Sailing yacht - what RPM to run engine at?

[hamburking]

Originally Posted by Ecos

I found that the windvane would steer the boat with full sail at 900 rpm. Any more than that and the Aries goes goofy. It's a Perkins 107 and the temp gets to 165-75 and fuel burn is pretty low. I can average about .3 gph. It's pretty quiet too.

I also like motor sailing at low rpm. The engine is under load, so the temp stays in the operating range. You can keep your speed up, and use hardly any fuel.

There is a difference between low rpm under load (which is okay) and low rpm because you are idling (which is not okay for extended periods).

[tarian]

Originally Posted by a64pilot

So how do these mechanics explain smaller high RPM motors that run for very long lives, do so in 1500 RPM generators?
Lot of Yanmar, Kubota, Mitsubishi etc powered generators

Each motor will have their own RPM setting to reach the 80% load as with newer diesels and small generators their loads may vary
I would be surprised though if these generators go beyond 10000 hours without major overhauls, my lovely perkins is 6000 hours and still purrrs like a pussycat , starts first time and just keeps on giving , and I always trust those that have more knowledge than me and have nothing to gain by passing on knowledge
Hey Im a trusting Guy

[tarian]

Originally Posted by hamburking

I also like motor sailing at low rpm. The engine is under load, so the temp stays in the operating range. You can keep your speed up, and use hardly any fuel.

There is a difference between low rpm under load (which is okay) and low rpm because you are idling (which is not okay for extended periods).

This is the Key to diesel engines they must have a load on them sitting charging your battery banks with no load is departmental to diesel engines.letting your diesel idle for anything more than a short duration is a bad idea because while the engine will use little fuel, what fuel it does burn will not combust completely because the operating temperature is too low. Unlike gasoline engines, diesels need to be under load to reach optimum operating temperature—if they’re not, unburned fuel can cause needless pollution and even can dilute the lubricating oil, increasing wear. Plus, every time a piston travels up and down a cylinder, the rings and cylinder walls wear just a little bit. The best rule is, if you’re not underway, turn off the engines.

2. You should let your engine idle for a few minutes to warm up before getting underway.
from another source .


T

[Aqualibrium]

Big Diesel engines will last forever if properly looked after, but that’s big ship stuff where everything is replaceable and engine revs can be 75 to 120 RPM
Low revs and low load will cause bores to glaze through inadequate combustion pressure, which causes increased gas blow past of rings and resultant degradation of lube oil, carbon loading hence black colour and acidification!
Changing filters and oil at an increased rate to recommended and running up to higher revs will prolong engine life significantly.
With small turbocharged engines your lube oil cools your turbocharger so make sure your heat exchangers are clean! Learn how to do a vinegar clean of your HE it’s quick easy and very effective.

[Compass790]

I reckon it's a bit of a guess but I wouldnt be afraid to run it at 2000rpm if you give it the occaisional burst higher. It depends on how the engine seems to run. If you have a feel for engines you will be able to tell if it's happy enough. I'm not familiar with that engine but lotsa yanmar manuals say you need to give them a hard run from time to time.

[a64pilot]

Originally Posted by hamburking

Its not strictly about RPM. Its about temperature. You want the engine to be running at operating temp, not cold. So if its under load, you should have no trouble. I like to cruise at 1500-1800 rpm, which is comfortable and quiet.



You get into trouble with glazing on the cylinders if you are just idling, not in gear, like to charge the batteries or run a watermaker.



And if in doubt, rev up ONCE PER DAY (not once per hour) for about 5 minutes.

I will motor sail as low as 1000 RPM, however I have an Autoprop and it’s a little different, it’s self pitching, so it will increase pitch when motor sailing and keep the engine under a load, it decreases pitch too though when I’m bashing against strong winds and waves.

[a64pilot]

Originally Posted by tarian

Each motor will have their own RPM setting to reach the 80% load as with newer diesels and small generators their loads may vary

I would be surprised though if these generators go beyond 10000 hours without major overhauls, my lovely perkins is 6000 hours and still purrrs like a pussycat , starts first time and just keeps on giving , and I always trust those that have more knowledge than me and have nothing to gain by passing on knowledge

Hey Im a trusting Guy

I’d be surprised to see any of these light duty Diesels go 10,000 hours, and if you get one that does, whatever you were doing is apparently the right thing to do.

[Rorzech]

You get better milage at 2,200 rpm than at 1,600 rpm
That chart makes no sense when you try to extrapolate the numbers.

[Q Xopa]

Originally Posted by Rorzech

You get better milage at 2,200 rpm than at 1,600 rpm
That chart makes no sense when you try to extrapolate the numbers.

Extrapolating is not really possible because this fuel usage/ torque and HP curve are, well a curve.
You may be correct for your own situation but it will depend.
My 4JH4-TE graph shows 1600 RPM burns around 2 Ltrs/hr. 2200 RPM is listed as 5ltrs/hr. That’s 60% less at 1600 RPM so, most boats wont be doing 60% less speed/ distance at 1600 RPM compared to 2200.
Using my own engine as an example, a Yanmar 4JH4-TE, 75HP.
As near as I can discern from the Performance graphs the-
Max Torque is at 2000 rpm. 1800 rpm is almost the same but either side Torque drops reasonably steeply.
Fuel consumption at 2000 rpm is list at 4 ltrs/hr. At 1800 rpm it is listed as 3ltrs/hr.
I would look at 1800 rpm as the best use of your diesel dollar$. At 2000 RPM you don’t get 25% extra Torque. The OPs figures tend to confirm this going 6.4 nm using 3 ltrs in one hour. Ie 2.13 nm/ ltr.
This would definitely give the most amount of alternator energy per liter of diesel burnt. Assuming of course that your alternator runs efficiently at that speed.
I have read studies done by one of the US Agricultural institute basically saying that the difference from a diesel idle speed to it’s peak Torque RPM is in the order of double. Ie at idle we use about double the amount of diesel per unit of work done to that at our peak Torque RPM. Perhaps some food for thought for those of us trying to maximize our diesel dollar$, and/or reduce the amount of times we have to visit the diesel dock.
It’s a little more complicated with motoring as it would depend on how efficiently the hull was at that particular speed. The OP was saying her boat is travelling at 6.4kts. A 50 foot boat is not too close to its hull speed. However if this was a smaller boat with a significantly slower hull speed it would not work the same. As we know getting close to our hull speeds the required propulsive power gets a lot more very quickly.
So I would suggest this is the engine sweet spot and you will get the Max work done for comparatively the least amount of diesel burned. This is definitely where you would want to be running if it were used in a generator operation. Even better if you could keep a reasonable load on it at that RPM.
The trouble with the average AC gen is that, even though it is kept spinning at 1500 or 3600 with or without a load. Some diesel AC gens can spend a substantial time spinning quite fast but little to no load. This is not good for an engine generally and especially your diesel consumption. DC Gens, or Inverter Gens are definitely ahead here. But that’s for another thread.
So on the topic of engine loads. As others have mentioned the combustion gasses sneak in behind the inside of the piston rings to push them out against the cylinder bore. This helps them seal better and reduces glazing.
Of course like everything there is a practical limit to all of this. Too much load, for too long will put more loads on our big end and main bearings etc.
Too much RPMs for too long will also increase the wear rate of and engine as there is obviously more piston speed against the bores etc.
Too cold and the design clearances are too big and more room for pistons and whatever to rattle and wear faster. Too hot and metals are getting softer and bigger, oils break down etc.
Manufacturers recommendations!
Re recommended operating speed according to Yanmar-
JH4 Series Operation Manual

P47.
CAUTIONS DURING OPERATION

7. When operating the engine at low
speed for long periods of time, race the
engine once every 2 hours.
Note: Racing the engine: With the
gear in NEUTRAL, accelerate from
the low speed position to the high
speed position and repeat this
process about 5 times. This is
done to clean out carbon from the
cylinders and the fuel injection
valve. Neglecting to race the
engine will result in poor exhaust
color and reduce engine
performance.
I cant post it but if you need electronic copies of these manuals, feel free to PM me.

[Hopefull]

I have an Oyster 53, with a Yanmar same as you 100bhp, currently 2500 hours, 2005.
Just motor sailed from Palma to Portsmouth..
Run at 2500rpm, burnt 1lt per nm, speed average 6.7

[Nicholson58]

We have the original 1984 Westerbeke 115 HP 6 cylinder. It was built by British Layland as a tractor engine. It is massively heavy. We run it 950to 1100 rpm. There is a Twin Disc 2:1 reduction transmission and a Hundested infinitely variable pitch prop 24inch three blade. We get about 4mpg at 6.5 knots in calm sea state. Motor sailing gets us up to 8mpg and 7-8 knots.

This rpm question comes up often. There is a lot of history here on the CF. The answer depends on a lot of issues relating to sea state, hull speed, motor maker, displacement, prop. I think it is probably common that the prop may be improperly paired with the system in many cases. We find that only a tiny variation in our pitch makes a lot of difference in performance. This is especially true going from motoring to motor sailing and sea state from flat to big.

[a64pilot]

I have over the years graphed speed vs fuel consumption on three boats, all with very accurate fuel flow meters.
What I found was that in all three cases the higher the RPM, the more fuel consumed per distance travelled, this was true at ALL RPM’s on all three boats. Idle was best fuel range.
I’m not abdicating idling for many reasons, but if for some reason you got stuck somewhere and absolute maximum fuel range was needed, remember the slower you go, the better your fuel mileage will be, unfortunately there are no “sweet spots” where you can actually speed up a little and get better fuel mileage.

Yes there are RPM ranges that an engine produces more work for less fuel, more efficient, but this is more than overcome by the higher drag of the hull through the water at higher speeds.

Without accurate fuel flow, you can’t really graph fuel consumption over speed, it would take forever picking one RPM, motoring for hours and refilling.

[a64pilot]

As far as Yanmars recommendation for racing the engine so frequently, I can’t explain it, unless they are talking about very low loading, like being used to charge batteries. Think about it, do you do this for Diesel engine automobiles if driving in town, ever two hours put it in neutral and race the engine? Do generators that run for thousands and thousands of hours at 1500 RPM do this?

I can only theorize they are speaking about engines not put under a load and not allowed to properly come up to full operating temp.

[FlightPlan]

I have similar concerns as the OP, regarding running up the engine to max RPM. I had a perkins 6.3 and never ran it at max longer than 2 to 3 minutes. I just have fear of throwing rods thru the block or floating the valves and stuff like that. I had to replace the Perkins after a service tech bled the injectors with the starter longer than necessary and flooded the aft cylinders.


So I searched for my replacement engine and looked at many and found that naturally aspirated engines are no longer sold in the US because of EPA standards. That put me into Tier II and beyond common rail, higher RPM engines and I think they all suck. Proprietary software, turbo's, after cooling the exhaust, etc. Anyway after reading all the manuals for the engines that would fit, they all state that the engine should be run at max RPM at some point in a 24 hour period. I installed a Cummins 6BTA5.9 210 reman engine and it state in my manual to run at max for 1 hour in 24 hours. Talking to the certified mechanics, they said it doesn't have to be done in the same hour, but should be done maybe at 10 minute intervals. I do it every 3 hours and at the end of a watch change works well. I still won't dare do it in neutral, but I'm old school. My engine maxes out at 2600 RPM and according to the curve, that burns 12.1 GPH and I don't like that. There is a formula on boatdiesel dot com that you can input your particulars like: displacement, LWL, transmission, prop configuration and it gives you a pretty nice chart and selects the optimum HP for engine selection. You can actually select the exact engine, transmission, and number of prop blades, and tell you the pitch required.


I'll be doing what the manufacturer recommends, but under load. Also, because this is my first experience with a Turbo and after cooling, I will leave it to the experts.

[A la carte]

When getting my floatplane rating, I was taught that, in water, a 1% increase in speed resulted in a 4% increase in drag. (You scientists, please correct me if this is not true in sailing). Thus, it would make perfect sense that increased speed would result in decreased fuel mileage. (regardless of RPM?)