So What Bad Happens to an R-21 EC at 2K RPMs?

[cruz-in]

Hi,

I have read that these 30 HP diesels are meant to be run at 80% WOT. Sevveral post simply mention that it is "not good" to run them for extended periods at 1800 to 2000 RPM.

Can anyone shed some light on why this is bad? What specifically will extended running at 1800 to 2000 RPM do to the engine?

Thanks
Dan

 

[Two killicks]

I read the same thing, but run mine at 2400rpm most of the time. This is the comfortable rpm for the vibration and noise level. On my tug, 80% should be at 2940 rpm. I run the boat at WOT minus 200 rpm for 5 minutes every hour. I am looking forward to read fellow tugger's opinion on this matter.

 

[Ahab]

Hi Dan,

In my experience with small diesel engines on our prior sailboats, I think that running at low rpm would tend to promote more carbon build-up in the exhaust mixing elbow and that by running "hotter" on occasion would help to prevent this. They say diesels shouldn't be babied. I would guess you'd be fine so long as you elevate it now and then.

Craig

 

[NorthernFocus]

I have read that these 30 HP diesels are meant to be run at 80% WOT. ...What specifically will extended running at 1800 to 2000 RPM do to the engine?

Presumably what you are referring to is an 80 percent service factor engine. That means that the engine is designed to run at/below 80 percent of full load averaged over time. Some interpret that to be on an hour by hour basis and some interpret it to mean on a daily basis (IOW run no more than two hours at WOT in a 10 hour operating day). Routinely exceeding service factor on engines/motors tends to reduce the operating life. By how much? It's pretty much a roll of the dice. Specifically, it'll run until it breaks.

Engines aren't typically designed intentionally for 80 percent SF. A lot of time/money goes into an engine design. A common marketing strategy is to tweak an existing, mature design to get more horsepower out of it rather than design an entire new engine. But during prototype testing if it is discovered that something major fails on the engine when run at full output, a simple way to salvage the idea is to downrate the engine to a load rating that it can survive. So for marketing purposes they advertise the max horsepower and put the service factor limit in the fine print.

 

[cruz-in]

Dan, thanks for that explanation.

This may in fact be where the often quoted "run the engine at 80% of WOT" comes from.

I totally get that hours and hours of idling are not the best thing for any engine.

However seems to me running these engines at 2000 RPM (ish) is similar to cruising at 55 in my Ford F250 Diesel. Truck is running at about 2000 RPMs (actually less) and can do that day in and day out.

Seems to me the Yanmar or Volvo should be able to run at 2000RPM (+/- a few hundred RPM) all day, week after week without harm. So I was surprised to read posts that indicated otherwise.

 

[NorthernFocus]

...Seems to me the Yanmar or Volvo should be able to run at 2000RPM (+/- a few hundred RPM) all day, week after week without harm. So I was surprised to read posts that indicated otherwise.

Internet forums are a poor place to get technical advice. For that matter so is your local mechanic. Find the manufacturers recommendations and operate within them. Certainly running an engine in the middle of its operating range isn't going to hurt it. Extreme high end and low end are where the problems typically occur.

 

[rt11002003]

I seem to recall Yanmar suggesting running at 80-85% of MAX RATED RPM. On my first R21EC, that rpm was on the data plate. I operated around 2800 rpm most of the time, except in "No Wake" areas where 1800 or so rpm seemed OK. My second R21EC also had the same Yanmar engine model. I operated it the same way. I also ran both those engines at WOT (out of gear was about 4000 rpm) for five seconds for five cycles, per operating manual.

I run my 2014 R21EC with the Volvo engine about 2400-2500 rpm most of the time. The max rpm in gear is 3000, which I assume is MAX RATED RPM. Again, the max WOT rpm, out of gear, is 4000.

MAX RATED RPM was defined as the rpm the engine could attain if fitted with the appropriate propeller. The appropriate propeller was defined as one that allowed the engine to exceed MAX RATED RPM by approx. 200 rpm to allow for accumulation of "stuff" over time.