'Cruising RPM' - What Does this Mean ?

[barnakiel]

I would think the 'crusisng' may be unrelated to fuel economy or what is best for the engine. It may be about what is best for the cruiser.

Local cruising rpm seems to be very close to max rpm, which makes me pause and think why otherwise sluggish people will drive their boats at such high speed. (Correct answer: To be back ashore for lunch!)

I drive our beast at half to 2/3 of max rpm. I try to go rather faster than rather slower. Max boat speed 6.8 knots, our cruising speed 5 knots (but only 4.4 if we are looking for best fuel efficiency.

There is some good engine / propeller / fuel efficiency / cruising speed read on HR site (look at the new HR31 page, bottom of the page - links).

b.

[Juniper]

My only question with all this is, what if you have an engine that is under/over sized for the boat?
I have a 35 footer( 25 foot LWL ) with a 3GM30F rated for 24 HP at full power and a clean bottom I can get hull speed ( 7.5 kn ) to a bit more at at 3300 rpm and I'm using about 1.5 gph, now if I had a 50HP engine I wouldn't be going much faster and would need to be well above hull speed to load the engine, or if I had a 10 HP, I would be full out and not hut hull speed. I'm still tring to what would happen straight in my head, anyone got any idea?

[barnakiel]

Not if you understand how the drag is distributed related to the speed. I believe it is not linear - someone said going 100% faster requires 4 times the energy. Imagine the boat making 1 knot, then 2 knots, then ...

But why would anybody put such a big engine into a sailing boat?

b.

[smurphny]

My only question with all this is, what if you have an engine that is under/over sized for the boat?
I have a 35 footer( 25 foot LWL ) with a 3GM30F rated for 24 HP at full power and a clean bottom I can get hull speed ( 7.5 kn ) to a bit more at at 3300 rpm and I'm using about 1.5 gph, now if I had a 50HP engine I wouldn't be going much faster and would need to be well above hull speed to load the engine, or if I had a 10 HP, I would be full out and not hut hull speed. I'm still tring to what would happen straight in my head, anyone got any idea?

Trying to push above hull speed always increases fuel consumption dramatically. I have the same engine in my 35 Alberg which has a bit more hull to push through the water and burn right around 1 gal/hr at 2600-2800 RPM. My hull speed is right around 6.5 knots. From experience, this seems to be the MINIMUM rpm to run without glazing the cylinder liners. Looking at the torque curve of this engine in the manual, it seems that 2800 is about top of torque before it falls off. The fuel consumption curve shows that consumption goes up dramatically above 3000rpm. Running at > 2800 has to decrease efficiency since it's requiring increasingly more energy to achieve each rpm. If you need to rev up to 3300 and are burning > 1 gph at hull speed, it may be that your prop needs a tweak to get to hull speed at a lower rpm, hence, less fuel consumption. With fuel prices as they are now, every little bit helps.

[barnakiel]

Originally Posted by smurphny

(...)

From experience, this seems to be the MINIMUM rpm to run without glazing the cylinder liners.

Is glazing related to "too low" rpm or is it related to loading (or not) the engine?

I was once told that the problem is in running the engine unloaded and especially so at high revs.

Just asking.

barnie

[barnakiel]

This:

Bore glazing

Not to say I think it is correct. But seems well based and makes sense (to me, I am not a diesel mechanic).

barnie

[smurphny]

That's a good explanation. It coincides with the Yanmar tech people told me. When I spoke to a Yanmar tech rep about the blow-by and smoking I was getting (engine with only 100 hours) he said the original owners probably ran it at low rpms during the break-in period. I wound up doing a ring job, luckily do-able right in boat to the tune of about $400 in parts. Now runs with no smoke at all. Will have to do a bit of research on proper oil grade to use. Have been using Rotella which I assumed was appropriate. By the sound of that article some of the modern oils can cause glazing.

[markpierce]

It's that "sweet spot" where fuel economy and speed are at the "best" (subjective) compromise. For example, for a 14-ton, thirty-something-foot waterline displacement-hull "trawler" it would be something like 6.5 or 7 knots. (Twenty horsepower consumes about a gallon of diesel an hour.)


S/L Ratio..... Knots..... HP
1 ...... ..........5.63........ 3.9
1.1............... 6.19....... 6.0
1.2 ...............6.75....... 9.5
1.25............. 7.03...... 12.8
1.3............... 7.32...... 17.3
1.35.............. 7.6........ 23.5
1.6................ 9.0 ...... 149.3

[barnakiel]

Originally Posted by markpierce

It's that "sweet spot" where fuel economy and speed are at the "best" (subjective) compromise. (...)

Mark,

With all due respect, I will disagree. Fuel economy will best at only one speed. I see it as objective, not subjective.

Much as most owners would like to go faster! (subjective here, true!)

Maybe boat makers (I will not call them builders ;-)) simply gave prospective owners a figure so that people would more or less understand they will not make it from A to B in two hours, even if the same distance could be covered in half an hour ... by car).

Cheers,
b.

[Juniper]

Originally Posted by markpierce

It's that "sweet spot" where fuel economy and speed are at the "best" (subjective) compromise. For example, for a 14-ton, thirty-something-foot waterline displacement-hull "trawler" it would be something like 6.5 or 7 knots. (Twenty horsepower consumes about a gallon of diesel an hour.)


S/L Ratio..... Knots..... HP
1 ...... ..........5.63........ 3.9
1.1............... 6.19....... 6.0
1.2 ...............6.75....... 9.5
1.25............. 7.03...... 12.8
1.3............... 7.32...... 17.3
1.35.............. 7.6........ 23.5
1.6................ 9.0 ...... 149.3

Markpierce
Do you have RPM values for above?
Some added fugures from above
gallons used...Kn traveled....Kn made for.....Gallons used for
at speed .......per gallon......8 hour run........8 hour run
0.20.................28.2...............45........ .........1.6
0.30.................20.6...............50........ .........2.4
0.34.................19.9...............54........ .........2.7
0.64.................11.0...............56........ .........5.1
0.87...................8.4...............59....... ..........7.0
1.18...................6.4...............61....... ..........9.4
7.47...................1.2...............72....... .........60.0

For the above boat between 6.75 and 7.03 looks to be the best speed/fuel use/time spot. But with a 150HP engine that is using less then 1/10th it's power,
So my question is, is it better to run at a higher rpm but with less load or to run at a lower rpm that is loading the engine at that speed?

[markpierce]

Originally Posted by Juniper

Markpierce
Do you have RPM values for above?

Sorry, no. That would depend on the particular engine, wouldn't it?

Your MPG figures go to show that, absent wind, waves, and current, the most fuel-efficient speed for a displacement hull is as slow as the engine can go, which definitely ain't cruising speed as proposed in an earlier post disagreeing with me.

[Juniper]

My way of looking at it is, at the point that not much more speed is gained for a a lot more fuel used.
5.63 Kn/h uses 1.6 gallons to go 45 Kn
6.19 Kn/h uses 2.16 gallons to go that same
6.75 Kn/h uses 2.25 gallons to go that same
7.03 Kn/h uses 4.10 gallons to go that same
7.32 Kn/h uses 5.34 gallons to go that same
7.60 Kn/h uses 6.90 gallons to go that same
9.00 Kn/h uses 37.59 gallons to go that same

So the above somewhere about 6.8Kn/h is the "Cruising Speed" as it is the best time/fuel efficient. I you have 9 hours to go that 45Kn go slow, if you need to be there in 5 hours there goes 36 extra gallons for 4 hours, now if you have 6 hours and 40 minutes it's only an extra 0.65 gal. for an extra 2 hour and 20 minutes of sipping drinks to go at 6.75 kn,

[smurphny]

The key is to run the engine at its optimum torque efficiency as close to your calculated hull speed as possible. When the engine is hitting the top of the torque curve, the boat should be at hull speed. This results in best fuel consumption. The rpms are less important than torque. An oversized wheel will never be able to produce max efficiency at hull speed because it does not reach the peak of its torque curve. An undersized wheel will never produce max. torque at hull speed because it does not move enough water at the peak of power available from the engine. I don't think there are any magic formulas to getting it just right. If I can get it close initially, erring on the large side, then it is pretty easy to fine tune later by repitch or cutting the diameter. Figuring best pitch and diameter, two, three, or four bladed, how much prop wash you want, etc. is a whole 'nother topic best left to prop manufacturers and people who really know your particular boat.

[barnakiel]

Originally Posted by markpierce

Your MPG figures go to show that, absent wind, waves, and current, the most fuel-efficient speed for a displacement hull is as slow as the engine can go, which definitely ain't cruising speed (...).

It is sort like we have the best fuel economy point (as per your statement and confirmed by good sources), then we have the necessity to load the engine properly (as per manufacturers recommendation - usually about 75% of max rpm) and next comes the owners idea of how fast they want to travel (subjective).

I think this explains why modern boats of our displacement will often have engines up to 3 (sic!) stronger than our little clunker. Too some extent one can use the power (e.g. a dupy alternator, an engine driven fridge, etc..) but otherwise it is due to the fact that most people do not understand a sailing boat is not a car.

b.

[SPARK]

It seems to me that the rpm is almost irrelevant to engine loading. It will depend on the specific propeller fitted. The ideal economy will be at peak toque. But most yachts are not propped for this. The simple solution is to fit a controlled pitch propeller. This way you can adjust to 75-80% of max engine load at any rpm. Just remember that you would have to do this with a minimum rpm that would have to be a percentage above idle speed to get efficiency and to get adequate splash lubrication.

hope this makes sense.