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Old 24-02-2015, 17:30   #1
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Over Prop?

We bought our boat last September and it's been low on RPMs since we got it. I thought the engine just might be down on power (and still think that's most likely) but after reading through some of the prop threads I thought maybe we're just over propped.

Our boat has a Campbell Sailor prop on it and I've read a few posts where people have been down on RPMs after installing one of these props. Right now we're only able to turn about 2300 @ WOT (should be 3k), however the engine doesn't bellow any smoke or overheat. My thoughts are that if we are over propped then we'd have other symptoms along with the low rpm. Does this seem like the right line of thinking?

Unfortunately I don't know the diameter or pitch of the prop as, other than for the survey, we haven't had the boat out of the water yet. Any other symptoms we should be looking for to indicate we're just over propped?
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Old 24-02-2015, 23:20   #2
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Re: Over Prop?

Any chance the prop and/or bottom have a lot of growth? You would be amazed how much impact it will have.
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Old 25-02-2015, 07:42   #3
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Re: Over Prop?

I don't think that's the problem. This is the hull when we pulled her out for a survey. It has a bit of slime growing, but no hard fouling and she wasn't growing a beard anywhere like a lot of the neglected boats around here.

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Old 25-02-2015, 07:51   #4
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Re: Over Prop?

It's hard to tell from the photo, but, although the bottom looks OK, it looks like there may be some hard growth on the prop?
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Old 25-02-2015, 08:35   #5
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Re: Over Prop?

Always seems to help to establish some sort of base line so; What model engine and gear? How high does the engine turn up in neutral? Wide open throttle at no load should be roughly 100 rpm over maximum rated rpm. Is throttle linkage adjusted for maximum travel?
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Old 25-02-2015, 08:39   #6
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Re: Over Prop?

If the engine cannot reach it's maximum governed speed, the hull is clean and the engine is healthy, then you are over-propped. It's really that simple.

The downside to being over-propped is a shorter engine life and the combustion chamber building up a layer of carbon. You need to be able to run the engine at full speed under a load periodically to blow the carbon out of the combustion chamber.

There is no economical advantage to being over-propped, contrary to some peoples opinions.

Some engine manufacturers will not honor the warranty of an engine that is over-propped.....it's that bad for an engine.

There are a few books out there where you can enter the numbers so that you can get an estimate of the correct propeller size for your boat. Making guesses at it can be ineffective and get expensive.

I would not go with propeller sizes generated in here unless someone has an identical hull, transmission ratio and engine.

I used this book when I re-engined my research boat. I more than doubled the horsepower so I needed new propellers and larger diameter shafts. The book nailed the correct size for me for a four bladed prop at 24x24 inches.



http://www.amazon.com/The-Propeller-.../dp/0071381767
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Old 25-02-2015, 09:14   #7
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Re: Over Prop?

Mine was overpropped to where all I could turn was 2600, engine rated for 3600.
Mine didn't blow black smoke or anything else either, your engine may be fine, it might just be overpropped. I've found overpropping to be real common with sailboats, people believe that they burn less fuel that way, some do it for motorsailing. In motorsailing, since a lot of the propulsion comes from the sails, the ideal is to have a higher pitched prop, course when just motoring, your overpropped.
I went with an autoprop so that it would self adjust pitch based on load, seems to work well so far, but is awfully expensive.
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Old 25-02-2015, 13:13   #8
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Re: Over Prop?

Quote:
Originally Posted by jimbunyard View Post
Always seems to help to establish some sort of base line so; What model engine and gear? How high does the engine turn up in neutral? Wide open throttle at no load should be roughly 100 rpm over maximum rated rpm. Is throttle linkage adjusted for maximum travel?
Fair enough, It's a westerbeke w21 with a 2:1 v-drive. I believe my max rpm in neutral is only 2700, but I haven't verified this with a laser tach like I did with the engine under load. I've verified the throttle linkage is adjusted for maximum travel. I have replaced all the fuel filters since I last tried to run the rpms up in neutral, so maybe I should try it again to see. I've also checked the exhaust elbow to make sure it wasn't clogged.

I know being over propped is bad, and I'm actually thinking about replacing the current prop with a folding prop. I mostly just wanted to see if there were other symptoms I should be looking for that are also associated with being over propped.
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Old 25-02-2015, 19:39   #9
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Re: Over Prop?

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Originally Posted by blinkerfluid View Post
My thoughts are that if we are over propped then we'd have other symptoms along with the low rpm. Does this seem like the right line of thinking?

Unfortunately I don't know the diameter or pitch of the prop as, other than for the survey, we haven't had the boat out of the water yet. Any other symptoms we should be looking for to indicate we're just over propped?
The prop looks clean enough in the picture. To know if you're overpropped....

FIRST, you must know what the manufacturer says is the maximum rated rpm of the engine.

SECOND, warm up the engine and wind it up to max rpm in neutral. It should reach maximum rated rpm + about 100. (Never mind laser tach, just read the analogue tach).

THIRD, take the boat for a spin under power on a calm day. Put the hammer down and, for a few minutes, wind up the engine to max rpm with the boat at hull speed. If the engine reaches the manufacturer's maximum rated rpm there, the prop is sized correctly. If it only reaches a few hundred rpm less than the maximum rated rpm there, it is overpropped.

From what you've said (It's a Campbell Sailer so it's been sized fairly recently, there's no black smoke and the mixer elbow on the exhaust is not plugging with black deposit) I'd bet a coffee and donut (not more!) that the prop is correctly sized.

=========

Here, there's a Yanmar 3GM30F with a 17 X 10" prop. The manufacturer's maximum rated rpm is 3600 rpm and it winds up to 3700 rpm in neutral.

(Now note carefully==>) At the dock, in still water, in gear the engine gets up to 3100 rpm, with smoke. (<==Did you see that?)

Cruising under power, at hull speed, the Yanmar happily winds up to 3610 rpm with a clean exhaust.

Now that is a perfectly sized prop. Lunenburg Foundries in Luneburg, Nova Scotia sized it and cast it. The engine pours all of its energy into the prop at hull speed, where it counts. If it reached max rated rpm at the dock it would be coasting a bit at hull speed, because of the slip of the water sliding past, with some energy to spare.
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Old 25-02-2015, 20:09   #10
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Re: Over Prop?

Your prop is covered with growth, I can see it in the picture. If our's isn't scraped once a month it effects the rpms. Don't even waste time considering the prop pitch until you scrape the prop completely clean.
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Old 25-02-2015, 21:34   #11
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Re: Over Prop?

The picture is from when we pulled the boat for the survey. The prop was cleaned up prior to putting the boat back in the water, so I dont think a dirty prop is the problem.

Sounds like I need to go wind it up in neutral again to see what I'm getting for rpms. The rpms at wot I mentioned above are while out on a calm day, definitely not just sitting at the dock.

I think we may be slightly over propped, but I still think the motor is down on power too. We'll check the valve clearances and probably pull he injectors to have them serviced.
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Old 25-02-2015, 22:36   #12
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Re: Over Prop?

Quote:
Originally Posted by blinkerfluid View Post
Fair enough, It's a westerbeke w21 with a 2:1 v-drive. I believe my max rpm in neutral is only 2700, but I haven't verified this with a laser tach like I did with the engine under load. I've verified the throttle linkage is adjusted for maximum travel. I have replaced all the fuel filters since I last tried to run the rpms up in neutral, so maybe I should try it again to see. I've also checked the exhaust elbow to make sure it wasn't clogged.

I know being over propped is bad, and I'm actually thinking about replacing the current prop with a folding prop. I mostly just wanted to see if there were other symptoms I should be looking for that are also associated with being over propped.
Okay then, the W21 makes 21 hp at 3000 rpm. If you don't turn up to 3100+ almost scary quick in neutral, there is a problem and you shouldn't be thinking about prop pitch until you find and correct the problem.

Without knowing any history, since you've just changed filters, I'd check; (in this order) intake restriction, valve clearance, exhaust back pressure, injection timing, compression and injector pop pressure and spray pattern. If the engine isn't smoking, knocking or overheating, chances are good that you only have a reasonably minor maintenance issue.
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