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Old 10-05-2015, 01:06   #1
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What's wrong with this picture

I have a southern cross 28. A pretty canoe stern sailboat that has a yanmar 2gm20f. Now upon taking my new boat out, i noticed that at 2000-2500RPM's i am not reaching hull speed. What are the possible causes? Keep in mind i had 3 kts wind, calm to no current. Based on the waterline of my boat i should be getting 5.9 knots on my clean, recently painted, hull. My Chart Plotter says i do 3-3.7knots. So what are the possible causes for this lack of performance?
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Old 10-05-2015, 01:15   #2
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Re: What's wrong with this picture

-propeller improperly pitched
-fouled propeller
-cones in transmission slipping
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Old 10-05-2015, 01:30   #3
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Re: What's wrong with this picture

Not running the engine hard enough!

2GM20 is rated at 18HP at 3600 peak, and 16HP at 3400 continuous.

At 2000RPM that works out at only about 3HP, at 2500RPM it is about 6HP.
Would you expect to get hull speed with a 3-6HP outboard?

You are running it well below it's optimum 3000-3200 RPM which will give you 10-12HP. How does it go at 3000?
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Old 10-05-2015, 02:15   #4
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Re: What's wrong with this picture

How many rpm are you getting at WOT?

It should be 3400 to 3600.

Hull speed should be reached well before this but knowing what rpm you are getting at WOT is helpful in determing what problem exists (if any).
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Old 10-05-2015, 03:47   #5
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Re: What's wrong with this picture

All good responses and there are other possibilities....prop is fouled or something is caught around shaft or rudder...and who knows if the GPS is accurate?...tachymeter could be off, fuel could be constrained somewhere...prop could be slipping, could be engine problems of one sort or another..but I agree that if rpm's are accurate, it needs to be revved up more to achieve full hp...
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Old 10-05-2015, 04:28   #6
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Re: What's wrong with this picture

You need about 3 hp per ton to reach hull speed. I'm guessing your boat weighs at least 5 tons, so you should be hitting hull speed at about 3400 rpm.
You may not get to hull speed with your present engine if your boat is heavier than 5 tons, or if the engine is worn with reduced compression.
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Old 10-05-2015, 04:41   #7
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Re: What's wrong with this picture

Quote:
Originally Posted by Kokanee View Post
You need about 3 hp per ton to reach hull speed. I'm guessing your boat weighs at least 5 tons, so you should be hitting hull speed at about 3400 rpm.
You may not get to hull speed with your present engine if your boat is heavier than 5 tons, or if the engine is worn with reduced compression.
Spot on. Specs say 8500lbs displacement which will need about 13HP on that rule of thumb. i.e. over 3200 RPM.
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Old 10-05-2015, 06:57   #8
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Re: What's wrong with this picture

"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.

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Old 10-05-2015, 09:24   #9
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Re: What's wrong with this picture

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? My baby is in Maryland and needs to come to new york this upcoming weekClick image for larger version

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Old 10-05-2015, 09:30   #10
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Re: What's wrong with this picture

In addition to the good advice above, was there any current against you when you took the reading?
What about reverse course? Different?
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Old 10-05-2015, 09:31   #11
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Re: What's wrong with this picture

Here are the curves for your engine. http://j30.us/ubbthreads/ubbthreads....put-curves.GIF
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Old 10-05-2015, 09:39   #12
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Re: What's wrong with this picture

Also does WOT mean?


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Old 10-05-2015, 09:43   #13
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Re: What's wrong with this picture

Wide open throttle


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Old 10-05-2015, 09:44   #14
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Re: What's wrong with this picture

Quote:
Originally Posted by senormechanico View Post
In addition to the good advice above, was there any current against you when you took the reading?
What about reverse course? Different?

Maybe .25-.5 knots difference


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Old 10-05-2015, 09:48   #15
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Re: What's wrong with this picture

Yanmars are a high rpm engine that like 2,600 rpms to run at all day, Diesels liked to be worked, running at low RPM does them no good but wear them out prematurely, on the other hand running them flat out is also hard on them. 3,400 to 3,600 RPM are maximum RPM. I have motored at hull speed @ 2,600 non stop from Mackinaw Is to bottom of Lake Huron on less than 20 gals of fuel in a J35 with a 31 year old 3GM, just remember to warm them up before putting them under load and cool them down after running. Now I have to learn how to run my 34 yr ols Perkins 4-107 in a 18,000 lbs boat.
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