Low RPM cruise

[freshalaska]

Not easy to get an accurate measure. You have winds, currents, even how dirty or clean your hull bottom is effect you miles per gallon.

[Kenomac]

Originally Posted by 907Juice

Dude, they are never gonna get it until they factor the time factor. In case anyone is still counting, I ran the numbers for 500 gallons. Quite an extreme number ran consistently just to show the difference. 5k @ 0.6gph gets you 4166 (n) miles 833hours. 6.5kn @ 0.8gph 4062 miles and 625 hours. So in 500 gallons, you only gain 100 miles but it takes you 200 hours to get there.

Juice,

Thanks for making a good part of my point to those in doubt. Motoring for an extra 200 hours to save 100 miles of range seems like a lousy investment to me. Motoring an extra 8.3 days to get 100 miles further.

[DumnMad]

Originally Posted by Kenomac

Juice,

Thanks for making a good part of my point to those in doubt. Motoring for an extra 200 hours to save 100 miles of range seems like a lousy investment to me. Motoring an extra 8.3 days to get 100 miles further.

Depends;
@ a quiet 1200rpm my boat could cross the Tasman without significant wnd assist,
@ the recommended noisy 1600rpm it would stop short.

[Delfin]

Originally Posted by Kenomac

Juice,

Thanks for making a good part of my point to those in doubt. Motoring for an extra 200 hours to save 100 miles of range seems like a lousy investment to me. Motoring an extra 8.3 days to get 100 miles further.

Again, it depends entirely on the boat, so doubt from some is warranted. A transit to Hawaii at 8 knots would take me around 10.5 days at 8 knots. At 6.5 knots, I would take an extra day and a half. However, I would burn $1,100 less fuel and have a quieter passage. That doesn't seem like such a lousy investment, at least to me.

[Singularity]

Originally Posted by Kenomac

Juice,

Thanks for making a good part of my point to those in doubt. Motoring for an extra 200 hours to save 100 miles of range seems like a lousy investment to me. Motoring an extra 8.3 days to get 100 miles further.

The problem is the that "5 knots at 0.6 or 6.5 knots at 0.8, my choice" numbers simply can't be correct in any system incorporating, in the least, gravity and/or resistance.

It's not possible (in this setting) that someone could want to lie about the numbers, rather they could only be mistaken. Don't remember why the lie talk came up. Only bad data.

X amount of diesel can move mass Y (acted upon by A,B,C...) for Z distance. The forces on the boat are simply not the same at 5 knots as they are at 6.5 knots, ceteris paribus.

IF those numbers are still believed, please describe another contraption that includes an internal combustion engine that achieves a 25% increase in speed with (only) a 25% increase in fuel consumption, ceteris paribus.

[Kenomac]

Originally Posted by DumnMad

Depends;
@ a quiet 1200rpm my boat could cross the Tasman without significant wnd assist,
@ the recommended noisy 1600rpm it would stop short.

Originally Posted by Delfin

Again, it depends entirely on the boat, so doubt from some is warranted. A transit to Hawaii at 8 knots would take me around 10.5 days at 8 knots. At 6.5 knots, I would take an extra day and a half. However, I would burn $1,100 less fuel and have a quieter passage. That doesn't seem like such a lousy investment, at least to me.

Again, we’re discussing low rpm cruising speeds 5 knots vs 6-6.5 knots, we’re not discussing traveling at 8 knots. Everyone also seems to be forgetting the fact that most of us are on sailboats, which won’t be motoring the entire distance across a sea or ocean, so if we end up short by 100 miles in our calculations, surely the remaining 100 miles can be sailed in under the 200 extra hours time required to save 100 miles worth of fuel. On our boat, that represents less than 15 gallons of fuel to motor 100 miles, so there’s no way I’d consider motoring for an extra 200 hours (8.3 days) just to save 15 gallons of diesel.

[Dockhead]

Originally Posted by Kenomac

Again, we’re discussing low rpm cruising speeds 5 knots vs 6-6.5 knots, we’re not discussing traveling at 8 knots. Everyone also seems to be forgetting the fact that most of us are on sailboats, which won’t be motoring the entire distance across a sea or ocean, so if we end up short by 100 miles in our calculations, surely the remaining 100 miles can be sailed in under the 200 extra hours time required to save 100 miles worth of fuel. On our boat, that represents less than 15 gallons of fuel to motor 100 miles, so there’s no way I’d consider motoring for an extra 200 hours (8.3 days) just to save 15 gallons of diesel.

No one said it's the wrong choice to motor faster. Everyone will make his own subjective choice about whether to burn more fuel or take longer to get there.


I get more than 3 miles per liter in very calm weather at 5 knots, going down to 2.3 or so at 6, and less than 2 at 7. Nevertheless, I never motor at 5 knots, and don't motor at less than 6 or 6.5 knots, almost ever, and I sometimes motor at 8, although that bring miles per liter down to less than 1 on my boat.


That's my personal choice, and others might make different ones.



Only if you are worried about running out of fuel -- in that case, the slower the better.

[bluenomads]

Originally Posted by Dockhead

I get more than 3 miles per liter in very calm weather at 5 knots, going down to 2.3 or so at 6, and less than 2 at 7.

Do you mean 3 miles per gallon rather than per litre? I find it hard to comprehend that a large monohull (your 54’?) would be getting 0.33 L/nm! I’ve never heard of that little consumption at 5knots, or your 0.5 L/nm at 7kn.
I seem to recall read in one of the magazines of one of the best larger power catamarans of about 40’ getting just over 1L/nm at about 7-8kn - which was deemed excellent/amazing (which would be double yours!)

[Dockhead]

Originally Posted by bluenomads

Do you mean 3 miles per gallon rather than per litre? I find it hard to comprehend that a large monohull (your 54’?) would be getting 0.33 L/nm! I’ve never heard of that little consumption at 5knots, or your 0.5 L/nm at 7kn.
I seem to recall read in one of the magazines of one of the best larger power catamarans of about 40’ getting just over 1L/nm at about 7-8kn - which was deemed excellent/amazing (which would be double yours!)

No, per liter. Caveat -- that's with a squeaky clean hull. The numbers are much worse with fouling. Second caveat -- absolutely glassy calm sea and no headwind.


My low speed efficiency may be better than some because I have a variable pitch prop. A fixed pitch prop does lose efficiency below a certain speed which can eat up some of the advantage of much less hull resistance at lower speeds.



I budget 1 liter per mile for longer distances of mostly economical cruising, because even a slight headwind, and especially head sea, can drastically increase consumption. I consistently beat this provided I don't have any significant episodes of high speed or headwind or head seas (or tailwind).



Which underlines this fact -- to compare consumption at different speeds, and compare apples to apples, meaning exact same conditions, you really need a fuel flow meter. You can't do it on the basis of an average over a whole tank because of the multitude of factors which can change the average over a tank.

[Kenomac]

Originally Posted by bluenomads

Do you mean 3 miles per gallon rather than per litre? I find it hard to comprehend that a large monohull (your 54’?) would be getting 0.33 L/nm! I’ve never heard of that little consumption at 5knots, or your 0.5 L/nm at 7kn.
I seem to recall read in one of the magazines of one of the best larger power catamarans of about 40’ getting just over 1L/nm at about 7-8kn - which was deemed excellent/amazing (which would be double yours!)

Our Oyster 53 just finished motoring most of the way north along the Croatian coast into a .5 knot current and always against a 5-15 knot wind, and averaged 7.5 miles per gallon of diesel @ 5.5 knots boat speed over a distance of 300 miles, 40gallons used. 100hp Yanmar 25 ton boat, hull... not very clean 7/10.

[Pete7]

Originally Posted by Kenomac

Again, we’re discussing low rpm cruising speeds 5 knots vs 6-6.5 knots, we’re not discussing traveling at 8 knots. Everyone also seems to be forgetting the fact that most of us are on sailboats, which won’t be motoring the entire distance across a sea or ocean, so if we end up short by 100 miles in our calculations, surely the remaining 100 miles can be sailed in under the 200 extra hours time required to save 100 miles worth of fuel. On our boat, that represents less than 15 gallons of fuel to motor 100 miles, so there’s no way I’d consider motoring for an extra 200 hours (8.3 days) just to save 15 gallons of diesel.

You would eat more food during 8.3 days than the price of 15 gallons of diesel. Plus the human factor, telling the wifey that you are going to spend over an extra week rolling around at sea to save £70 isn't going to go down to well.

[Delfin]

Originally Posted by Kenomac

Our Oyster 53 just finished motoring most of the way north along the Croatian coast into a .5 knot current and always against a 5-15 knot wind, and averaged 7.5 miles per gallon of diesel @ 5.5 knots boat speed over a distance of 300 miles, 40gallons used. 100hp Yanmar 25 ton boat, hull... not very clean 7/10.

What RPM at that speed?

[Kenomac]

Originally Posted by Delfin

What RPM at that speed?

1600-2200rpm on average. Sometimes more.

[Delfin]

Originally Posted by Dockhead

No, per liter. Caveat -- that's with a squeaky clean hull. The numbers are much worse with fouling. Second caveat -- absolutely glassy calm sea and no headwind.


My low speed efficiency may be better than some because I have a variable pitch prop. A fixed pitch prop does lose efficiency below a certain speed which can eat up some of the advantage of much less hull resistance at lower speeds.



I budget 1 liter per mile for longer distances of mostly economical cruising, because even a slight headwind, and especially head sea, can drastically increase consumption. I consistently beat this provided I don't have any significant episodes of high speed or headwind or head seas (or tailwind).



Which underlines this fact -- to compare consumption at different speeds, and compare apples to apples, meaning exact same conditions, you really need a fuel flow meter. You can't do it on the basis of an average over a whole tank because of the multitude of factors which can change the average over a tank.

Delfin's underbody is full displacement with more wetted surface than most sailboats. When a new CAT is installed, they send a tech to make sure you can reach max rpm to verify that the boat isn't over propped, since that can cause premature, within warranty, issues. She could, but that was with a completely empty vessel since I bought her without a stich of cabinetry, doors, wiring, etc. etc. Once I added about 15 tons of stuff she was then over propped. Go figure.

Over propped, I would have argued that poking along at 5 knots was less efficient in terms of distance traveled, time, and fuel consumption than running at 6.5 knots since at the lower speed the engine was laboring trying to turn a 44" wheel at low rpm.

My point is that physics dictates fuel consumption of a diesel pushing a displacement at different prop loads. Over prop the boat and the prop load increases at a given rpm burning more fuel, so my guess is that a vessel that shows a small difference in fuel consumption and kmpg at low speeds vs. kmpg at higher speeds is probably over propped, which is very bad for the engine. Displacement hulls that are correctly pitched will see a big reduction in fuel consumption with small decreases in speed. That's just physics.

[Delfin]

Originally Posted by Kenomac

1600-2200rpm on average. Sometimes more.

Sounds like you motored around 54 hours to travel 300 miles at 5.5 knots, burning 40 gallons. That would indicate fuel consumption per hour of 3/4 gal/hour. The fuel consumption graph for a 100 hp Yanmar indicates that you would have to motor at 1400 rpm to have that fuel burn, and at 2200 you'd burn around 2 gph. At 2600 rpm it's 3 gph. Not trying to be argumentative or doubt your numbers, but you are getting pretty amazing fuel economy that isn't really explainable based on what Yanmar publishes for that engine.

https://www.yanmarmarine.com/theme/y...eet_4JH110.pdf