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Old 20-04-2016, 08:45   #16
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Re: At what rpm to you cruise?

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Originally Posted by a64pilot View Post
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
Interesting. Having Kiwiprops I can adjust pitch but hesitate as one often hears that you don't want to overload a diesel, specifically, that one should be able to reach max rated rpms at WOT.
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Old 20-04-2016, 09:21   #17
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Re: At what rpm to you cruise?

Depends on how old the engine is. We had an old German engine that ran at 300 rpm day in and day out. The new "recreational" diesels work best at 1200-2800 rpm. You can retro many diesels to work with a two speed gearbox so you can operate at lower rpm and save a ton of fuel. Its not so much the rpm but the load that affects a diesel. They like to work under loads, not free wheeling.
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Old 20-04-2016, 09:41   #18
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Re: At what rpm to you cruise?

I run my Perkins 4.236 at 1800 rpm when cruising.
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Old 20-04-2016, 09:44   #19
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Re: At what rpm to you cruise?

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Originally Posted by mikereed100 View Post
Interesting. Having Kiwiprops I can adjust pitch but hesitate as one often hears that you don't want to overload a diesel, specifically, that one should be able to reach max rated rpms at WOT.

Forever the prevailing wisdom has been the engine should reach redline RPM at full throttle.
This ensure three things.
1. Maximum power will always be available if you need it.
2. under normal conditions, it just won't be possible to overload or "lug" your engine.
3. No training or understanding or instrumentation required to operate the engine, any one can do it.

For most of us, the ideal situation is moderately overpropped, meaning most of us want enough load on the engine so that at half RPM she will hit full operating temps., but that also means you can't operate at full throttle for long as the engine will be overloaded, what that means is excessive fuel to air.
See Diesels of course have no throttle valve, they pull in a full cylinder full of air at each revolution whether at idle or full bore (atmospheric engines).
What you do is add fuel, not air and fuel like a spark ignition engine, just fuel. Too much fuel and you quickly get to what is known as the smoke point, the issue with being there is exhaust temperatures are excessive, aluminum pistons will melt right at about 1400 F, but other damage is done as well before then from over temperature operation. Engine oil and or water may not overheat, but the temps in the combustion chamber are dangerously high

So if your moderately overpropped, you can motor along happily at half RPM or less forever and life is good, you just have to accept that your 40 HP engine is now say a 25 HP engine, and never let yourself get into a position where 40 is needed, cause it's not there anymore.
Overpropped is especially useful when motorsailing as if your not overpropped on engine alone, your underpropped under engine and sail.

So, you want your cake and eat it too? That is a controllable pitch prop, like a car in overdrive as your cruising along on a long flat road, need power? Downshift for the acceleration, then back to overdrive to cruise. Think of motorsailing as going down a long hill, a really high gear would be useful then.

I believe though for most of us a true controllable pitch prop doesn't exist, the closest I know of is the Autoprop that will increase pitch until an engine is loaded even at low RPM and motorsailing, but you don't control it and like any automatic thing, it works, but if you could control it and knew what you were doing and had the instrumentation, you could do better manually.
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Old 20-04-2016, 10:17   #20
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pirate Re: At what rpm to you cruise?

Sailing and if need more electricity run the Yanmar 4JH3HTE 100 hp at 1000 rpm. Rarely engine at 2000 rpm if no wind which is rare in the Carib.
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Old 20-04-2016, 10:29   #21
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Re: At what rpm to you cruise?

Find the specs sheet for YOUR engine and the most efficient rpm is pretty obvious.


https://www.yanmar.com/media/global/...0datasheet.pdf


This engine is most efficient 3,300 to 3,600 rpm. (not marine) but a good example of HP/ Torque curve.

Usually where Torque curve flattens, peaks and starts to drop off as soon as you get in the horsepower curve your fuel consumption spikes up.

http://www.globalspec.com/ImageRepos...f91f7021e0.gif


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Old 20-04-2016, 12:17   #22
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Re: At what rpm to you cruise?

Mine cruisers along at 500 rpm doing 7.4 knots very nicely thank you
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Old 20-04-2016, 12:50   #23
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Re: At what rpm to you cruise?

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Mine cruisers along at 500 rpm doing 7.4 knots very nicely thank you
Steam engine?
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Old 20-04-2016, 12:57   #24
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Re: At what rpm to you cruise?

Almost its a gardener 6lbw which is the size of a small car and a very nice piece of kit from the good old days when things were built to last
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Old 20-04-2016, 14:07   #25
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Re: At what rpm to you cruise?

It's important to heat up the oil to 160°F, and 180° would be better to get the water out. Running at a lower than peak torque and hp will extend engine life, especially in engines with turbos. 3600rpm generators don't last as long as 1800rpm generators don't last as long as 1200rpm generators for example.
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Old 20-04-2016, 14:46   #26
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Re: At what rpm to you cruise?

I have a Perkins 4108 and seems it likes 1400-1600 I get over 6.5 knots. I was struggling with temperature issues but new raw water pump fixed that. Interested to see if others running higher. Previous owner told me that was the sweet spot and I think that is pretty close to hull speed so sticking with what works!
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Old 20-04-2016, 15:50   #27
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Re: At what rpm to you cruise?

It depend a lot on what engines you have.

I have two chev 350 v8 petrol engines and cruise at 2000 rpm doing 13 knots with ease with both engines running.

The fuel consumption is not very good but all that power is a lot of fun as well as handy when needed.
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Old 20-04-2016, 16:03   #28
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Re: At what rpm to you cruise?

"Ungvar" is on the money! All motors have a sweet spot run it at that. Revving the crap out of it, just because "the book" says so is just rubbish . It is called "mechanical Sympathy " some people have it some dont!
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Old 20-04-2016, 16:29   #29
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At what rpm to you cruise?

Quote:
Originally Posted by a64pilot View Post
Forever the prevailing wisdom has been the engine should reach redline RPM at full throttle.

This ensure three things.

1. Maximum power will always be available if you need it.

2. under normal conditions, it just won't be possible to overload or "lug" your engine.

3. No training or understanding or instrumentation required to operate the engine, any one can do it.



For most of us, the ideal situation is moderately overpropped, meaning most of us want enough load on the engine so that at half RPM she will hit full operating temps., but that also means you can't operate at full throttle for long as the engine will be overloaded, what that means is excessive fuel to air.

See Diesels of course have no throttle valve, they pull in a full cylinder full of air at each revolution whether at idle or full bore (atmospheric engines).

What you do is add fuel, not air and fuel like a spark ignition engine, just fuel. Too much fuel and you quickly get to what is known as the smoke point, the issue with being there is exhaust temperatures are excessive, aluminum pistons will melt right at about 1400 F, but other damage is done as well before then from over temperature operation. Engine oil and or water may not overheat, but the temps in the combustion chamber are dangerously high



So if your moderately overpropped, you can motor along happily at half RPM or less forever and life is good, you just have to accept that your 40 HP engine is now say a 25 HP engine, and never let yourself get into a position where 40 is needed, cause it's not there anymore.

Overpropped is especially useful when motorsailing as if your not overpropped on engine alone, your underpropped under engine and sail.



So, you want your cake and eat it too? That is a controllable pitch prop, like a car in overdrive as your cruising along on a long flat road, need power? Downshift for the acceleration, then back to overdrive to cruise. Think of motorsailing as going down a long hill, a really high gear would be useful then.



I believe though for most of us a true controllable pitch prop doesn't exist, the closest I know of is the Autoprop that will increase pitch until an engine is loaded even at low RPM and motorsailing, but you don't control it and like any automatic thing, it works, but if you could control it and knew what you were doing and had the instrumentation, you could do better manually.

A great explanation. I also overpropped for same reasons after discussing with Nigel Calder at the boat show">Annapolis boat show 2 yrs ago. He advocates for this for people who are smart enough to realize you won't get full hp or rpm of engine without risking damage. Benefits are greatly reduced fuel consumption, noise, and possibly extended engine life of moving parts.

I run my 4jhe at 2200 rpm for 7 kts, 2000 rpm for 6.6 kts and quiet.


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Old 20-04-2016, 16:34   #30
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Re: At what rpm to you cruise?

Next topic for discussion : how long is your piece of string?
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