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Old 19-04-2016, 16:45   #1
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At what rpm to you cruise?

I was just reading my engine manual (Nanni3.30) which states "cruising" rpm should be 2880-3240. We have been cruising for years at 2000-2400, doing 4-4.8 knots on one engine. Anything higher and the noise becomes intrusive and the fuel burn goes up from less than 2 liters/hr to over 3 liters/hr. Just curious what other folks are doing.
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Old 19-04-2016, 16:56   #2
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Re: At what rpm to you cruise?

I typically run between 2400 and 2600 rpm, low noise and my w27 sips fuel.
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Old 19-04-2016, 17:03   #3
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Re: At what rpm to you cruise?

4JHE Yanmar, usually 2000 to 2200, for the reasons you give. I get 6.5 kts at 2000 and 7kts at 2200 with a clean prop and bottom.
RPM above that is obnoxious from a noise standpoint, and I'm rarely in that big a hurry anyway.


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Old 19-04-2016, 17:03   #4
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Re: At what rpm to you cruise?

Yep, I usually run 2200-2500 depending on engine sweet spot. Cant deal with that 3600 rpm airplane noise. And incidentally, both Yanmars I ran at high rpm as they suggested failed prematurely. Coincidence? Cracked pistons.
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Old 19-04-2016, 17:06   #5
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Re: At what rpm to you cruise?

Everything I've read, by those whose opinions I value, indicates that our small diesels like to be run hard. I run the Universal M25XP in my Catalina 28 at about 80% of maximum rpm (2500 cruise / 3100 max).

On a typical daysail, I have to spend a fair amount of time at lower speeds, going through the marina and out the channel to Lake Michigan, but as soon as possible, I raise the revs.

When we cruise, it's tempting to reduce rpm to alleviate the noise, but we don't.
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Old 19-04-2016, 17:13   #6
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Re: At what rpm to you cruise?

My tach has been toast since I bought the boat, but my engine sounds sweet at about 6-6.3 knots. She can run around 7.3 knots without the temperature climbing. Perkins 4.108, 24000 lb sailboat with a 30' waterline.

My philosophy is if it sounds good, it must be good.

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Old 19-04-2016, 17:33   #7
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Re: At what rpm to you cruise?

Two years ago I made an age-related switch from sail to power. My 34' trawler has a big Cat diesel... single engine. At 1200 rpm I'm able to average close to 8 knots with fuel consumption at 2 gph. Moving up to 1400 rpm, fuel consumption doubles with only another knot or so of speed. Being frugal and in not too much of a hurry, I'm happy to remain at 1200 rpm.
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Old 19-04-2016, 17:34   #8
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Re: At what rpm to you cruise?

If I'm covering a lot of ground and conserving fuel to do it I run at 1400-1600 rpm. Day sailing I typically run 2000-2200 with occasional bursts to 2800-3000 just to heat things up.

The engine is a Kubota d1105


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Old 19-04-2016, 17:36   #9
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Re: At what rpm to you cruise?

It really depends more on the pitch of your prop and load on the engine.
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Old 19-04-2016, 17:44   #10
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Re: At what rpm to you cruise?

Normally about zero. That's what the white flappy things are for.
If we motor, 900 rpm, 5 knots, 66,000 lbs 5 Litres an hour.
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Old 19-04-2016, 17:47   #11
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Re: At what rpm to you cruise?

Yeah, some boats are overpowered and some boats are underpowered. I assume small engine makers sell like a ~25 HP 3GM30 thinking you need the HP. So they recommend you run up at RPM where that HP is produced.
Bigger diesels are sold by usage. The same engine can be rated for continuous use, recreational use or in between. The interesting thing is for higher usage they rate the engine at a lower RPM. So an engine for 24/7 use might be rated at 2400 max rpm. But for Recreational intermittent use rated at 3600 RPM.
I used to go to commercial boat expositions in my work. The same engine would have a placard on it with different rpm/usage details on that placard.

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Old 20-04-2016, 06:25   #12
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Re: At what rpm to you cruise?

Quote:
Originally Posted by siamese View Post
Everything I've read, by those whose opinions I value, indicates that our small diesels like to be run hard. I run the Universal M25XP in my Catalina 28 at about 80% of maximum rpm (2500 cruise / 3100 max) ...
Actually, diesels like to be run at design operating temperature.
Whatever RPM gets your engine up to temp. is ok, and will depend upon several variables (hull form, cleanliness, prop, etc).
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Old 20-04-2016, 08:07   #13
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Re: At what rpm to you cruise?

You guys are both right, but we usually don't have the instrumentation to determine operating temps.
However you can get close with an IR thermometer, it's oil temp more than anything that is a good indicator, I'd like to see oil temp above 180F, but my Yanmar just won't ever get there, both fresh water side and oil temp rise to about 165F and that's as hot as they will get, even if I run it hard.
On my engine, on my boat, 2000 RPM will get my oil and water temp to 165F, but I have an Autoprop which does load an engine more at lower RPM than a fixed prop set to allow the enigne to reach full operating RPM does.

Same engine having different ratings is very common, the basic IO-540 aircraft engine is rated in different application from 235 HP to 300 HP.
Guess which one has the reputation for lasting almost forever?


Engine wear rate is proportional to engine RPM, but it's not linear, I believe as you double the RPM, you square the wear rate.

Low RPM and or low load builds carbon and possibly varnish, that is why you want to avoid long, low loads on a Diesel, and being fixed props there isn't much you could do about it, but if you could increase load then you don't need to avoid the lower RPM range
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Old 20-04-2016, 08:34   #14
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Re: At what rpm to you cruise?

I like to run my Volvo 2002 (18hp) about 1800 rpm...quiet, not too much vibration. Often I'll use 2000rpm for a full 5knots of speed. Rarely rev higher, as the fuel consumption and noise both go up excessively.

If I'm motorsailing, I rev as low as possible.

So long at operating temp is maintained, you can run at any rpm...just don't run cold.
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Old 20-04-2016, 08:45   #15
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Re: At what rpm to you cruise?

Kind of a silly question since every boat will be different RPMs depending on engine, displacement, prop pitch, sea conditions and other factors.

Also there are very few sailboats that can run an engine at any speed and not have annoying sounds or vibrations!

I usually run my sailboat at 80% of rpms, which produces a good speed to fuel consumption ratio. That equates to 3/4 gallon per hour at 7.5 knots in smooth water with a clean bottom.
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