Clarification on cruising RPM

[Porchhound]

I read the thread  (June 20,2005) on cruising rpm. Ron made this statement: "With the engine in gear, note what MAX rpm is at the boats fastest speed and then multiply that rpm by .8"

My max RPM during the sea trial was 3400 indicated, but I don't know what a good speed number was due to sails also being up. If I use Ron's technique, do I use the eighty percent of the RPM noted when the speed has stopped increasing, or the WOT RPM of 3400, even though the top speed will be associated with a lower rpm?

I've read several discussions with people using anywhere from 1800-2400 rpm. I know it has to do with the individual boat/set-up. I also noted that the max torque range is between 1800-2000rpm. Is there any point in pushing an engine beyond this max torque range?

Also Stu referred to a link in that discussion, but it is dead (at least to my computer).

[Stu Jackson]

Dave,

You mean this thread?  https://c34.org/bbs/index.php/topic,2257.0.html

The link is dead because it pointed to an old bbs thread which died.  FWIW, the thread is from 2005, the bbs was older.

Most of our boats have a WOT of 3000.  Your 3400 could be from a number of things, usually a different alternator if the tachometer hadn't been recalibrated to it, or it's just plain off.

Some of the basics still obtain:  80% of WOT is good to BEGIN to develop a BASELINE for your use.  Check them against STW.  Some would suggest trying fuel consumption, but that will NOT help.  Why?  Because I've been measuring my fuel consumption for two and a half decades and it's always with % points of 0.5 gph for my M25.

When I lived in SF and my first 18 years with my boat I motored out of the Oakland estuary for at least 45 minutes every time I used the boat.  I'd run anywhere from 2500 to 2700.  0.5 gph.

When we sailed her up to British Columbia in 2016 I kept the pedal to the stops at 2800 and 2800+ every day we moved (over six weeks) and got the same 0.5 gph.  Unless all you do is run your boat at idle, you WILL NOT see any difference in fuel consumption based on reasonable use and rpms.  Period.

This is a real world experience that indicates that trying to find a "sweet spot" rpm to save fuel by balancing speed is a fool's errand.

If your question only is about torque ranges I just don't know.  But at 1800 rpm with a clean bottom I'd probably go 4 knots, at 2500 I go 6 knots, and much over that is just pushing water.  On the trip up I was going 7 at 2800+ and some days had to do that because of the 13 hours between harbor entrances.

[Porchhound]

Thanks, Stu. That makes sense to me. There is a higher output alternator added two owners ago. I think I'll start with 2400 and see how that goes. I've read running a low RPM is not good for the engines, lugging them down. It's still a mystery but time and study will make it clearer.

[Ron Hill]

Porch : I don't know when or what I said that???  I've always stated that cruise RPM is 80% power for a diesel.  I use 2600RPM.

A few thoughts

[KWKloeber]

You might eyeball these.
https://c34.org/search_gcse/?q=%22engine%20rpm%22

Diesels are better under load than high rpm, unfortunately, there is relatively little load at the prop compared to, say, a highway truck pushing uphill.  There is nothing detrimental to running a diesel in a boat at low rpm!

[Porchhound]

Ron it was an early 2000 discussion I believe and I probably have the statement out of context. Thanks.

[Porchhound]

So the trip from Olympia to Bremerton (55nm) went well with the engine at 2500 rpm indicated. It was an eight hour run with the tides helping most of the trip. A few things here and there to fix but we are very happy with fades the boat. Unfortunately we didn't have time to run up the sails but we will soon.

Thanks to everyone for all the assistance!

[waughoo]

Congrats on your arrival!