RPM's for Yanmar 4JH3TE

Steve Dion · Jun 15, 2011

The normal cruise RPM is about 85% of the max rated RPM or 3200 rpm for your engine if this data is correct. Anything between 80-90% is a good range.

http://www.marinedieseldirect.com/repower/specs/yanmar/4JH3_TE.pdf

Bill Roosa · Jun 16, 2011

I would have to differ in my interpretation of the data.
Clearly 3800 RPM is max RPM.
3700 is max continuous RPM and you can run the engine forever at any speed below that. Between 3700 and 3800 you can run the engine for an hour max.
What would be a better piece of info to know is the best rpm for max fuel range or as we say in the business best specific fuel consumption. The chart does not have that directly but you can calculate it by graphing fuel consumption/engine power for several RPMs and find the "low spot" in the curve. The low spot is the max power for a given amount of fuel and you should prop your boat to reach 95-100% of hull speed at that engine RPM.

Bill Roosa · Jun 16, 2011

I'm going to the boat tonight (splash next week!!!!) I'll get the engine manual out and check the number. For my normally asperated version it is 3400 RPM and we ar proped for 95% hull speed. I took off the mixing elbow this winter to check andit had NO evidence of sooting up. None whatsoever. As you probably guessed I'm not a huge fan of the 85% rule as it is hard on engines and wastes fuel.

Steve Dion · Jun 19, 2011

Bill:

According to the Yanmar experts running the engine at lower speeds is what is damaging to the engine. They claim glazed cylinders. According to mechanics I have spoken to these engines are run at 4000-4500 at the factory.

So what should we believe?

As far as wasting fuel, we are burning approx. .6 gal. per hour on our 3jh2e. How much more fuel can I expect to save?

Bill Roosa · Jun 20, 2011

drchpete
I run mine at 2800 RPM

Steve
they do test the engines at significanlty higher PRMs at the factory to make sure they are up to snuff. as they injection pump is governered running at over 4000 ROM would not even be possble with modifying the pump. As for "glazed cylinders" that is an old trick that happens when the mechanic needs a new pair of shoes for his kids. My experience is that automobile deisels operate a low speeds (and high) all the time and provided you "blow them out" once in a while they don't care how fast you run them. I would also note that auto gas engines don't get glazed cylinders and they don't run at 85%++ all the time either. The worst case is the exhaust is going to get carboned up. you can avoid that by operating the engine in a propery loaded condition. That means having the prop, trans, and engine configured correctly so you are pushing the boat at hull speed in calm water and still air (requires a certian HP) is matched to the engine output at that RPM. I believe that is the "engineer method" Then as the wind and waves pipe up you have a power reserve to allow you to maintain speed. this assumes that your engine is "properly sized" HP wise for the hull

Samaral · Jun 22, 2011

From the Yanmar operation manual: "The JH series engines are designed to be operated at maximum throttle (3000 to 3200 rpm) for less then 5% of total engine time(30 minutes out of every 10 hours) and cursing speed (2800 rpm or less) for less then 90% of total engine time(9 hours out of 10 hours)"

richard965 · Jun 27, 2011

all yanmar diesels have a factory governor that does not allow for over revving the engine, therefore you should run at high rpms and even full throttle . I have had 4 yanmar diesels and ran all of them at full throttle when cruising between ports...sometimes for 30 hours or more at a time. Never had a problem and certainly no deposits. A little diesel smoke stain on the transom however that is easily scrubbed off