What RPM Should I Run At?

agallant80

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I was speaking with a few people at the dock this weekend. We all have cruisers around 25 feet and we all had V8 motors in our boats. Some the 305 some the 350 some Merc some Volvo. The thing that keeped on coming up was what RPM should you operate at. The opinions varried. One guy said he would never go above 3500 and one guy said he runs at 5000 when conditions permit, there were a few people in the middle. I myself run around 4K when I can. What are the thoughts on this?
 

Levinz11

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Re: What RPM Should I Run At?

I run mine 95% of the time between 2500 (20-23 mph) and 3700 (35+/- mph). Lower and I'm not on plane really....higher and I feel like I'm beating on her. Granted the engine is 25 years old.
 

HT32BSX115

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Re: What RPM Should I Run At?

I was speaking with a few people at the dock this weekend. We all have cruisers around 25 feet and we all had V8 motors in our boats. Some the 305 some the 350 some Merc some Volvo. The thing that keeped on coming up was what RPM should you operate at. The opinions varried. One guy said he would never go above 3500 and one guy said he runs at 5000 when conditions permit, there were a few people in the middle. I myself run around 4K when I can. What are the thoughts on this?

Every marine engine is recommended by the manufacturer to be "propped" so it will turn in the recommended WOT RPM range(preferably at the top, lightly loaded) when at FULL throttle.

For yours, that would be around 5000 RPM. (your rev limiter is set slightly higher than the max recommended RPM)

Normal operation turns out to be approx 75-80% of that.....or around 3500-4000 RPM. Other than for short periods, no manufacturer suggests you should operate at MAX RPM or full throttle continuously.

The slower you turn any engine [continuously], the longer it will "last"......... one of the reasons diesels last longer.

Marine engines rarely wear out due to operation..... They usually "wear out" from non-use and other environmental problems (rust, freeze damage, etc)
 

Bondo

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Re: What RPM Should I Run At?

Ayuh,.... General Cruisin', I run 'bout 3200/ 3500 rpms,...

Wot rarely, 'n not for long,...

4200 if I'm in a hurry, 'n the water's flat enough....
 

tpenfield

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Re: What RPM Should I Run At?

My 24 ft cuddy, I would try to run it 3000-3200. It all depended on what the ocean Gods had in mind that day . . .

On my 33 ft, I'm going to try to keep it more towards 2800-3000, but we shall see. Once you get above 3500 RPM, then fuel economy starts to take a dive.

You could see if there is a BoatTest report on your particular boat, or something very similar. You want to cruise where you are getting the best fuel mileage. Usually that is between 3200-3500 RPM for most boats.

4K RPM is a real fuel burn on most boats and above that is totally unnecessary for any sustained period.
 

agallant80

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Re: What RPM Should I Run At?

ccording to boattest.com it looks like 3500-4500 is the good spot for economy, spped and range. It looks like anything in the 2K range is a fuel hog.

RPM MPH Knots Total GPH MPG NMPG Stat. Mile NM KM KPH LPH KPL dBA
650 3.4 3.0 0.9 4.00 3.48 252 219 406 5.5 3.41 1.7 62
1000 5.2 4.5 1.4 3.68 3.20 232 202 373 8.4 5.3 1.56 65
1500 7.1 6.1 2.5 2.88 2.50 181 158 291 11.4 9.46 1.22 72
2000 8.5 7.4 4.1 2.07 1.80 131 114 211 13.7 15.52 0.88 73
2500 10.9 9.4 7.0 1.55 1.35 98 85 158 17.5 26.5 0.66 81
3000 17.0 14.7 8.9 1.90 1.66 120 104 193 27.4 33.69 0.81 78
3500 24.2 21.0 10.1 2.40 2.08 151 131 243 38.9 38.23 1.02 79
4000 30.7 26.7 12.3 2.50 2.18 158 137 254 49.4 46.56 1.06 82
4500 36.4 31.6 15.6 2.34 2.03 147 128 237 58.6 59.05 0.99 84
5000 40.8 35.5 21.4 1.91 1.66 120 105 193 65.7 81.01 0.81 86
5200 41.5 36.1 22.6 1.84 1.60 116 101 187 66.8 85.55 0.78 86
 

aerobat

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Re: What RPM Should I Run At?

i think at most planing boats the sweet economy spot is the lowest speed where the boat keeps up on full plane.
 

QC

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Re: What RPM Should I Run At?

Volvo recommends 75% of WOT RPM for cruise speed. Seems reasonable to me.
 

emilsr

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Re: What RPM Should I Run At?

ccording to boattest.com it looks like 3500-4500 is the good spot for economy, spped and range. It looks like anything in the 2K range is a fuel hog.

RPM MPH Knots Total GPH MPG NMPG Stat. Mile NM KM KPH LPH KPL dBA
650 3.4 3.0 0.9 4.00 3.48 252 219 406 5.5 3.41 1.7 62
1000 5.2 4.5 1.4 3.68 3.20 232 202 373 8.4 5.3 1.56 65
1500 7.1 6.1 2.5 2.88 2.50 181 158 291 11.4 9.46 1.22 72
2000 8.5 7.4 4.1 2.07 1.80 131 114 211 13.7 15.52 0.88 73
2500 10.9 9.4 7.0 1.55 1.35 98 85 158 17.5 26.5 0.66 81
3000 17.0 14.7 8.9 1.90 1.66 120 104 193 27.4 33.69 0.81 78
3500 24.2 21.0 10.1 2.40 2.08 151 131 243 38.9 38.23 1.02 79
4000 30.7 26.7 12.3 2.50 2.18 158 137 254 49.4 46.56 1.06 82
4500 36.4 31.6 15.6 2.34 2.03 147 128 237 58.6 59.05 0.99 84
5000 40.8 35.5 21.4 1.91 1.66 120 105 193 65.7 81.01 0.81 86
5200 41.5 36.1 22.6 1.84 1.60 116 101 187 66.8 85.55 0.78 86

Is that for your particular boat/engine combination? If so, I think you have your answer.

Our last boat had a 5.0mpi. I could run just on plane (about 23-24mph) at 2500rpm and it was very miserly on fuel at that speed.
 

agallant80

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Re: What RPM Should I Run At?

I noticed that like the chart I posted the boat does not do well in the 2K range so i try to stay out of it. The chart above is for the 5.7 I have the 5.0 but from what I can caculate it seams to be pretty much the same. When I started this thread I was thinking more along the lines of shortning the life of the motor rather than fuel economy.
 

emilsr

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Re: What RPM Should I Run At?

Mercruiser recommends running at no more than 3/4 throttle for extended periods. You'll be fine at 4k rpm....maybe even a little more.

That's a good motor; will run for a long time and it will be several years before you need to do much maintenance other than oil changes.
 

QC

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Re: What RPM Should I Run At?

Mercruiser recommends running at no more than 3/4 throttle for extended periods.
This one has always been odd to me. How do you determine 3/4 throttle?
 

QC

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Re: What RPM Should I Run At?

i guess roughly by the position of your lever .
Seems arbitrary, but agree this is the only practical way. Probably over-thinking, but what else is a forum for? :nerd:
 

aerobat

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Re: What RPM Should I Run At?

Probably over-thinking, but what else is a forum for? :nerd:

what about boaters who never had the b@lls to push the lever briefly into WOT - how can they judge where the 3/4 positon is ? :)
 

HT32BSX115

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Re: What RPM Should I Run At?

i guess roughly by the position of your lever .

In aircraft engines the manufacturer determines 75% "power" from a "family" of engine operating curves.

These curves are provided at several (different) RPM, manifold pressures and altitudes.

With a variable pitch (governed) prop you can vary the throttle at a fixed RPM and of course, you can select many different RPMs and throttle settings.....

The power curves are always a percentage of maximum rated power and you can determine actual HP from them at various throttle settings (manifold pressure) , ENG RPM's for many different altitudes.... etc.

Since a boat has 2 of those parameters "fixed" I.E propeller pitch and (pressure) altitude, the operator can only vary the RPM directly with the throttle.

If you have a marine engine with a prop pitch selected to allow MAX (rated HP RPM) at full throttle, then you can only estimate the 75% point by assuming 75% of max RPM approximately equals 75% HP.

That may not be an accurate assumption, but it's all we have.

The only way to accurately determine the exact 75% HP point (assuming you're actually getting FULL rated HP at WOT) would be to find that point using a dynomometer, run the engine to max rated HP at full throttle and then pulling it back to 75% of that number. (and note the RPM and throttle [manifold pressure] setting) AND, you would have to try to approximate the same load that would be present while operating the boat. you couldn't vary the load (total boat weight) while doing it either...............

This would probably not be worth doing.

I suspect that Volvo and Mercruiser are just recommending using 75% of max rated RPM as a suggested approximate continuous operating RPM instead of just an arbitrary throttle position.

Seems arbitrary, but agree this is the only practical way. Probably over-thinking, but what else is a forum for?
nerd.gif
And I may be "over-thinking" this!:D
 

QC

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Re: What RPM Should I Run At?

And I may be "over-thinking" this!:D
In a word, yes . . . :D But that's what got me to this point. For me it's fuel flow, and that's not available to most. I like Volvo's because it is something concrete. Merc's is too open to interpretation.
 

aerobat

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Re: What RPM Should I Run At?

If you have a marine engine with a prop pitch selected to allow MAX (rated HP RPM) at full throttle, then you can only estimate the 75% point by assuming 75% of max RPM approximately equals 75% HP.

i don,t think so. water resistance surely goes up in sqare and not linear. running 75% rpm in a boat results in less than 75% power output of the engine.

fact is indeed the boater does not has any instruments to judge correctly the actual power output , so the recommendation is only very rough , the fuel flow meter might be the best indication some boats have. i do not have such an instrument on my volvo , but the sea ray 240 ( 5.0 MPI ) of my parents has. when you drop the hammer the flow goes to nearly 100 liters / hour , easing off to roughly 3700 rpm results the boat is still on plane but the flow goes back to 45-50 litres.
 

agallant80

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Re: What RPM Should I Run At?

If they say 75% and the max RPM of my motor is 5K then would 75% not be 3,750RPM?
 
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