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Jack Long · 19-07-2008, 22:03

My boat has a Yanmar 2MQ15... What is the proper sustainable cruising RPM?

Danke,
J

19-07-2008, 22:37

Quote:

Originally Posted by Jack Long

My boat has a Yanmar 2MQ15... What is the proper sustainable cruising RPM?

Danke,
J

Do you mean a Yanmar 2QM15?????

If so here is the result of the google search you could have done...

Yanmar 2QM15 Specification

Rated for continuous output at 3000 rpm, the exact rating actually depends on the propellers power curve.

Jack Long · 19-07-2008, 22:55

Okay Ketch, you win. Let me try again.

I have a Yanmar 2MQ15 and, being as how I am new to all this, I want to be really super careful to avoid causing expensive damage. I want to know a good cruising RPM that I can sustain for a couple of 10+ hours stints.

I tried searching Google with strings like "yanmar 2mq15 cruising rpm" and the first page or so of hits did not lend me a lot of confidence. You know how anybody can post any stuff on the internet without any real filters... So, since I have vetted this community and...

A) feel pretty good about the competence and honor levels
B) know a lot of you have similar boats and motors

... I thought posting here would be a good idea!

However, if my question offends you or if typos offend you or if you would rather people not ask things conversationally and only use this forum for data farming- just ignore me and don't reply. If I make a lot of posts and nobody responds I will eventually take the hint and go away. I promise.

**Alan Wheeler** · 20-07-2008, 01:03

Hi Jack, Firstly a question. Does the engine rev to 3000RPM or close when in gear. If it does, then thats great news and the actual speed is where ever your best speed through water is. What I mean by that is, if you push the boat too hard, it will try an bury its rear end down in the water and you end up burning fuel for little extra speed. So there is always a point where fuel to speed ratio is spot on. And if all has been done correctly, that will be the best RPM as well. It is better for a Diesel to work hard than idle along. So you certainly can not do harm running at higher RPMs, as long as the engine is not overloading.

**GordMay** · 20-07-2008, 01:52

Continuous duty will be approximately 80-85% of your wide open throttle RPMs.
Hence, if you can achieve 3,000 RPM at WOT, then your cruising speed will be 2,400 - 2,550 RPM (NO black smoke).

**Hud3** · 20-07-2008, 04:10

Jack,

Here's a good website for "real world" info on Yanmar engines: Yanmar Marine Engine Help

Good stuff there, plus it has a QM series discussion forum, and you can email the site manager for help.

p.s. Gord is right about cruising rpms!

**GordMay** · 20-07-2008, 04:37

Thanks for reminding us of the great Yanmar Help Site, Hud.

Excerpted from their Engine Operating Guidelines at Yanmar Marine Engine Help

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

Chief Engineer · 20-07-2008, 08:36

How long is your boat.

Determine the theoretical hull speed.

If you try to push faster than that, you will get black smoke and burn more fuel.

AnchorageGuy · 20-07-2008, 08:44

Quote:

Originally Posted by GordMay

Thanks for reminding us of the great Yanmar Help Site, Hud.

Excerpted from their Engine Operating Guidelines at Yanmar Marine Engine Help

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

Gord, What is with that site? When i try to get on the discussion forum it tries to save a MV file to my computer.

**GordMay** · 20-07-2008, 09:03

Quote:

Originally Posted by Chuck Baier

Gord, What is with that [Yanmar Help] site? When i try to get on the discussion forum it tries to save a MV file to my computer.

Beats me; but I see what you mean.

Sandero · 20-07-2008, 09:19

It would seem that the way to figure this out for your boat and prop would be to determine what RPMs will drive the boat a hull speed in flat water. Obviously any more is wasteful.

My boat engine (Penta) combi can push to around 2800 or so but that is way to high, the stern squats and the boat is trying to drive up the back of the bow wave. I find that 2000 works well for motoring in calm seas and push it higher if plowing through seas. When at all possible if I have to motor I try to use the sail as well because it gives a but of speed and stability I think the optimal fuel economy is about 1800.

Jack Long · 20-07-2008, 11:40

Thanks guys- this gives me some good stuff to think about.

I will be leaving for the boat in a few minutes- expect to hear from me late in the week... If not I am lost at sea.

Ketch, sorry I was snarky.

Chief Engineer · 20-07-2008, 16:36

The offhand formula for THS is 1.34 times the square root of the WATERLINE length

Sandero · 21-07-2008, 03:37

You need to consider that some hullls will have a different waterline length when moving though the water as the stern squats and this effectively lengthens the waterline length.

Shiva has about a foot or so of the stern out of the water at anchor and moving slowly but the transom does touch the water when moving quickly making her LWL 12 inches or so longer.