Just curious as to what RPM others cruise at? Our boat is a 386 with a 3jh3e Yanmar and a 3 bladed prop.. At 3000 rpm we get between 6 and 6.5 knots. If I push tp 3400 we get about 7 knots. That extra speed is great on some of our 8 hour motors down Chesapeake Bay but am I pushing the diesel too hard and losing fuel efficiency?
Cruising RPM.
- Thread starter Waltsailor
- Start date
Anything lower results in carboned up valves, glazed cylinders and quite likely E.D.
You are definitely losing fuel efficiency since you have a displacement hull if you define fuel efficiency as miles traveled per gal of fuel consumed. I generally cruise at approx 75% of rated 3600 rpm on my Hunter 40.5). That is 2700 rpm. I go to wide open (3600) for 5 minutes towards the end of each sail or daily if on long cruises with a lot of motoring time. First, can you do "rated full rpm" for your diesel on your boat with your prop? There have been other threads on this subject and I would encourage you to read them. For my boat at 2700 rpm and a clean hull/prop I get about 6.5 knots and burn about 1gallon/hr. Converting that to miles per gallon, that yields 6.5 nautical miles per gallon of fuel used. At 3600 rpm I get about 7.5 knots and burn about 1.75 gal/hr. That converts to about 4.25 nautical miles per gallon consumed or a 35% decrease in miles per gallon.
Is it good to run a diesel at full rated rpm for long periods of time. Probably not. Is it good to run then lightly loaded for long periods of time, definitely not. There is a sweet spot and you'll have to determine what that is.
Is it good to run a diesel at full rated rpm for long periods of time. Probably not. Is it good to run then lightly loaded for long periods of time, definitely not. There is a sweet spot and you'll have to determine what that is.
You should run between 75 to 80% of your real MAX RPM. That will change from rated based on your prop pitch. Run your boat up to full RPM out cruising and let it stabilize, the difference between rated and actual is your Prop pitch effect. Take 75% of your MAX stabilized RPM and that should be your target Cruise RPM. I use 2,750 RPM and that gives me between 6 to 6.2 knots Cruise on my boat - my load, wind, waves, bottom condition all impact my speed thru water.. My max is 3,450 RPM with the setting on my MaxProp. Slower and you build up carbon, faster and you are pretty inefficient on fuel. If you are getting soot on your stern, then you aren’t burning all the fuel. That’s building up carbon and indicates an issue. Usually start with a fouled or Over-pitched prop. I had my MaxProp get worn gradually, didn’t notice obverse time. We went on a long Cruise and built up soot, burned oil, fuel consumption up. Changed injectors, had my timing checked - pulled the boat and the blades were somewhat loose - cone gear worn out. Effectively the loose prop blades over pitched my prop and I was running way over my target %. Since I rarely went to full throttle, I had not noticed the RPM going down. Was running 2750 at 3200 RPM until figured this out. That was 86% RPM way too high. Sent MaxProp off for reconditioning and back to normal.
More to the point, if you cannot hit very close to the engine's max RPM (e.g., within about 100 to 150 rpm of max) then you have an issue with your prop, the engine not making its rated HP, or both. That is an issue that should be addressed.
So right @Alan Gomes . If you can't make max. RPM with everything in order, all bets are off.
Run your 3jh3e Yanmar about 75-80% of 3800 RPM (around 3000 RPM) and that's it for cruising speed.
There should be a plate on your engine that indicates max rpm and continuous rpm. Yanmar recommends running at continuous. However, your best fuel efficiency may be a little lower than continuous. Frankly, as little fuel burned saving a little on fuel should not be an issue. Keeping our engines in best condition should be a priority. So going with the manufacturers recommendation is a good thing.
I don't believe Yanmar recommends running it at its continuous rating all the time; this would be too high. Although you *can* run it at that without blowing up the engine, it is higher than what you should run it for everyday use.
My 1GM has a max RPM of 3600 and a "continuous" rating of 3400. A good cruising RPM for that engine would something like 2800 or maybe 2900, not 3400. But I do open it up from time to time to blow the crap out of it.
If you have a statement from the Yanmar documentation saying that they actually *recommend* running the engine at the continous rating all the time, it would be interesting to see that. The closest I can find in the Yanmar 1GM manual is that the engine should be run at 3400 rpm *or less*.
I talked to a Yanmar rep at a boat show and he said run it at continuous RPM all the time and at max RPM about 30 minutes now and then.
Well, the Yanmar rep's comment notwithstanding, in the case of my 1GM that would be running it at 95% of its max RPM (3400 = continuous; 3600 = max). Again, that sounds too high to be running it that way all the time. Conventional wisdom for diesels is something like 80% of max as typical cruising RPM; not sure why a Yanmar would be different.
Anyway, I'd like to see that recomendation in their printed literature before I did it. But that's just me. I guess if that's the continuous rating then you *can* run it at that, well, continuously! But whether one ought to do that is a different question. I run mine at 80% or sometimes a bit more and then run it at WOT occasionally to blow out the carbon, and that has worked well for me (as far as I know).
Your boat, your choice!
I cruise my 410 with a 4jh3e at 2000-2400 rpm. That is the sweet spot for speed and noise to me. But every few hours i run it up to 3000 for a while. This is how the manual says to run the engine. You people can just keep the crazy “run it hard” theories. No mechanical device likes to run “hard” all the time.
Btw far as efficiency goes. I have found that for my boat the mpg between 5-6.5 knots is so close to being the same that it isn't worth thinking about.
Btw far as efficiency goes. I have found that for my boat the mpg between 5-6.5 knots is so close to being the same that it isn't worth thinking about.
It's not an emotional thing, they're designed to run hard. Strong cylinder walls, beefed up connector rods, properly sized valves. The manufacturer could have just as easily designed it to run slow but why would you want reduced power from the same sized package ?
Don't run it hard and suffer the consequences ........ that's according to the manufacturer, the guy who designed the engine.
I think you are quite right, Ralph. Lightly loading a diesel engine is not good for its longevity.
The point I was pondering earlier is whether "running it hard" in terms of its normal, everyday cruising RPM is the same as running it at its "continuous rating," which for my 1GM, at least, would be 95% of max RPM. As I said, I've tended to run my engines somewhere around 80% of max RPM. I consider that to be "running it hard," punctuated by occasional bouts of "running it *really* hard" (WOT) to blow out carbon and so forth.
In the example to which you are responding, the 4JH3e has a max RPM of 3800 and a continuous rating of 3650. Were it my engine, I'd cruise it at 3000 RPM. Running it primarily at 2000-2400 is surely not doing it any favors.
If I were in the market for a boat of any significant age and the owner told me that the engine has "super low hours" (i.e., he rarely ran it) and that when he did run it he "babied it," these are not selling points and I'd figure the cost of a repower into what I offered.
My take is that you CAN run at the continuous rating but you fuel economy will suffer
This thread might prove helpful for those who are concerned about not damaging their engines by consistently under loading them.
My hunter 356 and yanmar 3gm30 cruises at 2850 RPM
This is thé manufactures curves where power (HP) and consommation of diesel (2liters/ hour) coïncides.
I get about 6 knots on smooth seas.
If i push the RPM to 3400 continuous the diesel consomption doubles!!!! For less than a knot more
This is thé manufactures curves where power (HP) and consommation of diesel (2liters/ hour) coïncides.
I get about 6 knots on smooth seas.
If i push the RPM to 3400 continuous the diesel consomption doubles!!!! For less than a knot more