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Old 20-04-2016, 16:36   #31
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Re: At what rpm to you cruise?

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

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Old 20-04-2016, 17:52   #32
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Re: At what rpm to you cruise?

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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.

If you can run those small blocks that slow, they will last a real long time, most turn them at 4 to 5 grand


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Old 20-04-2016, 17:58   #33
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Re: At what rpm to you cruise?

2000 rpm volvo MD17D... 2 blade fixed prop
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Old 20-04-2016, 17:59   #34
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Re: At what rpm to you cruise?

I like to run at about 2000 RPM on my Volvo MD22 which is rated for 4000. But RPM higher than that is intrusive due to noise and the fuel consumption suffers to boot. However I acknowledge that running a diesel engine lightly loaded for extended periods is a big no no. the money saved in fuel is likely to be offset by premature engine overhaul. Therefore I do what many do and that is to over pitch the prop somewhat. Better to run loaded at lower RPM and sacrifice the ability for higher power than ruin the engine. So a simple survey of cruising RPM may not be very useful. It also really depends upon your vessel and the style of use. My boat performs well in light winds and I hate to motor. If I had a motor sailor I would likely over pitch it more to save noise and fuel as I would be using the engine far more.

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Old 20-04-2016, 18:06   #35
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Re: At what rpm to you cruise?

Volvo Penta D2-55 with 3 blade folding props. 2400 rpm at 7knots
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Old 20-04-2016, 18:27   #36
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Re: At what rpm to you cruise?

It all depends. For me, 1800 RPM with a 2400-max-RPM engine is a "sweet spot" providing one-knot below hull speed.

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Old 20-04-2016, 19:00   #37
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Re: At what rpm to you cruise?

120HP 1981 Ford ; 1100 rpm is sweet. Temperature is Ok at 900 rpm but only do that if needed to save fuel on passage. Give it a blast every now and then at 1600.
Handbook says 1600 but its a bit noisy and that is for trucks cruising on highways.(Unless its uphill or into a strong head wind the highway truck load is not the same as on a boat.)
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Old 21-04-2016, 08:41   #38
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Re: At what rpm to you cruise?

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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?
As a perfect example, my Nanni 3.30s rated 29hp at 3600 rpm have as their base engine a Kubota 1100 rated 24hp at 3000 rpm.

I started this thread because I was wondering if anybody actually followed manufacturer recommendations on cruise speed for small diesels on boats, which seem very high to me. As so often happens, I am learning quite a bit more than I expected!

Currently, I can reach 3400 rpm at WOT, 200 rpm less than rated, which means that either I am already slightly overpropped or that my alternator-based tachometers are not 100% accurate. Either way, I will adjust the pitch today, aiming for a max rpm of 3000, and will let the forum know the results.
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Old 21-04-2016, 09:22   #39
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Re: At what rpm to you cruise?

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?
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Old 21-04-2016, 09:22   #40
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Re: At what rpm to you cruise?

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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.
Controllable pitch props are available and very useful on a sail boat or motor sailer. Most of them can go to reverse pitch so it is possible to have a simpler gear box but some don't have enough travel to both feather and go clear to reverse thrust. Steve and Linda Dashew used them on some of their sailboats. The big thing is to not exceed the max exhaust gas temp. It is easy to put one on. The modern John Deeres have a fancy engine monitoring system that shows % of power being used. On a turbo charged engine long as the ratio of manifold pressure to rpm is in the safe range you are good to go. The modern field tractors (with the fancy $$$ transmissions) will pick the lowest rpm that will safely produce the required power and change ground speed ratios to keep the most efficient operation going.

Kasten recommends them on some of his designs.

Controllable Pitch Propellers
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Old 21-04-2016, 19:01   #41
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Re: At what rpm to you cruise?

The father inlaw tells me diesels are best run hard otherwise they smoke and gum up. He's been a master mariner for 45 years and has rebuilt countless big diesels himself. Never idle I have his rule.

When we bought our boat I was told she smoked. Turns out she was being run too slow. His advice was to run her at higher rpms and miraculously the smoke went away. My brother inlaw marine engineer says the same as do several others with great cred.
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Old 29-04-2016, 11:25   #42
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Re: At what rpm to you cruise?

I increased the prop angle by 1.5 degrees on my Kiwiprops to overprop them a bit. I now get 3200 rpm WOT, as opposed to 3600 as recommended by Nanni. Speed has increased by about .6 knots at all rpm and the motors still seem comfortable at 2200 to 2400 rpm with no smoke. I understand that I now have 20 hp max at 3200 rpm as opposed to 29 hp max at 3600 rpm, but the reality is that in 6 years and 2800 engine hours we have never run the motors at more than 2600 rpm so I expect this will not be an issue.
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Old 29-04-2016, 11:42   #43
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Re: At what rpm to you cruise?

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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 water pump">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!
My 4-108 liked 2300-2400. That was the sweet spot. 44 Ft cutter. Of course it depends on the boat size and weight etc. You sound a bit low to me though.
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Old 29-04-2016, 11:43   #44
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Re: At what rpm to you cruise?

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Next topic for discussion : how long is your piece of string?
Mine's better than yours... mine is traditional cotton string.
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Old 29-04-2016, 11:47   #45
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Re: At what rpm to you cruise?

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?
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