Cruising rpm's - SailNet Community
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post #1 of 28 Old 10-26-2009 Thread Starter
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Cruising rpm's

I have a 3 year old Yanmar 2GM20FC. I understand that the maximum rpm's are ~3500. How do you determine what is the best cruising rpm's.
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post #2 of 28 Old 10-26-2009
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The cruising RPM for your Yanmar is 2000 -2800. This depends mainly on your needs and the boat's hull speed. Normally it is best to attain speeds near to hull speed without too much load on the engine. My cruising rpm's are 2200 -2500 for my Yanmar which means 5,5 -6 knots.
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post #3 of 28 Old 10-26-2009 Thread Starter
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My boat is a Niagara 31. I'm not sure of the hull speed but I think it's approx 5.5 knots. She seems to motor along nicely at 2800 rpms at 5.5 knots. But a friend told me I should only be cruising at 2000 rpm's.
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post #4 of 28 Old 10-26-2009
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Look at the performance curves. One will be torque and RPM - Another will be Fuel consumption, although it may be in grams of diesel & you will have to convert it.

A max torque will end generating max HP to the Prop and you can see what is best cruising RPM for your yacht. Always will be a compromise. Revs or speed


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post #5 of 28 Old 10-26-2009
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around .75 * max RPM ...

Over the years sailing OPBs, most folks ended up cruising at around 75% of max RPM or whatever got them to hull speed, whichever was lower.

My 30' Islander Bahama goes about 6.5 knots at 2700 RPM, which is 75% of my 2GMF's 3600 max RPM. My hull speed is 6.64 knots, so that's about right.

Those who do not know their hull speed should check out this Sailcalc page. It shows you all kinds of interesting statistics about your brand, size, and model boat (assuming it is on the list). I have no idea how accurate it is overall, but it was pretty close when I spot-checked a couple of its statistics for my boat using the original formulae.

Tom

T. P. Donnelly
S/V Tranquility Base
1984 Islander 30 Bahama
Pasadena, MD
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post #6 of 28 Old 10-26-2009
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Quote:
Originally Posted by celenoglu View Post
The cruising RPM for your Yanmar is 2000 -2800.
What is that RPM range based on?

Thanks

Dale

S/V Elnora
Pacific Seacraft "Crealock" 37 #312

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post #7 of 28 Old 10-26-2009
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By Ear! The boat will tell you where she is the most "comfortable." I have the same engine in a Beneteau. 2100 rpm's gives me 5.4 knots and I burn 1/4 gallon a hour. 2800 rpm's I am moving along at 6.4 + knots, but I am burning 1/2 gallon a hour.

Depending on wind and waves she will settle in somewhere between those two, but you can "feel" where she (the boat) wants to be.


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post #8 of 28 Old 10-26-2009
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Quote:
Originally Posted by bubb2 View Post
By Ear! The boat will tell you where she is the most "comfortable."
Bubb2 is totally right! An old Yanmar rep from Down Under told me - "Run her where she sings to you"

Our old Tanzer 28 - 2GM20F - 2400 RPM
Our "new-to-us" Irwin Citation 34 - 2QM20H - 2500 RPM

And yes, she does sing when it is good - she "shakes, rattles and rolls" when it is not good.

Cheers

Rik

Irwin Citation 34
Mystery
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post #9 of 28 Old 10-26-2009
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I agree you should run where it seems comfortable. If you are not sure of your hull speed here's the formula: 1.34 x square root of waterline length. Example: 25' waterline, square root is 5 (5x5=25). 5 x 1.34 = hull speed which is approxmately 6.7.
Brian
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post #10 of 28 Old 10-26-2009
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my nimble 30 express has a yanmar 2GM 20F . LWL 25.8 beam 9.4 displacement 6,500 . 15"X11" pitch - perfect pitch prop. i cruise at 2200 RPM & 5.5kn at 3600 rpm (top) 6.9 kn.

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