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Thread: Prop Knocking

  1. #21
    Join Date
    Oct 2012
    Location
    Southminster, essex
    Posts
    9,602

    Default Re: Prop Knocking

    Quote Originally Posted by Praxinoscope View Post
    Wow, something somewhere is seriously out of alignment, I hope it hasn’t been vibrating like this since installation.
    Dunno how long it has been like this. I bought it from the Royal Corinthian YC last year & they must have had issues. But no problem. It was bought at a good price & I will sort it.
    It is all down to the fact that my wife does not understand me !!

  2. #22
    Join Date
    Oct 2001
    Location
    Solent
    Posts
    197

    Default Re: Prop Knocking

    As Spyro has written, I wonder if this is the shaft hitting the stern tube sides. These are heavy propellers and at low rpm the shaft can oscillate in the stern tube but centres at higher rpm. I have cured a similar problem by replacing the rubber-mounted face seal with a conventional stuffing box on a stiffer rubber tube. This eliminated the knock at low rpm.

  3. #23
    Join Date
    Jan 2005
    Location
    Dorset
    Posts
    4,530

    Default Re: Prop Knocking

    Quote Originally Posted by Norman_E View Post
    Other possibilities are engine mounts that are too soft or tickover too slow once in gear and a bit irregular.

    One question: Is the propeller pitch the same in forward and reverse? If they are different that might explain it only happening in forward, and a pitch adjustment might solve the problem.
    That's what I am going to ask Darglow later today.

  4. #24
    Join Date
    Jan 2005
    Location
    Dorset
    Posts
    4,530

    Default Re: Prop Knocking

    Quote Originally Posted by RobF View Post
    Out of interest, does it also make this noise when the boat is moored against a pontoon or similar and there is plenty of resistance to stop the folding / unfolding? I suspect it solely makes it whilst going along.

    You may have some joy with some white grease, which should dampen the fold / unfold speed. Might be worth applying this to the prop the next time the boat is out of the water??
    Tried it tied up in a marina and just the same. No engine vibration at all. Take it up to 1100 rpm and all sounds sweet.

  5. #25
    Join Date
    Jan 2005
    Location
    Dorset
    Posts
    4,530

    Default Re: Prop Knocking

    Quote Originally Posted by neilf39 View Post
    I can't see how the engine slowing between strokes would cause it. If the propshaft is rotating at all there is positive water pressure on the blades holding them in place. They don't have any mechanism trying to push them to feathering. Badly worn hub bearings I guess could cause it but if it is new that seems unlikely. The hub did have grease injected with a grease gun I presume. I have a centaflex coupling and don't have any issues. I have an 8x15 three blader on a Konsort with the beta 25. There is a small clunk as you engage gear if you are going in the opposite direction to that last used but I would expect that.
    I see you Have a Westerly Konsort I have a Storm. It's nice to here that the combination of a Darglow featerstream and a beta 25 can work.
    The boat is now out of the water for the winter. Up until two weeks ago I was certain it was the cutless bearing, but wear on it is minimal.
    I had a left handed 2 blade prop on before, 18x15 which I was told was a bit to big, by Darglow, however it worked well with the old VP2002. There is plenty of clearance all round.

  6. #26
    Join Date
    Nov 2001
    Location
    scotland
    Posts
    3,457

    Default Re: Prop Knocking

    You refer to a flexible coupling do you also have flex mounts and stern gland?

  7. #27
    Join Date
    Jan 2005
    Location
    Dorset
    Posts
    4,530

    Default Re: Prop Knocking

    Quote Originally Posted by scottie View Post
    You refer to a flexible coupling do you also have flex mounts and stern gland?
    I have an R&D flex coupling joined to a shaft that goes through a radice seal. I thought all mounts flexed.
    I have to emphasize, the clattering/knocking sound does not happen in reverse, or neutral, only at about 900 rpm in forward, as soon as the revs hit 1100 it purrs along nicely.

  8. #28
    Join Date
    Nov 2001
    Location
    scotland
    Posts
    3,457

    Default Re: Prop Knocking

    Quote Originally Posted by KREW2 View Post
    I have an R&D flex coupling joined to a shaft that goes through a radice seal. I thought all mounts flexed.
    I have to emphasize, the clattering/knocking sound does not happen in reverse, or neutral, only at about 900 rpm in forward, as soon as the revs hit 1100 it purrs along nicely.
    Speak to R&D their couplings often are able to break the two out of three flex rule but worth checking with them

  9. #29
    Join Date
    Jan 2005
    Location
    Dorset
    Posts
    4,530

    Default Re: Prop Knocking

    Well well, just had a long talk with Darglow, who make the propeller.

    They are aware that there can be a problem with engines that have a light flywheel, like the smaller Beta range. They even have a name for it "Chattering".
    From my understanding, it sounds as if It's when there is not too much thrust and the boat is moving forward, the prop tries to feather then goes back into drive, and as it does it clatters on the stops.

    He said why not ideal it doesn't damage the prop, but can cause concern if you don't know what it is.
    He went on to say some people just lived with it, some had been cured by an increase in the pitch, so he was going to sort out a new pitch cassette for me, which is relatively easy to change.

    On the whole Darglow a very friendly and helpful company, and the quality of their props and backup is superb.
    My only criticism is if they new this "chattering" could happen with certain combinations maybe a word of warning would have taken away any uncertainty I had during my summer sailing.
    Plus the expense of going into a marina and getting a diver down twice to look at things.

  10. #30
    Join Date
    May 2017
    Location
    Norn Iron
    Posts
    113

    Default Re: Prop Knocking

    Very interesting.
    I've got a folding prop and a Yanmar 2GM20, and at certain revs there's a distinct knocking from the stern of the boat.
    I replaced the stern gland this year and that hasn't made any difference, so it'll be a winter investigation, but in general I just work around it. I've spent long enough riding motorbikes that vibrate like crazy at certain revs to know that the best solution, generally, is not to ride at the revs that cause the vibration!

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