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  Topic Review (Newest First)
10-14-2010 09:20 AM
Maine Sail My own rules for my own engine, a Westerbeke, is that the prop allow the engine to turn max rated RPM as Westerbeke strongly suggests. I then take her to hull and back off the throttle just ever so slightly. This usually puts me at 75-80% of max rated RPM on my Westerbeke. Yanmar is rather un-clear on the difference between max rated and max continuous, but on the 3YM-30 it is only 100 RPM +/-. If your boat can only turn 3200 at WOT then the prop is certainluy over pitched as you should see at least 3489 (3500) and perhaps sizing for 3600 would be better? It should ideally be double checked with a digital photo tach to make sure your tachometer is correct.

Universal, Westerbeke and Beta are all fairly strict on what they want to see the prop be able to turn at WOT. Yanmar is less clear, but I would think you should be able to at least turn 3500 +/- at WOT then run at 75-85% of that number..

When I used to work with a surveyor we always tested the engine/prop combo against the engines rating during sea trials. On nearly ever boat over propped the engine was burning or using more oil and some had seen multiple exhaust elbow replacements vs. those with properly sized props. The transom soot was always a dead give away as to an improperly sized prop. We had some boats so over propped that they would do FOUR knots at idle!!! Kind of tough on docking....

I am over on the Ericson site because my friend Tim R. had invited me over there a few years ago during a thread, which I have since forgotten. Tim is no also no longer an Ericson owner, owns a Caliber 40 LRC now, but it is a great group of guys. I still participate on many forums of boats I used to own such as the C-36 and Cape Dory forums but mostly here, SBO, cruisers and a few others.
10-14-2010 09:14 AM

For prolonged running, a pleasure boat engine a good operating speed is around 85% of maximum continuous rated RPM (see the engine data plate)
assuming the propeller size is correct and the engine can reach maximum rpm).
See this list for approximate rpm:

GM - 4JH - 3/4JH2 series 2900rpm
YM series Depending on WOT rpm 2600-2900rpm
4LH and 6LY series 2650rpm
6LP and 3/4JH3 series 3000rpm
3/4JH4 series 2400rpm
6CX series 2200rpm
10-14-2010 08:56 AM
teejayevans I run at 2400 rpm which gives me 5-5.1 knots, 2 blade prop. Running above 3000 just causes much higher fuel consumption & stress on the engine.
I understand you'll get glazing at lower temps, but the thermostat insures its running at it's minimum temp.
The 1 time I max it out for any extended time (trying to make a bridge opening), I overheated after 5-10 mins.
I do follow the manual's advice and run the engine at high RPMs every so often to clean out the carbon deposits.
10-14-2010 08:07 AM
FishSticks Rod - you might find it useful to study a copy of Dave Gerr's Propeller Handbook.
10-14-2010 07:45 AM
Thanks to all

Maine Sail: I read what you posted on the Yanmar site, which is why I was asking for real world advice on this site. (by the way, my previous boat was an Ericson, and I was a frequent reader of your elaborate postings on the Ericson site. Glad to know you peruse this site as well).

Stevemac: Thanks for the input from Larry Berlin. I had forgotten his name, but he is the person who certified the installation of the engine in my boat. I will give him a call about this. Note that the Yanmar site says that the prop size DOES matter.

Another question: what speed does the boat cruise at????

Rod Johnson
10-14-2010 06:43 AM
Maine Sail From Yanmar

"For prolonged running, a pleasure boat engine a good operating speed is around 85% of maximum continuous rated RPM (see the engine data plate)
assuming the propeller size is correct and the engine can reach maximum rpm).

Operating for extended periods below 80% rpm may increase longevity but the cylinder bores could glaze causing an increase in oil consumption and blue smoke especially when new."

Max continuous RPM on the 3YM-30 is 3489
Max No Load Engine RPM is 3600

85% of 3489 puts your cruise RPM at 2965 RPM or just about 3000
10-14-2010 06:37 AM
stevemac00 I have 3gm with the maxprop. I can hit around 3400-3600. I just talked to Larry Berlin at Mac Boring and he said you should run at max a few hours each month to burn off the varnish. he also said the tachs are notoriously inaccurate and you should have a handheld to calibrate at different rpms.

I usually run mine around 2700 as that provides me with the best speed/efficiency ratio. For me that's a little over 5 kits and burning a little over a half gallon/hr. But I'm always loaded down into the water line stripe. If I take it up to 3200, it's a lot louder and burns a lot more fuel for marginally faster speed. I do run it high once a while as recommended.

Forgot to add, your dealer is wrong. The maximum rpm is for the engine - not a prop/engine combination.
10-14-2010 05:37 AM
FishSticks I have a 3YM30 with almost 900 hrs. I run typically at 2750 (the recommended optimum), but have run all day at 3000 quite a few times. I have a 3-blade MaxProp set at 16X11 and the reduction gear is 2.62:1. The boat is about 19,000 lb. The RPM peaks at about 3500 (indicated RPM) under load and drives the boat at about 6.8 kt in smooth water when the bottom is clean . Hull speed is 7.7 kt. This is a sweet engine and I have had zero problems with it.
10-14-2010 05:20 AM
lsbrodsky This sounds a little strange to me. The boat should have a prop that allows max engine RPM. Your dealer may mean that normal cruising rpm should be about 3200, which is just about right. Too many people under rev their engine thinking they are extending its life when just the opposite is happening. You can run right up to engine max with no harm and cruise all day at 3200.
10-13-2010 01:55 PM
Yanmar 3YM30 in a PS 31 cruising RPM

I am reading and hearing different recommended cruising RPM on my Yanmar 3YM 30.

Maximum RPM from Yanmar is "3200-3600".

The dealer told me that my max RPM is 3200 (I guess for the specific boat and prop I am using).

So is my cruising RPM 75, 80, or 85% of 3200?

Again, dealer told me 75% of 3200, which is 2400 rpm, but I have read higher.

This seems a little slow, and the boat moves at 4.5-4.75 knots.

I would like to think I could run a little higher RPM, and a little better boat speed.

Any other opinions out there? Especially with the same hull and engine.

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