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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #1  
Old 05-29-2008
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Prop question.....

I have a C&C36 of 1989 vintage with a 3 bladed fixed prop and a perkins 3 cylinder 30hp diesel. I have noticed that at 2000 revs my speed is only about 4 knots. I have been out on two other boats of similar length recently, one using the same engine and at the same speed they were both virtually ticking over at 1000-1200 revs. I guessing my prop has been changed at some point to one with the wrong pitch. Am I right to assume i need one with a coarser pitch ?
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Old 05-29-2008
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Do you know if the prop is clean and in good shape? Four knots in a C&C36 at 2000 rpm seems slow for sure.

Hard to believe one would live with those conditions for long - did the previous owner have the same experience?

A fouled prop (and/or rather dirty bottom) would produce this kind of problem.
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Old 05-30-2008
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Well it's my first boat and so I didn't realise anything was wrong until I went on the other boats recently.
I took a dive under last weekend and sure enough there were quite a few barnacles on the prop which I scraped off. The hull was antifouled about 10 months ago and is still quite ok with no weed growing on it. After this there was an improvement but still nothing like it should be.
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It does sound like the prop pitch is a bit fine. If you have the same motor (or similar) to the other boats, then find out what prop they are using. It would be a good start.
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Prop manufacturers are pretty good about recommending diameter and pitch. Give one a call and get their recommendation, then compare to what you have. That will tell you if the prop specs are in the ballpark.
If you decide to buy a new prop, remember that prop selection is still as much art as science. Call the manufacturer and try to get to talk to one of the older guys there. The young ones seem to place too much reliance on the computer programs output. The older guy will know what direction to adjust the program output. This is a situation where experience really does matter.
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Have you motored beside these other boats to see if you are indeed that slow? Is the 4 knots indicated boat speed or GPS groundspeed?

I'm sure you can feel the difference in boat speeds, (between 4 and 6 knots, for example) but part of your problem might be instrumental (knotmeter or tach??).
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Old 05-30-2008
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If you have barny's on the prop your bottom is probably not as clean as you think.
Boat length and engine size is not the only variables - boat shape and weight also come into play as do current and wind strength/direction.

Entire books have been written on propeller selection etc.. (David Gerr's for one) - the bottom line is ask your perkins dealer or C&C forum users what they are going with. You do need to find out more about your prop - being 3 bladed isn't enough to work with - you need both pitch and diameter to work the calculations.
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Old 05-30-2008
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The way to determine if you have the correct prop is to check your WOT (wide open throttle) rpm. It sounds like you may have a Yanmar 3GM, I think the WOT is supposed to be 3600 (check your maunual). Your engine should reach the WOT in forward but not much higher. If it goes above that rpm, you are underpitched, if it won't rev up that high you are overpitched (or the bottom is fouled, prop covered in barnacles, etc.) and you lugging your engine all the time.

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Sorry, missed the part about you having the Perkins.
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Old 05-30-2008
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I think the post regarding WOT is not quite correct. It may be true that if you are over-pitched you won't reach the engines spec'd WOT RPM's; but if you are under-pitched it won't matter because a diesel engine is speed governed. It won't rev above the max RPM because the fuel injection pump will restrict flow at some point and the engine will only run at the factory pre-set max RPM.

It's possible that your RPM gauge is out of calibration; and while you think you are at 2000 you may really be at 1500 or less. That's what was happening with my boat; and it was a simple procedure to adjust the gauge.

There should be an adjustment screw on the backside of the tachometer. In order to do the calibration you will need a handheld or small fluroescent lamp that has a non-electronic ballast. An electronic ballast will have a strobe frequency that is faster than 60 cycles per second. A standard fluroescent ballast will flash 120 times per second; that's what you will need.

Place a piece of white tape on the engine main pulley. Run the engine at 1800 RPM and you should see 4 semi-stationary white marks while illuminating it with a fluroescent lamp. You will need to close/cover the hatches to block out ambient light or do the test at night. If your tach is out of calibration you will need to adjust the engine throttle until the 4 marks appear; then adjust the tachometer to read 1800 RPM. Then if you run the engine WOT while underway it should read the governed max RPM of the engine +/- 100 RPM.


Here is the article that details how this test is done:

C36IA - Diesel and Propeller Tutorial
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