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Old 29-04-2016, 12:38   #46
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Re: At what rpm to you cruise?

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Silly question but with twin engines I assume your adjusting the props so you can reach full rated RPM with both engines?

I've only had planing hull power boats with twin engines, and when properly propped for both engines, neither could plane on one engine, meaning of course on one engine I was severely overpropped. A non planing hull is of course different, but I was thinking if your propped for full RPM with both, are you a little overpropped on one, you may already be where you want to be?
Strangely, it does not seem to matter if I am running one or two engines, max rpm remains the same, although I gain roughly 1 knot by running both.

I usually run on one engine with the (perhaps mistaken) belief that I am burning less fuel to achieve the same speed. Also, I feel (again perhaps mistaken) that it is better to run one engine hard than to run both and divide the load between them. I think most cat owners tend to run on one engine but I have met some that run on both and I am sure that in boat reviews they are running both to achieve "cruising speed".
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Old 29-04-2016, 13:09   #47
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Re: At what rpm to you cruise?

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Originally Posted by mikereed100 View Post
Strangely, it does not seem to matter if I am running one or two engines, max rpm remains the same, although I gain roughly 1 knot by running both.

I usually run on one engine with the (perhaps mistaken) belief that I am burning less fuel to achieve the same speed. Also, I feel (again perhaps mistaken) that it is better to run one engine hard than to run both and divide the load between them. I think most cat owners tend to run on one engine but I have met some that run on both and I am sure that in boat reviews they are running both to achieve "cruising speed".
In my cat I used to motor sail with the lee engine at low rpm. It made the boat point maybe 15 degrees higher and was really fast just loping along with almost undiscernable engine noise.!
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Old 29-04-2016, 13:59   #48
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Re: At what rpm to you cruise?

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Strangely, it does not seem to matter if I am running one or two engines, max rpm remains the same, although I gain roughly 1 knot by running both.



I usually run on one engine with the (perhaps mistaken) belief that I am burning less fuel to achieve the same speed. Also, I feel (again perhaps mistaken) that it is better to run one engine hard than to run both and divide the load between them. I think most cat owners tend to run on one engine but I have met some that run on both and I am sure that in boat reviews they are running both to achieve "cruising speed".

Max RPM remains the same due to slippage, I know it exists of course, but don't know how much there is, I assume it's another of those things that vary a little from one boat to another, prop efficiency etc.
You will burn a little less fuel making say 30 HP out of one as opposed to 15 out of two, although of course there are other variables like prop drag, slippage, transmission losses etc. so it's not purely as straight forward as that.

In my opinion, perhaps the biggest gain is the number of hours your putting on your engines is half of course if you run one at a time as opposed to both together.

But I only have one, so what do I know


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Old 29-04-2016, 14:10   #49
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Re: At what rpm to you cruise?

I takes very little HP to move a boat at hull speed... in flat water anyway. Likely the reason many old boats have smaller diesels. They were more "auxiliary" engines then. Now, more people motor if the winds not right. I tested a 29 ft power boat during seatrials. In about a 1 ft chop the 8HP kicker moved it at hull speed fine into the wind. No doubt a low wetted surface cat will move fine with one engine, although controlling it in tight quarters can be tough for sure in some wind. Been there done that!
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Old 29-04-2016, 14:43   #50
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Re: At what rpm to you cruise?

VP-2003 - cruising at 2000rpm - 6.6kts cruising , Jeanneau Sun Liberty 34, pop: 17x13
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Old 29-04-2016, 15:45   #51
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Re: At what rpm to you cruise?

Perkins 4-108 25k lbs boat
Run at 2600 rpm 180 deg temp 6.8knts
Wide open is 2900 rpm 200 deg 7.5 knts
About to install larger HE
I hope to be able to run 27/2800 at 180deg
These are higher RPM than most but boat is under powered at lower rpm.
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Old 30-04-2016, 01:36   #52
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Re: At what rpm to you cruise?

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In the old days, 3/4 throttle was considered optimum for a small diesel. Guess times have changed. We were taught to keep the load constant and to avoid going towards max rpms. Wonder what Perkins teaches now?

It's not. Yanmar recommends 80% which is essentially 75%. What's different is the max rpms, so 80% for my engine is about 2900 rpm.
Older engines typically had lower max rpms.
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Old 30-04-2016, 06:25   #53
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Re: At what rpm to you cruise?

Right- yanmar recommends 80% because if the boat isn't overpropped then you have to get it up to those rpms to run it under load.

But if you overprop and essentially "detune" the engine, with a new redline, then running it slower under load should make it quieter, more efficient, and longer lasting....


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Old 30-04-2016, 07:17   #54
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Re: At what rpm to you cruise?

Yanmar 3GM30 sweet spot seems to be 2600-2750 and uses about .5 gph (6 knots)
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Old 30-04-2016, 15:15   #55
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Re: At what rpm to you cruise?

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Right- yanmar recommends 80% because if the boat isn't overpropped then you have to get it up to those rpms to run it under load.

But if you overprop and essentially "detune" the engine, with a new redline, then running it slower under load should make it quieter, more efficient, and longer lasting....
Thanks Malbert. It was my gut feeling that my boat was loading the motor at low revs to the same degree a truck might when cruising on a highway - using maybe 30 to 50% of HP capable. Trucks that do a million mile without problems.
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Old 30-04-2016, 15:27   #56
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Re: At what rpm to you cruise?

Trucks do a million miles with a couple of engine rebuilds... If they are operated properly.

Overloading but reducing rpm isn't good.

The trucks can select from 12, 13, 16, 18... (more) gears to adjust engine load so the engine is running in its most efficient rpm band without overloading.

Your boat might in some cases have 2 or 3 speed transmission, but most are just single speed. You adjust the load by picking the right prop.

If the engine is "lugging" from load... you need to adjust the load.
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Old 30-04-2016, 15:37   #57
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Re: At what rpm to you cruise?

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Originally Posted by malbert73 View Post
Right- yanmar recommends 80% because if the boat isn't overpropped then you have to get it up to those rpms to run it under load.

But if you overprop and essentially "detune" the engine, with a new redline, then running it slower under load should make it quieter, more efficient, and longer lasting....


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I have been reading this thread with interest. I have a 54hp regular Yanmar (Non-turbo). Had a folding prop turning at 3200 max and then I switched to a 3 blade fixed. now max is 2800-2900. The folding prop moved the boat faster than the fixed prop at highest rpm's. But from what I'm reading here is that perhaps the big fixed prop is better for long longevity as the rpm's are lower?

Good info and definitely adding to my understanding. The only thing is the folding prop vibrates way less and is faster under sail so seems better overall.
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Old 30-04-2016, 18:13   #58
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Re: At what rpm to you cruise?

My little 2.8litre diesel truck has a recommended limits on the rev counter 1400rpm & 2800rpm - only 5 gears but a pretty big range. Normally operate in the middle half of this range but still a wide range of rpm that is acceptable.
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Old 01-05-2016, 03:59   #59
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Re: At what rpm to you cruise?

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I have been reading this thread with interest. I have a 54hp regular Yanmar (Non-turbo). Had a folding prop turning at 3200 max and then I switched to a 3 blade fixed. now max is 2800-2900. The folding prop moved the boat faster than the fixed prop at highest rpm's. But from what I'm reading here is that perhaps the big fixed prop is better for long longevity as the rpm's are lower?

Good info and definitely adding to my understanding. The only thing is the folding prop vibrates way less and is faster under sail so seems better overall.

There is a huge difference in fuel consumption based on the data




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Old 01-05-2016, 09:57   #60
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Re: At what rpm to you cruise?

For years, I ran 2650 with WOT RPM of 3,450 on my Hunter 356 with a 3GM30F with a MaxProp. During a cruise 2 years ago, heavily loaded in terms of clothes, food, extra fuel etc, I was burning more fuel than normal and smoking. later discovered that my MaxProp is worn, resulted in being way over propped. I could only get 3100 RPM at WOT and once realized what the problem was, reduced to 2500 RPM, smoking stopped and fuel consumption returned to more normal levels. Problem was that it reduced my cruising speed from 6.2 Knots to 5.5 Knots. I am pulling the boat this summer and will get my MaxProp rebuilt. In the future I will run at WOT occasionally just to see what is going on with my prop. The prop change was so gradual, I didn't notice the change. I have 1263 hours on my engine now.


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