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Old 02-03-2011, 05:00   #1
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'Cruising RPM' - What Does this Mean ?

Just what is this "crusing rpm" I'm always reading about so I can know when I'm at 80% of it?

My engine manual says 3600 rpm max and 3000 rpm continous. I know from the past (because I ran out of fuel) that if I run around 2400-2600 rpm and do about 6 knots I use about 1.25 gals/hr. But if I run about 2000 rpm at 5 knots I burn about .8 gal/hr. It's also easy to tell by ear alone that the engine is more comfortable at the 2000 rpm.

If I look in my Yanmar manual it suggests from the curves that the best efficiency (for the engine, which isn't the same as for the boat as a whole) would be around 2600-2800 rpm.

So what would be considered the "cruising" rpm? And if this 80% thing is so much better why don't we refer to that rpm as the "cruising" rpm?
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Old 02-03-2011, 05:07   #2
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Re: "cruising rpms" - what's this mean

Don,

"crusing rpm" is the RPM that use's the least amount of fuel. On my Catalina 380 with a 40 HP Yanmar the "crusing rpm" was 2,300. Its boat specific and has a lot to do with your hull speed.

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Old 02-03-2011, 05:13   #3
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Re: "cruising rpms" - what's this mean

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Originally Posted by Cotemar View Post
Don,
the RPM that use's the least amount of fuel.
Mark

"least amount of fuel" . . . per hour or per mile or per . . . . ?
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Old 02-03-2011, 05:18   #4
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Re: "cruising rpms" - what's this mean

"least amount of fuel" in gals/hr
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Old 02-03-2011, 05:25   #5
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Re: "cruising rpms" - what's this mean

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Originally Posted by Cotemar View Post
"least amount of fuel" in gals/hr

Well, not quite. The least of amount of fuel in gal/hr would probably be at idle, which is certainly not the cruise RPM.

In the end, fuel efficiency will take into account the distance traveled on a gallon.
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Old 02-03-2011, 05:33   #6
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Re: "cruising rpms" - what's this mean

So if the "cruising rpm" is the most effecient for the boat (not the engine), why would people be talking about running at 80% of this?
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Old 02-03-2011, 05:34   #7
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Re: "cruising rpms" - what's this mean

This thread got me thinking about engines, hours, wear and so forth.

First, for all practical purposes in my experience I don't motor for more than a single period of daylight unless I am becalmed off shore and with a long passage ahead. So in the vast amount of my "long term" motoring it is involved with coastal cruising and keeping a schedule. Under these circumstances the consideration is getting to the anchorage on time (good light, good spot?) and fuel consumption doesn't come into my equation. The wasted fuel differential doesn't amount to much. However engine wear and noise are a consideration with my aging MD17D (26 yrs).

Once I throttle past about 2100 or so I don't go faster, just squat the stern and the boat it then trying to climb up a larger bow wave. Not happening. On the other hand at 1800 she's slowed a knot or more below hull speed and so that extends the trip time 15% or so.

The other point we don't see discussed with respect to engine wear is the nature of the engine wear. A diesel run at no load and idle will show more wear than one run under load at a few thousand RPM. So few entire rotations can equate to MORE engine wear. I would guess that the same would apply running the engine constantly at the "red line". Each of those three engines running the same time would show different wear with the idle and the redline engines showing more wear.

I'd be curious to see some sort of plot of engine speed, load no load vs engine wear. Are there any out there?

And why don't we have more sophisticated metering showing total number engine revs (as opposed to hours)? I noticed that my hr counter was wired to the key switch and so if the key was in the on position without the engine turning over it was counting hours! I fixed that one.


[The question about engine wear at different RPM's has been reposted in a separate thread here: Engine wear at different RPM's]
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Old 02-03-2011, 05:36   #8
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Re: "cruising rpms" - what's this mean

The best place to find the "crusing rpm" for your Hunter 410 is to go to your Hunter 410 forum and ask. Make sure that the responders have the same prop as you.
This data usually has already been done on a spreadsheet by a few owners.
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Old 02-03-2011, 05:41   #9
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Re: "cruising rpms" - what's this mean

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Originally Posted by Don Lucas View Post
So if the "cruising rpm" is the most effecient for the boat (not the engine), why would people be talking about running at 80% of this?
With a properly sized prop your engine should be able to attain max rated RPM per engine manufacturers specs. As I was told by a Westerbeke engineer when I had a an incorrectly sized prop, size the prop correctly then take her to hull speed and then back off 100-200 RPM.

Yanmar's guidance might be different from Universal/Westerbeke. Westerbeke will not honor warranty if your prop is not within spec and the engine can't hit max rated RPM.

If the prop is sized correctly all else should fall into place well. The hull speed minus 100-200 RPM is nearly spot on the 80% suggestion for cruise RPM for our current boat/engine combo and has been on multiple other boats as well.
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Old 02-03-2011, 05:58   #10
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Re: "cruising rpms" - what's this mean

There is "Max RPM" = momentary RPM... (3,600 on my Yanmar) Ideally, your prop should be of a size that allows the engine to just reach this.

There is "cruising RPM" which is the maximum RPM that the engine can be run at (long term), without damaging it. (3,400 on my Yanmar) NO ONE really does this!

The important RPM # would be neither of the above. It would be called the "sweet spot". This is a point where the engine is running fast enough to not be damaged, slow enough to not be damaged, pushes the boat well, and gets the best MPG.
(2,950 on my Yanmar)

Yanmar engines, like other modern, light weight, high RPM engines, DO NOT LIKE TO BE RUN LONG TERM AT LOW RPMS!!! (A minimum speed for my engine would be around 2,600 RPMs) Below this, and they tend to carbon up.

If one needs to run more slowly than this for some reason, "Yanmar America" suggest that every hour, you rev it up to over 3,000 for 5 minutes, to minimize the above problem. It takes getting used to the idea, but running too slow is more likely to wear them out than right at the "sweet spot".

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Old 02-03-2011, 06:00   #11
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Re: "cruising rpms" - what's this mean

I've been thinking a lot about this lately.

I would respectfully disagree with Cotemar that for most people the best cruising RPM is the one at which you burn the least fuel. Since the energy required to drive a sailboat through the water goes up geometrically with speed, the least-fuel RPM will be, for most boats, something not far above idle. In my opinion a better definition of cruising RPM might be that speed at which you achieve the optimum balance between speed and fuel consumption in most cases according to your own particular values.

My Yanmar 4JH3HTE is redlined at 3800 with continuous power at 3700. It is sometimes said that cruising RPM's should be 75% (or 80%, depending on who you talk to) of redline. That would be 2850 RPM. My first year with this boat, I usually motored at 2500 to 2600 RPM

Unless I was in a hurry to make a tidal gate or something like that (in which case 3,000 RPM or even 3,200 RPM). Or another case for a higher cruising speed is if you are bucking an adverse tide -- the slower you go through the water, the more miles you lose to the current, so a false economy appears at some point of slowness.

So at 2500 RPM I get about 8 knots in flattish water. According to Yanmar, I should be consuming about 7 liters/hour at that speed.

At 3000 RPM, I get about 9 knots. But the fuel consumption is now 12 l/h, almost double. So obviously, the fuel consuption measured in liters per mile goes up in a very steep curve -- only 12% more speed for almost double the consumption.

So what about if, on the contrary, I slow her down? That's what I've been doing this year. At 2200 RPM I still get about 7 knots, or maybe even a little more. Fuel consumption is about 5.5l/hr, about 20% less fuel burned per hour for about 12% less speed.

At 2200 RPM I'm already getting into diminishing returns -- I'm burning a little less fuel per sea mile but nothing like the huge economy from throttling back from 3000 to 2500.

Another reason to throttle back is if you have a fair tide. This is the opposite case of bucking a foul tide. When you have a favorable current, the slower you go through the water, the proportionately more work the tide does, instead of your engine. So this exaggerates the fuel economy gain from lower RPMs.

Of course the biggest fuel economy gain comes from putting up the sails, let's not forget


Concerning the curves in your Yanmar manual: Be sure to correctly understand what is "specific fuel consumption". Specific fuel consumption is the efficiency of the engine at converting grams of fuel into horsepower/hours of work. This is not all that significant to us, especially if you notice that the scale is very shallow -- the SFC curve for my engine goes from 152 to 175 g/hp*h -- not a very big range.

On the contrary, the number of horsepower needed to drive our boats varies over a huge range depending on speed -- from 20 horsepower at 2300 RPM to 95 horsepower at 3800 RPM according to Yanmar (I'm assuming that the engine puts out its maximum power for a given RPM at that RPM, but that's probably a reasonable assumption considering my Bruton self-pitching propellor). So it's optimizing the quantity of horsepower used which is really important, not optimizing the efficiency with which each horsepower is produced.

You should therefore get fuel curves which show liters/hour -- absolute fuel consumption, not specific fuel consumption. Again, these assume maximum power at each particular RPM. For a fixed pitch propellor, you may be using somewhat less than maximum power and so will consume somewhat less fuel at lower RPMs, than the fuel curves suggest.
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Old 02-03-2011, 06:08   #12
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Re: "cruising rpms" - what's this mean

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Originally Posted by defjef View Post
This thread got me thinking about engines, hours, wear and so forth.

The other point we don't see discussed with respect to engine wear is the nature of the engine wear. A diesel run at no load and idle will show more wear than one run under load at a few thousand RPM. So few entire rotations can equate to MORE engine wear. I would guess that the same would apply running the engine constantly at the "red line". Each of those three engines running the same time would show different wear with the idle and the redline engines showing more wear.

I'd be curious to see some sort of plot of engine speed, load no load vs engine wear. Are there any out there?

And why don't we have more sophisticated metering showing total number engine revs (as opposed to hours)? I noticed that my hr counter was wired to the key switch and so if the key was in the on position without the engine turning over it was counting hours! I fixed that one.
Strictly speaking, this fascinating question is off topic. Shall we create a separate thread?

Thread created here: Engine wear at different RPM's
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Old 02-03-2011, 06:34   #13
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Re: "cruising rpms" - what's this mean

The old saying to kill a diesel with kindness is true. Your diesels are quite capable of much higher RPMs than 3400-3800. Trying to baby them at low RPMs and finding by ear what sounds good many times leads to issues with glazing and improper engine temps. The natural tendency is to believe that an engine that is run slower and under less load will last longer. Not so with diesels. Getting the best fuel economy needs to be weighed against the cost of engine damage and repair.
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Old 02-03-2011, 06:36   #14
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Re: "cruising rpms" - what's this mean

I'm not a diesel mechanic, but I think the 80% rule of thumb (75%, etc) is not about fuel efficiency. It's to maintain a diesel's compression, avoid carbon buildup, achieve the optimum engine operating temperature, etc.

Here's my thinking: my engine's operating costs are a combination of fuel/oil, maintenance, and the amortized cost of a new engine or rebuild. If I plan for a "new" engine after 4000 hrs at a cost of $10,000 (an arbitrary figure that lies somewhere between the cost of rebuild and replacement), that's $2.50 per hour - less than my fuel bill, but not by much. If I also consider the benefits to engine health of running my diesel at 75-80% RPM, I realize that it's penny-wise / pound-foolish to consider only fuel efficiency. I cruise at 75% RPM, just below hull speed, at a reasonable 3/4 gph.

The above reasoning is common in operating aircraft, where higher fuel costs argue for a slower cruise speed (but no lower than a "best range" setting) and higher maintenance costs (per hour) argue for a faster cruise speed (up to the equivalent of an aircraft's "hull speed," to put it simply).
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Old 02-03-2011, 06:49   #15
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Re: "cruising rpms" - what's this mean

Although I am a true advocate for running a diesel hard and cruising at 80% with a correctly matched propeller I would also add that 'Cruise Speed' is the speed that the engine/transmission/boat (vibrations and harmonics etc) allow for a smooth, quite ride, outside of allowing for sea and weather conditions. All boats will be a little different, but there will be a 'sweet-spot' that the engine sounds and feels best at. I would also suggest every hour or so, or whatever period you choose, to max out the revs for a minute or so and then back to the 'sweet-spot'
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