Any outboard props have 17 splines? Volvo folding prop conversion away from alloy.

Discussion in 'Props' started by DennisRB, Feb 12, 2016.

  1. DennisRB
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    DennisRB Senior Member

    Short story: I have the dreaded Volvo 3 blade folding props on my SD20 saildrives powered by 3GM30s. I want to eliminate the alloy part of the hub by casting polyurethane into the bronze hub casing to surround a 17 spline insert so the result would be much like an outboard propeller.

    I am searching for source for 17 spline inserts preferably in stainless steel, but anything other than Al would do! Where can I get these? Does any outboard have 17 splines? Seems they top out at 15 splines?

    Long story:

    The hub insert is made out of aluminium, so the splines are Al. After talking to a few people and reading reports on the internet these often have horrific corrosion issues and extreme ware rates on the prop anodes and eventually the splines. A prop guy told me "they should last a few seasons". A few seasons? This is unacceptable IMO.

    My boat came with Al fixed blade props which do not have any corrosion issues. There is no electrolysis issue. Its just the folders which have the prob. The saildrive anodes last 1.5 years.

    My boat came with the folders as "spares". They were swapped out by the previous owner for some reason, but I think I can guess. I do not want fixed blade props on my boat as I value sailing performance so went about installing the folders. I have tried both props and the folders performance as good if not better in forward even though the folder has to push against the drag of the fixed prop on the other hull. When I first relaunched the boat about 1.5 years ago I had one Al hub insert replaced for $450 as it was very loose on the splines due to corrosion. This was at a prop shop. Volvo said this was not even possible and said the hub is just a ware item and I should pay $950 for a whole new hub. The one I had replaced is about ready to fail if I was to guess.

    I expressed concern about the other one too as it was loose as well. The prop guy told me that is just what happens with these and I might get another season out of it before it needs doing. Anyway it stripped out recently while cruising and I swapped it with the fixed prop again. Huge performance loss (up to 2 knots) to windward when the fixed prop is on the leeward side due to the drag of the prop itself, but it also kills the flow over the rudder meaning huge rudder angles are required.

    I talked to a shop who casts polyurethane for all sorts of industrial jobs including drive shaft couplings on massive mining machines. He guessed the job might cost about $50! If I can centre the spline insert in the hub and braze a few ribs inside the bronze hub for grip I think this is worth a shot. Not only will this be cheaper but the result will be permanent and probably stop the corrosion issues caused by having an Al hub!

    Thoughts?
     
  2. gonzo
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    gonzo Senior Member

    You can press-fit a polyurethane spline and use two or three set-screws for safety. There isn't that much torque generated by a small diesel, so it should work fine.
     
  3. DennisRB
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    DennisRB Senior Member

    My idea was not to make the splines out of polyurethane, but to bed a splined hub insert into the bronze outer hub casing of the folding prop using setting liquid polyurethane. This would not require any machining if I can find a splined insert to suit, compared to making a custom poly insert of exactly the right dimensions so it would work as a press fit.

    I could just used the sail drive splined output shaft as a mould and make poly splines but I doubt this would last long. There is a torque multiplication in the saildrive.

    So I still just need a 17 spline hub insert. They are commonly available for outboard props but it seems they only go up to 15 spline. So I am having trouble locating a source for 17 spline inserts.
     
  4. waikikin
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    waikikin Senior Member

  5. DennisRB
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    DennisRB Senior Member

    Thanks, I did eventually find that. Also I found this with pics. It has some 17 splines, but all with rubber fused to them. I assume the grey ones are Al which I do not want. Would it be safe to say the gold ones are bronze? I will contact Austral.

    http://www.hmshubs.com/HMS Picture Index.pdf
     
  6. DennisRB
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    DennisRB Senior Member

  7. philSweet
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    philSweet Senior Member

    About that "braze in some ribs" idea -

    from here - http://img2.tapuz.co.il/forums/1_146973682.pdf
     
  8. Chuck Losness
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    Chuck Losness Senior Member

    Have you thought about having a prop shaft made with 15 splines which would fit the 15 spline inserts? Any machine shop should be able to do this. Or could a 15 spline prop shaft be modified to fit or mated to the volvo prop shaft?
     
  9. DennisRB
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    DennisRB Senior Member

    Phil. The ribs would just be small pieces on the inside of the bronze hub so that the liquid casted polyurethane would have something to grip on. The poly would act as an insulator. At the moment I have perfect anode life on the sail drive. Its just the volvo props which have the issues.

    Another idea would be to have a bush machined up and press fitted into my larger bronze hub. This would fill the gap allowing the Yamaha 350hp cushion lock hub to be pushed in as usual. The issue would be the folding props are much heavier so it might not hold up to it. But then again I only have 27hp not 350hp!n Seems this hub is very expensive, 15 spline and 19 spline versions are cheap. 17 must be a rare size.

    Chuck, if I had to resort to getting something machined up I would get a 17 spline insert machined up. That makes more sense than a new shaft.
     
  10. Barry
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    Barry Senior Member

    Will the the inside of the prop hub be smooth?
    Ie are you going to rely on just the adhesion of the urethane to the hub interfaces?
     
  11. DennisRB
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    DennisRB Senior Member

    No, thats what I was talking about re brazing ribs inside it. But if I go for the rubber cush lock press insert it would rely on friction. The volvo hub will be too big for it to fit so I would get another plastic sleeve press fitted into it first. This would rely on a tight press fit, but I wonder if those type of hubs could handle the heavy weight of these props as they would probably be much heavier than even a 350hp outboard prop.

    https://www.youtube.com/watch?v=PDrVPRwCa2M
     
  12. philSweet
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    philSweet Senior Member

    Okay, I had imagined you were centering the hub with the ribs.
     

  13. DennisRB
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    DennisRB Senior Member

    That might be a plan too. Centring will be critical.
     
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