RPMs for cruising

[Mark G]

With a 3-blade prop, my max RPMs with a 1991 M35 show about 2,600 under load. A little more, actually. I can hit about 7.2 knots on the GPS in flat water during a short test at that engine speed. I cruise at just a touch over 2,000 RPMs and have no trouble maintaining 6.2 to 6.6, depending upon the seas. Should I use 75-85% of my max RPMs under load for a cruising speed or max RPMs in neutral? I know that the tachs are notoriously inaccurate, but I figure the relative numbers are what really matters.
Thanks!

[Roc]

Mark,
I have an M35B (2000 model year).  I cruise at 2500 rpm.  Under load, full throttle, my rpm will go to 3100.

[Roc]

Oh, I have a 3 blade fixed prop....

[Stu Jackson]

Mark

You're right on the money with your concept.  This, of course, has been discussed previously and a good highlight from "search" on "rpm" finds this:

http://www.c34.org/phpbb/viewtopic.php?t=1446&highlight=rpm

It IS all relative and not absolute.  Also depends on what "sounds" and "feels" good as there are all sorts of RPMs that, at least on our boat, sound horrible, compared to other smoother turns.

[Ron Hill]

Guys : As Stu said "it's all relative"!!!  
Some people have 2 bladed, or 3 bladed, or feathering props from different manufactures that are completely different from each other (with different pitches!).  
Some have cleaned off 15+ years of bottom paint, some have not.
Some carry more than one anchor with an all chain rode, while some have multiple anchors & rodes.
Some are stripped for racing while others are loaded for cruising and others are every stage in between.

The only given is that you want to run the engine at about 80% power.  How do you determine that? - as alternator rpm signals differ drastically.  
With the engine in gear, note what MAX rpm is at the boats fastest speed and then multiply that rpm by .8   That should be about 80% power for YOUR boat.  Simpliatic, but close.  

[Mark G]

I appreciate the feedback. The engine feels good at the 80% level and the speed under power is everything I had hoped to achieve with a move up from the C30.
--Mark