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Old 10-05-2015, 09:51   #16
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Re: What's wrong with this picture

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Originally Posted by Subway Sailor View Post
Im assuming I'm not going to get 1/2 gallon /hr fuel consumption?
If I recall correctly, the 2GM20F on my (previous boat) C&C33 would push us at or near hull speed around 2800-3000 RPM; cruising around 2600 it drank 1litre/hour, say 1/4 gallon/hour (fudging US-vs-Canadian gallons).

How many Canadian litres in a US litre? (please no one answer that).
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Old 10-05-2015, 10:17   #17
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Re: What's wrong with this picture

You need to find out where your WOT is.
That will determine how your prop is pitched.
Personally, I would avoid running that engine above maybe 3000, as they have a history of piston issues.
But if you end up needing the HP you may have no choice.
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Old 10-05-2015, 10:18   #18
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Re: What's wrong with this picture

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Originally Posted by RainDog View Post

Forgive me i don't really know how to read that type of stuff,

I only assume certain things but can't confirm if I'm right or wrong..


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Old 10-05-2015, 11:27   #19
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Re: What's wrong with this picture

apparently I'm killing my Yanmar 2QM15 on my 1966 columbia 29 mark2. 8400 lbs, 8' beam. Bought Ola 2 years ago from some yahoos that used her as a cabin here in the Morro Bay State Park marina. Lots of non-use. Always a pain in the butt to cold start. Best remedy so far has been to heat gun the injectors and pump for two minutes and sometimes jiggle the throttle while cranking. Always starts right up the rest of the day and runs smoothly. Three blade prop moves her at 4knots at 1400 rpm on flat water. Rarely has the motor seen 1700. Raw water coolerd in this typically below 60F water, the engine never gets over 100F. No smoke, no smell. Seems to use no fuel. Why she has a 30 gallon tank I don't know. I've put on 300-400 hours. It was rebuilt by a locally renowned mechanic about 7 years ago. Maybe it will die next week.
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Old 10-05-2015, 12:07   #20
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Re: What's wrong with this picture

I had a yanmar 2gm. Those around 1980-1986 have a design problem with the transmission clutch cones...they slip. The more they slip, the more smooth they get, and they slip even more.

To test...looking at the prop shaft, shift from neutral to forward. There should be a nice solid "clunk" and the shaft should be turning at engine rpm instantly. If the shaft slowly increases speed, thats slippage.

This problem cost me about $1000 to fix (paid a sleezy mechanic to rebuild the trans) on my 1982 C&C27.

Other causes could be a prop too small...do you have a 2 blade? If the boat was designed for a gas engine, the prop would have been designed for higher rpms. This can be fixed by getting a 3 blade prop.

Another problem I have personally seen, is boat too heavy. Have you loaded up with all chain rode, extra anchors, extra tankage, etc? My friend loaded up his bayfield 29 with so much gear he had to use full throttle, and the boat was still way below hull speed.

And finally, check you waterline line length. A boat your size, I would only expect 5 knots tops.

Hope this helps
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Old 10-05-2015, 12:13   #21
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Re: What's wrong with this picture

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Originally Posted by Subway Sailor View Post
Wow how interesting, you guys are blowing my mind...

so i thought it was bad to run the engine past 2000-2500 and that motors where selected for boats in order to have hull speed met in that. RPM range...

so from what i'm understanding its ok to do this? Im assuming I'm not going to get 1/2 gallon /hr fuel consumption?
I disagree. I had the yanmar 2gm in my 1982 C&C27. I rarely revved over 2000, and enjoyed speeds around 5 knots. Once I took it up to 2500 and was going 6 knots. At 1800-2000 rpm I would burn only 1/2 a litre per hour....wonderful. These numbers aren't from books, they are from my personal experience.
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Old 10-05-2015, 12:28   #22
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Re: What's wrong with this picture

I will get back to you with my WOT results on Tuesday weather permitting! Damn tropical storm Ana


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Old 10-05-2015, 13:47   #23
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Re: What's wrong with this picture

When cranking up the engine past the revs you normally work with keep an eye on the temperature gauge. If it gets up to 160 or so then don't push any more revs until it levels out. You can go higher temperature but be cautious the first few times you push it. 180-185 max.

Your engine should be at least 140 to be more efficient.
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Old 10-05-2015, 14:38   #24
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Re: What's wrong with this picture

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Originally Posted by Shawn67 View Post
"At 2000RPM that works out at only about 3HP, at 2500RPM it is about 6HP."

That seems low.

HP = (Torque x 5252) / RPM
Torque = ( 5252 * HP) / RPM

Diesels have pretty flat torque curve and peak torque is probably not at max RPM. At 3600 RPM if it is making 18hp it is making just over 26 ft/lbs of torque. If you use 26 ft/pounds at 2000 the engine is putting out almost 10hp and a little over 12hp at 2500 rpm.

Shawn
My figures were based on the common formula "HP = Max HP times the cube root of the ratio of rpm to max rpm".

I've generally found that that ( and the other "rule" of 0.225 litres per hour per HP) works out pretty well when estimating speed and fuel burn.
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Old 10-05-2015, 14:50   #25
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Re: What's wrong with this picture

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Originally Posted by hamburking View Post
I disagree. I had the yanmar 2gm in my 1982 C&C27. I rarely revved over 2000, and enjoyed speeds around 5 knots. Once I took it up to 2500 and was going 6 knots. At 1800-2000 rpm I would burn only 1/2 a litre per hour....wonderful. These numbers aren't from books, they are from my personal experience.
Your 3000lbs less displacement and fin keel probably made a big difference
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Old 10-05-2015, 15:00   #26
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Re: What's wrong with this picture

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Originally Posted by hamburking View Post
And finally, check you waterline line length. A boat your size, I would only expect 5 knots tops.
The LWL of a SC28 20.17ft according to sailboatdata.com, so around 6 knots would be right based on the common 1.34 x sqrt(LWL)
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Old 10-05-2015, 15:06   #27
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Re: What's wrong with this picture

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Originally Posted by Subway Sailor View Post
Forgive me i don't really know how to read that type of stuff,

I only assume certain things but can't confirm if I'm right or wrong..
Look at the propellor power curve in RainDog's link.

That tells you that at 2500 RPM you are getting around 6HP delivered at the propellor (just what I calculated using a basic formula in a previous post )
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Old 10-05-2015, 17:00   #28
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Re: What's wrong with this picture

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Originally Posted by bailsout View Post
apparently I'm killing my Yanmar 2QM15 on my 1966 columbia 29 mark2. 8400 lbs, 8' beam. Bought Ola 2 years ago from some yahoos that used her as a cabin here in the Morro Bay State Park marina. Lots of non-use. Always a pain in the butt to cold start. Best remedy so far has been to heat gun the injectors and pump for two minutes and sometimes jiggle the throttle while cranking. Always starts right up the rest of the day and runs smoothly. Three blade prop moves her at 4knots at 1400 rpm on flat water. Rarely has the motor seen 1700. Raw water coolerd in this typically below 60F water, the engine never gets over 100F. No smoke, no smell. Seems to use no fuel. Why she has a 30 gallon tank I don't know. I've put on 300-400 hours. It was rebuilt by a locally renowned mechanic about 7 years ago. Maybe it will die next week.
Apologies to Subway Sailor for the thread drift but perhaps Bailsout isn't aware that his 2QM15 is qutie a different beast to the 2GM20. The QM series are much older, a bit heavier and are slow reving. Max rpm IIRC is 3000 and normally operated around 2000 IMO.
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Old 10-05-2015, 19:17   #29
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Re: What's wrong with this picture

Subway:
Not to disagree with all the some of the other replies but having been a Yanmar rep for over 15 years and sailboat propeller business for another 15 years, I have to put my two cents in.
Your Yanmar 2GM20F in a 28' boat should push that boat along at over 4 knots at those RPM, especially with a newly painted bottom. You should have the reserve to WOT to fight into head winds and seas at approximately the same speed.
You don't mention the propeller at all, number of blades, diameter, pitch or transmission ratio. This will, like they say make a difference to your wide open throttle rpm.
In neutral at the dock you should be able to reach 3800 - 4,000 rpm to show you have throttle settings correct and no restrictions. Scary to do but best way to check the linkage.
Underway at WOT you should as they suggested be able reach 3400 - 3600 rpm. Naturally it would be preferable to do it after the engine is warmed up. If it goes over the 3600 your pitch/diameter is probably too low and could be increased. That would also give you higher boat speed at your optimum running rpm that you are presently using. If it only reaches 3200 or less your problem is not likely to be resolved with the propeller as it may be a transmission or engine power problem since you aren't getting the speed at low rpm with an over pitched prop.
At any rate running at the 2000-2500 rpm range is not a problem but you should run it up over 3000 rpm for at least 10 - 20 minutes each time you use it to heat up the exhaust and blow out the carbon and it also can help to keep your hull clean. The water mixing exhaust elbow needs this particularly as it can close up and restrict exhaust flow and cause low power problems in your engine.
I am assuming that this is not a new installation and it has had a previous owner.
Another point never use your engine for charging battery by idling it at 1200 - 1500 rpm as it is not loaded up, as others have suggested and it does need to work. Put the engine in reverse at anchor if that is when you need to charge or at the dock in gear. At the dock better yet, plug in and use shore power to charge. Or even better change anchorages and motor over charging at the same time.
More than two cents, I know but once started hard to stop.
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Old 10-05-2015, 19:35   #30
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Re: What's wrong with this picture

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Originally Posted by StuM View Post
Look at the propellor power curve in RainDog's link.

That tells you that at 2500 RPM you are getting around 6HP delivered at the propellor (just what I calculated using a basic formula in a previous post )
That tells you you are getting 6hp delivered by the propeller. The amount of power to the propeller shaft is a little over 10hp. The engine itself is generating over 10hp at 2500rpm and fuel consumption is going to follow 10hp, not 6hp. I believe the engine is specced to burn approx. 1.75 liters an hour at 2500 rpm.

Redoing the calculations (in the formulas I posted earlier) using the DIN6270B rating of 15hp (not 18hp) at 3600 that is 21.8 ft/lbs

Using 21.8 ft/lbs at 2000 that is 8.3 hp and at 2500 that is 10.37hp. Both follow the graph, the 2500 calculation is slightly low due to the engine producing more torque at 2500 than it does at 3600 or 2000. Max torque will be where the engine has the greatest volumetric efficiency and fuel efficiency (most power per fuel) will be highest there too.

Shawn
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