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  #1  
Old 07-21-2013
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Prop walk in FORWARD?

Short story:

I have a LOT of "prop walk" or "weather helm" in forward and darn near none in reverse.

Is this normal?

Long version:

In the four years that I've had my 28' fin keel sailboat .. my first boat .. I never did a whole lot of motoring. Open water was 1 minute from the marina.

What motoring I did do was at lower RPM; the Yanmar REALLY didn't want to go above 2500 RPM.

This year, I replaced the CLOGGED exhaust mixing elbow and noticed that the motor now revved freely well above 3000 RPM and motoring speed went from barely 5 kn or so working hard to 6.5 kn or better sounding happy!

So far so good.

Recently though, I had occasion for a longer stint of motoring; NO wind AT ALL and far from home.

I noticed that there is a VERY decent push on the tiller to port, enough so that even a few minutes on the starboard side and your arm gets pretty tired. I spend the whole trip on the port side of the tiller using my thigh to keep the tiller centered.

This does not seem normal, but I really can't remember if it's always been like this or not.

Weirder still, I have NO prop walk at all in reverse. My boat backs up nice and straight, even at low speed.

The prop is a 3 blade Campbell sailing prop, and I did notice at the last pullout that one of the blades seemed SLIGHTLY out of line with the rest. The test was imprecise .. pencil on a stand near a blade, spin the prop, note any difference in gap. One the blades DID seem out of whack to the others.

Could this account for the "pulling" ?

I'll add one more interesting event that may or may not be related. While sailing in big wind one day, we heard a rhythmic rapid thud thud thud, as if we were hitting a whole series of small logs perfectly equally spaced.

It turned out that at some point, the throttle had been knocked into reverse, completely unbeknownst to me. The "thuds" were the prop trying to start the engine in reverse!

After half a minute and maybe 30 thuds, we figured it out, plunked it back into neutral and problem solved. However, when pulling into the marina at the end of the day, the motor seemed rougher and noisier. It may be just my imagination, but my wife kinda agrees; rougher and noisier. However, it still pulls fine, there's lots of power in rough seas, it starts fine and so on.

So, I don't know if the funny starting event is related or not.


But my engine, which I used to have complete confidence in now has me wondering a bit.

The wondering became a wee worry today, Sunday. Wednesday, I have to deliver a friend home from Vancouver to Saltspring Island, about 8 hours away, and then come back the next day for work on Friday. The weather looks like the trip will be 90% motoring, meaning the whole engine thing is .. on my mind.


Any thoughts, opinions, similar experiences gratefully accepted.


Alan
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Old 07-21-2013
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Re: Prop walk in FORWARD?

If the prop is right in front of a spade rudder (like it usually is) the "vortex" of the prop wash hits the rudder on one side, causing it to turn the rudder the other way.
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Old 07-21-2013
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Re: Prop walk in FORWARD?

BTW diesel engines do like to be run hard for long periods of time. The engine gets up to temp and helps burn out gunk and crap making it run smoother...
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Old 07-21-2013
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Re: Prop walk in FORWARD?

Silly question, is it an offset prop?
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Old 07-21-2013
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Re: Prop walk in FORWARD?

I don't know the Campbell prop, but if you mean it is a typical folding prop, in reverse the blades do not always fully extend. This is particularly the case at low power as you would probably use in reverse. So the prop walk going astern may simply be masked by the prop's own inefficiency in reverse. Any single-screw installation will have a torque reaction whether going forward or astern that will pull the boat one way or the other. If your engine is a bit over-powered for the boat, it will be noticeable but nothing to worry about. Better too much power than not enough! OR, as the previous post suggests, some boats do have the shaft offset i.e. sticking out to one side of the boat's centerline. The amount of offset was originally intended to ensure that the torque reaction was best balanced going ahead. These set-ups are often terrible in reverse. You don't see it on modern boats for that reason, and on old boats it was probably optimised for a low horsepower gasoline engine. BUT, if your boat was re-engined at some stage of its life, the direction of forward rotation may have changed. With an offset shaft that would really exaggerate the prop walk going ahead.
Re the accidental selection of reverse while sailing: It is unlikely it could have damaged the engine in any way. If there was a clunking sound it may have simply been the prop folding in and out. If the prop is not a folder and was attempting to rotate the shaft it may not have been doing the gearbox a whole lot of good, but if you got on to it quickly it is doubtful that anything serious could happen in such a short time.

Last edited by arvicola-amphibius; 07-21-2013 at 09:47 PM.
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Old 07-22-2013
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Re: Prop walk in FORWARD?

I'd expect the vibration/roughness to be somewhere south of the engine, the prop in particular, that whacked blade specifically. It's possible you managed to get your engine a little loaded on diesel if it was actually being turned by the prop, some of which (being unburned) slipped past the rings and diluted your crankcase oil. This could also be some of the vibes you're feeling. Do an oil and filter change and re-evaluate.

Last edited by seabreeze_97; 07-22-2013 at 12:39 AM.
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Old 07-22-2013
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Re: Prop walk in FORWARD?

I don't reckon there is a prop out there capable of turning its own-sized diesel when the boat is sailing. Unless it is a very small diesel that somehow happens to turn a really oversize prop and the boat can sail at 20 knots. Or it is one very sick (as in no compression) diesel!

I have experienced that clunking noise from folding props at certain speeds when the blades don't quite have enough water flowing past to hold them in, or when under power, not enough is being delivered to throw the blades all the way out. Also when they get old, the pins that hold the blades to the hub, or the holes in the blade roots, get worn and can rattle a bit.

Last edited by arvicola-amphibius; 07-22-2013 at 01:23 AM.
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Old 07-22-2013
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Re: Prop walk in FORWARD?

Thanks to all who replied! I'll answer in one message here to save bandwidth.

1. Not an offset prop, completely conventional right down the middle prop shaft.

2. Not a folding prop, but a somewhat unusually designed three blade prop. Well regarded in most publications as a low drag "sailor's" prop.

3. Prop IS a bit oversized maybe, compared to a buddy with a similarly sized boat and same engine, and to other sizing charts I've seen.

Now, I too find it HARD to believe that a prop could start an engine. However, the engine was warm, having been run just a few minutes earlier and when warm, the engine starts like an electric motor; no cranking at all, press the button and instant diesel rattle.

And there was NO doubt that taking the boat out of reverse stopped the thumping right that second. And the thumping was loud, darn near the same volume as when the engine is running, just much lower in frequency .. 60 RPM as opposed to 600 RPM.

It was not a rattle, it was a THUMP. I'm not kidding, it felt like we were sailing through a log boom.

But there's the chance of low compression being involved .. the motor IS 30 years old after all. Maybe it does all add up, as weird as it seems; low compression, fast starting when warm, big prop ...

4. Good advice on the oil change, hadn't thought of that. Will get on that tonight.


Thanks again to all.


Alan
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