really believe all this.

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.

[SailRedemption]

Originally Posted by markpierce

Rate of fuel consumption is a measure of load. Comparing fuel consumption rate to maximum rate of fuel consumption measures relative load.

I definitely agree with that, but it's hard to get accurate fuel consumption numbers on small mechanical diesel engines like what you see in the size boats we all have. Sure you could figure it out if you have a constant RPM over a long period of time and measure fuel. But temp, and boost if equipped, is a better solution for instant accurate data for figuring load.

The modern common rail, computerized engines can give you load and fuel rate. But most of us avoid those engines for various reasons covered in many threads.

[dfelsent]

I think everyone here might be right for their own boat.

That’s why I have to make careful measurements for my particular situation before I can claim to really know.

And then it will be only for my engine/boat combo.

[emoji1]

[transmitterdan]

Originally Posted by Kenomac

I’m not giving up just because I’ve been 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.

Ken,

For a "properly sized" boat engine the efficiency (miles/gallon) increases as speed increases from 0 to about 1/2 hull speed then mpg decreases thereafter. The peak in efficiency is a function of the size of the engine. A big engine peaks its mpg efficiency at a higher speed but its less efficient overall than a smaller engine at same speed. So a big engine just obscures the physics it doesn't render physics wrong. A hugely oversized engine might keep increasing efficiency until hull speed is reached but that doesn't make your point valid. Almost no one has such a stupidly oversized engine as that.

[CatNewBee]

I can confirm this, that motoring at half throttle (1500rpm) streches the distance significantly - in fact doubles the distance you can go compared to running at 2900rpm. and in speed it is 5kn vs. 7-8kn (on our boat). You gain time wise only 35% by the higher speed.

In the manual it is about idling below 1000rpm, I do not consider motoring at 1500rpm as idling, the engine is warm enough to burn out the residues. It is sufficient to run it once a week or so to 90..100% rpm for maintenance (after long motoring and a warm engine, not on a cold engine! and also do not turn it off immediately after the abuse, let her cool down in lower rpm a little)

[jkleins]

Originally Posted by CatNewBee

I can confirm this, that motoring at half throttle (1500rpm) streches the distance significantly - in fact doubles the distance you can go compared to running at 2900rpm. and in speed it is 5kn vs. 7-8kn (on our boat). You gain time wise only 35% by the higher speed.

In the manual it is about idling below 1000rpm, I do not consider motoring at 1500rpm as idling, the engine is warm enough to burn out the residues. It is sufficient to run it once a week or so to 90..100% rpm for maintenance (after long motoring and a warm engine, not on a cold engine! and also do not turn it off immediately after the abuse, let her cool down in lower rpm a little)

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

[Kenomac]

Originally Posted by transmitterdan

Ken,

For a "properly sized" boat engine the efficiency (miles/gallon) increases as speed increases from 0 to about 1/2 hull speed then mpg decreases thereafter. The peak in efficiency is a function of the size of the engine. A big engine peaks its mpg efficiency at a higher speed but its less efficient overall than a smaller engine at same speed. So a big engine just obscures the physics it doesn't render physics wrong. A hugely oversized engine might keep increasing efficiency until hull speed is reached but that doesn't make your point valid. Almost no one has such a stupidly oversized engine as that.

Then you’ve confirmed my point. Our 62 has a Perkins 225hp low rpm tubo diesel detuned to 185hp, hull speed is just under 10 knots and the sweet spot for fuel efficiency is 5-6 knots or approximately 1/2 hull speed. At 5 knots, gorilla is just taking it easy and smoking a cigarette while pedaling. He puts out the cigarette, pedals slightly harder, the turbo kicks in for a little more efficiency and presto.... he’s going slightly faster using less energy thanks to the turbo.

[ontherocks83]

Originally Posted by Kenomac

Then you’ve confirmed my point. Our 62 has a Perkins 225hp low rpm tubo diesel detuned to 185hp, hull speed is just under 10 knots and the sweet spot for fuel efficiency is 5-6 knots or approximately 1/2 hull speed. The gorilla is just taking it easy and smoking a cigarette while pedaling.

Oh man, now we are adding in smoking diesels, isn't that going to screw up your sweet spot when that soot builds up? Better get that gorilla pedaling full speed to clear out its lungs, before the argument turns to best carboned up fuel economy.

[Kenomac]

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

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!

[Kenomac]

Originally Posted by ontherocks83

Oh man, now we are adding in smoking diesels, isn't that going to screw up your sweet spot when that soot builds up? Better get that gorilla pedaling full speed to clear out its lungs, before the argument turns to best carboned up fuel economy.

The gorilla is pedaling “piano” in the peloton.

[CatNewBee]

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!

OK, I am talking about Yanmar diesel 3JH5E 40hp with saildrives, originaly it was about a Yanmar 4JHE (44hp)

[Kenomac]

Originally Posted by CatNewBee

OK, I am talking about Yanmar diesel 3JH5E 40hp with saildrives, originaly it was about a Yanmar 4JHE (44hp)

Sailorboy and I are comparing monohull fuel efficiency, I’m sure things are different with a catamaran such as using one engine or two, increased windage and then of course hull design. Too many differences to compare to a monohull.