Cruising RPM?

[druid]

For those who have an engine with max RPM around 3600 (like my new Beta 25): What's your typical cruising RPM? I'm used to the lower-rpm engines and a cruising RPM around 2200-2400, but I'm finding that a "slow cruise" (just over 5 knots) is 2500, "fast cruise" (over 6 knots) is about 2800, and "getting somewhere in a hurry" (almost 7 knots) is 3000.

I rarely do the 3000 though. Should I be doing the 2800 most of the time? what RPM do you find yourself at most of the time? (I'm asking what YOU do, not for advice on changing my system)

druid

 

[woodster]

it is recommended that you run at 80% of top rpm to keep the engine healthy with short term burst of higher rpm's from time to time

 

[Terry Cox]

Lots of posts on this topic and an opinion with each. The cruising RPM for our Yanmar 4JH2-TE with a fixed three blade is 1,800-2,000. That will push the boat to six-seven knots SOG. Even at that RPM the engine reaches its operating temperature, and does not change regardless.

Some will say that Yanmars like to run hot at the higher RPMs. Perhaps, but I've been cruising at this RPM for years with no apparent ill effects. I figure as long as the engine reaches its operating temperature, has a good quality diesel rated oil and filter, that should keep the engine clean.

 

[Benny17441]

Turbo charged engines experience less carbon build up and unburned fuel deposits than normally aspirated engines. The increased compression of the fuel mixture makes for a more complete combustion. That is probably why Terry has not experienced any difficulties running at low RPM. Normally aspirated engines do not seem to handle them that well. All engines have a sweet spot at which they seem to run smoother and that is probably the preferred RPM range for most but we have to be careful that if it is too slow then we should periodically race the engine under load to clear out excess carbon. I'm not much for warm up periods preferring to warm up underway but a cool down period especially after a high RPM run is a must.

 

[dlochner]

80% of wide open throttle seems to be sweet spot. Much higher than that you are just pushing a lot of water and burning a lot of fuel with little gain in speed.

 

[Rich Stidger]

My Volvo MD22L has a 3000 rpm top speed. I cruise it at 2200-2400. The east coast Volvo tech rep told me that I could back off 400 rpm from the max and then run it 24/7. That would be 2600 rpm but I don't push that hard.

However he also told me that on each use- a day, a weekend, a week, or so I should run the engine at WOT under load for 10 minutes. The purpose was to clean out the deposits and also to assure that should such speed be required, the engine, drive train, prop, etc can actually perform. His reasoning was that in an emergency you don't want to find out that your prop is fouled or that there is some problem with the engine. I do actually run at WOT per his advice.

 

[JamesG161]

My Volvo MD22L has a 3000 rpm top speed. I cruise it at 2200-2400. The east coast Volvo tech rep told me that I could back off 400 rpm from the max

I heard the same, but I understood it as 400 rpm off maximum speed of the boat, not ultimate 3000 rpm.
In calm seas, I can push 7.8 knots (SOG) in an emergency. That was 2600 rpm for my bit longer (hull speed) boat than Rich's boat (I have the same engine and model year).
I understood that was the "sweet spot" for delivered Hp versus fuel consumption, but Rich's prop is different than mine.

So... 2200 rpm gives me ≈7 knots

What's your typical cruising RPM?

I would suggest testing and listening to your engine. If you have fuel flow, it can be plotted to find the max fuel flow versus speed. Find the peak speed of your boat and then back off ≈400 rpm as Rich said.
Jim...

 

[Scott T-Bird]

Sue and I recently did a trip, heading north with very light wind on the nose and very flat water, we ran almost 4 hours at 3,000 rpm. Our engine is Yanmar 2YM15 with top, unloaded rpm at 3,900. WOT is about 3,500 rpm but I never run her at WOT because it doesn't seem worthwhile. Whenever I'm going somewhere I almost always run at 3,000 which gives me about 6.5 Knots on our small boat and the engine and boat feels smooth. The fuel consumption on that trip seemed about right. It was less than I anticipated. That just doesn't seem like a hurry to me ... all sailboats are slow to begin with! . I almost never run less then that unless in a confined channel or we're raising sails. That said, coming back from our trip, I did about 1/3 of the trip at 3,000 rpm into a 20 knot wind on the nose with choppy conditions. Getting tired of the spray, I backed down to 2,800 rpm for the last 2/3 which made the trip a little more comfortable, the engine seemed to be smoother, and the time it took to complete the trip was not significantly different that it took going the other way. For cool down, when the engine has been under load for at least an hour, I give it at least 10 minutes in idle. I really have no idea what it takes to do a full cool-down since I have no temperature reading on the control panel.

 

[JamesG161]

For cool down, when the engine has been under load for at least an hour, I give it at least 10 minutes in idle. I really have no idea what it takes to do a full cool-down since I have no temperature reading on the control panel.

That caught my curiosity and hopefully not to far off topic.

But I have never thought of cooling down my engine in idle.
Is this a normal procedure? Have I been doing something wrong if no idle cool down?

My only Yanmar is my 16 Hp genset engine (RPM constant, load varies), which like yours, Scott, has no Temperature gauge.

Heat produced internal in engine is by combustion of fuel. Gears and bearings are heated, additionally, by load, than in idle. There are still engine bearing associated with RPM in idle.
Jim...

 

[Ron20324]

Like Rich said... When I get out of my creek into the Severn and the engine has warmed some, I go wide-open. If I can't make 3600 (3YM20) I let it run for some minutes to see if it will build up (in flat water). I always thought the engine ran rough, but I had nothing to compare it to. I ran at 2800, but after 9 years I found 2400 to be the smooth, "sweet spot". That makes me more comfortable.

Yeah, let the engine cool down. IIRC Yanmar says for a few minutes in idle.

 

[druid]

Once again: I'm not asking for advice as to what my cruising RPM SHOULD be - I'm asking what YOUR cruising RPM normally is. But as for the suggestions...
I've found the Yanmars are more picky about running at higher RPMs (ie they carbon up and basically run badly if they're not run at close to WOT), while the Kubutas (Universal or Beta) seem to be happy running at lower RPMs. I'm not familiar enough with the Volvos to see where they stand. After all, they're tractor engines, and tractor engines rarely run WOT.
As for listening: my engine is louder and uses more fuel as the RPM goes up, but doesn't seem to be lugging or "uncomfortable" - it seems happy at 2500 or 3000. Cruising at 2500 is very comfortable, quiet, and uses very little fuel, but only gives me maybe 5.3-5.5 knots in calm water. 3000 is very loud (but smooth) and uses a lot of fuel, but doesn't give me much more speed than 2700-2800.
As for cooling down: I do that, but there's no indication on the temp guage that anything is happening.

(as an aside: should we be talking about "throttle" when the "fast-slow control" doesn't actually involve throttling the air/fuel mixture in a diesel? Discuss...)

druid

 

[Scott T-Bird]

Well, the speed control on a diesel engine seems to actually be a "governor" to limit the fuel flow. I fail to completely understand the significance of the difference between "throttle" and "governor" but throttle seems to be more universally understood, so I tend not to fight city hall.

 

[Jalepeno]

I cruise at about 2,500 to 2550 RPM on my Yanmar YSM12. That is about 80% to 85% of the maximum loaded speed rating.
What you call the "throttle" is called the "regulator" by Yanmar. Think of it as your car's cruise control speed (RPM) setting if you had only one gear. It sets the target motor RPM. The governor will adjust the fuel control to try to maintain that target motor RPM speed with a variable load.
A gasoline (outboard or inboard) motor does not use a speed governor. That is why a gas motor will try to speed up to infinite speed if run at no load.

 

[Johann]

Well, the speed control on a diesel engine seems to actually be a "governor" to limit the fuel flow. I fail to completely understand the significance of the difference between "throttle" and "governor" but throttle seems to be more universally understood, so I tend not to fight city hall.

The way I understand it, a "throttle" controls engine speed by regulating the amount of air available for combustion. On a diesel, engine speed is controlled by fuel flow. The "governor" is not a speed controller, but a speed maintainer. The diesel "throttle" (speed control lever) actually sets an RPM, and the governor adjusts fuel flow up or down, based on load, to maintain the selected RPM.

 

[dlochner]

FWIW, the current issue of Practical Sailor addresses this very issue.

 

[Jumpstart]

I ran my Yanmar 2GMF at 2750 RPM for 22 hours straight coming home from Mackinac. Boat speed was at speed of 6.3 knots average and fuel consumption was .6 gallon/hour for my C&C 29. The engine really liked that RPM and ran very smoothly. Best speed over ground was 10.8 knots in the St. Clair River after going under the Blue Water bridge. Gotta love that current going downstream as Lake Huron funnels into the St. Clair River

 

[dave_and_dd]

I run my 3ym20 typically at about 3300-3400 for much of the 15 min passage in and out to open water. I do let it cool down at idle for a few minutes before shutting down and can see the temp needle go a bit cooler in response to that.

As per the manual, I also rev it up a couple times right before shutting down.

Dave

 

[druid]

I was being a bit facetious about the "throttle" comment. A gas engine has an actual "throttle": a butterfly valve in the carburator that restricts ("throttles") the air/fuel mixture going to the cylinders. The throttle control moves that throttle valve. A diesel does NOT have a throttle valve. I guess you could say it "throttles" the fuel going into the cylinder, but it does it in a different manner than by using a throttle valve.

Just muses for a boring Monday (it's TUESDAY???)

druid

 

[jssailem]

Just completed a 59nm motor (no wind beautiful sunshine) at 2500 rpm. Above that the bow would rise and the stern would settle in the water putting the exhaust port below water. Speed between 6-7 knots SOG. My 1974 40hp Perkins diesel purred. No idea how much fuel was sipped. But it was a fun christening after 8 months of Marina dock refitting.