Using pourable urethane in partners and maststep

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  • 11 Jun 2015 14:23
    Reply # 3382853 on 3341055

    Tammy Norie has soft wood wedges (details, pictures, and diagrams here) that work very well but creak quite a bit. I'm thinking of molding two quarter-circle chocks from high density polyethelene, otherwise known as melted milk cartons. (Here's a nice video of someone making a mallet head from the stuff.) I think the density and strength are about right, and the surface is naturally very slippery and so I hope it won't creak. The other main advantage is that I can mold it in place and get a perfect fit.

    Last modified: 11 Jun 2015 14:34 | Anonymous member
  • 14 May 2015 12:46
    Reply # 3343249 on 3341055

    Jerry, if you are going to use wedges to displace volume in the pourable PU, make sure they are secure or they'll float out. 

    Rubber O rings of precisely the right size can be made using rubber cord cut to length. Gluing the ends together is not necessary, as you're going to squeeze it up. I got mine from Polymax - their 30mm silicone cord has a hardness of 60 Shore, which I think would stiffen up on being squeezed. Anything harder probably wouldn't compress easily enough. I intend to form 2 rings sandwiched between ply discs drawn together with 8 bolts. The O rings need to be contained within ply rings, leaving a gap around the mast equal to the thickness of the O rings. I reckon that this would allow the rings to be compressed by 6mm vertically. 

    I don't claim any expertise, so I'm probably not the right person to provide advice. I got the idea from a visit aboard Ivory Gull, a Maurice Griffiths Lone Gull, fitted out and rigged by her original owner, David Tyler. Her mast partners are constructed in this way - the mainmast is carbon fibre and doesn't need a mast coat to keep the rain out. The foremast is alloy and does need a coat, to prevent water lodging on the O rings and causing corrosion.

  • 14 May 2015 10:30
    Reply # 3343183 on 3341055

    Thanks all for the suggestions.

    Asmat, I had toyed with the idea of sliding loose fitting wedges into the gap between mast and partners whilst pouring the urethane to displace much of the volume, and with David's warning about overheating that sounds like a wise strategy.

    Where did you source your 30mm O-rings, and how many need to be sandwiched between the plywood flanges?

    I've just been re-reading PJR's section on partners and mast step and note the recommendation for a 1 degree angle for the partner taper and wedges. When measuring the taper on my partners I noticed the angle is more like 3 degrees....not sure how that happened but is it likely to cause a problem? I can see why a very slow taper is best but I would, of course, add small plywood pads to the wedges after installation to prevent them riding up and loosening. Am I needlessly wittering or is a 1 degree angle critical? I also note that PJR recommend a 2 degree angle for mast step wedges, presumably to ease the job of removing them when unstepping the mast.




  • 14 May 2015 01:07
    Reply # 3342792 on 3341055

    On Arcadian I used a flat rubber sheet about 6 mm thick and about the density of car tire rubber. This was wrapped around the mast and joined and fixed in place by a piece of vinyl tape used to join damp proof membranes under concrete slabs, and was the depth of the mast collar. I then blocked the base of the gap with a short piece of rope of a diameter slightly larger than the gap and filled above with thickened epoxy resin. 

    This has worked very well with no squeaking or groaning.

     If the space between the mast and collar needs to be able to be opened up for a larger section of mast or a base plate, then a second rubber sheet could be taped around the collar and the gap between the two filled with thickened epoxy. The rubber would probably need to be thinner, say 3 mm, so that the total join allowed about the same amount of compression as a single layer of rubber. 

    The vinyl tape is easily removed and the join between the mast and rubber collar is not glued so should be easily removable. I waterproofed the collar on Arcadian with two layers of canvas bedded in silicone mastic. Not the prettiest but has proved watertight to date. All materials used cost a total of about $30.00 NZ.

    Last modified: 14 May 2015 01:12 | Anonymous member
  • 14 May 2015 00:36
    Reply # 3342764 on 3341055
    At the risk of horrifying the less technically-challenged and more competent craftsmen in the JRA, I, somewhat hesitatingly, offer my bodge-up solutions.

    David T, (Pom, not Kiwi) quite rightly, told me to get my partners tapered when I had them welded up.  (Why did I go for stainless steel partners in the first place, you may well ask.  Dunno.  Probably because I didn't trust my woodwork to make wooden ones.)   It was a shockingly expensive choice and the wrong one, too, because the metalworker, when he realised how much it was going to cost, said that the additional time involved in making it tapered was going to mean I'd need to take out a bank loan to pay for it.  However, I ended up with a parrallel-sided mast in ditto partners.  I had the mast out a few months ago and with a little help from a friend, put it back with foxed wedges, ie one from top and one from underneath.  This did a pretty neat job and I then dribbled some Gorilla glue around them to keep them in place.  Seems to work perfectly, even if it's not the most elegant solution.

    The mast step is a much snugger fit.  I put lolly sticks (popsicle sticks/small tongue depressors) vertically around this.  After the first couple of sails they worked into place and I added one or two more until I couldn't fit another one.  The mast step is lined with some thick, hard, black plastic (don't ask me what exactly) to obviate (I hope!) the risk of corrosion between mast and partners.  All very satisfactory and, as the lolly sticks are around the forward side of the mast, inoffensive.

  • 13 May 2015 20:15
    Reply # 3342498 on 3341055

    Jerry, I can't see any advantage in using polyurethane rubber for the mast step. Although wedges might be used here, I'm using 30mm silicone rubber O rings compressed between plywood flanges to grip the heel, another Tyler innovation. This might be a bit more complex than wedges, my thinking was that this system would be quieter. Other people say their wedges are silent and junk rigs are noisy anyway, with creaking batten parrels etc. sawing at the mast.

    If you are concerned about the expense of the polyurethane jollop, perhaps you can combine it with wooden wedges, not necessarily driven firmly home, just embedded to reduce the volume needed by displacement.

  • 13 May 2015 16:59
    Reply # 3342239 on 3342073
    Jerry Stebbing wrote:

     This means I wouldn't be able to reduce my current partner diameter by much at all, requiring a huge amount of urethane to fill the 25mm all round gap.

    Just one more question, is it necessary or even desirable to plug the base of an aluminium mast. My 222mm dia, 5mm wall thickness, spun taper lighting pole section seems strong enough without an end plug, but what does everyone else do?

    The urethane that I've used needs a maximum thickness of 1/2in, to avoid overheating.

    Not necessary nor desirable, I think. You need to have any water that trickles down make an easy exit through drain holes in the step. My mast just stands on the heavily-epoxied plywood.
  • 13 May 2015 14:58
    Reply # 3342073 on 3341055

    Thanks for the advice. I've just purchased some epoxy resin from MB Fibreglass so I'll know where to go for the urethane if I decide to use it instead of conventional wedges.

    Just one thing occurs to me. As my mast is parallel from base to well above deck level I would need to keep the internal diameter of the partners large enough to pass the base of the mast with its bonded on urethane chock through when unstepping/restepping the mast. This means I wouldn't be able to reduce my current partner diameter by much at all, requiring a huge amount of urethane to fill the 25mm all round gap.

    As I see it, the only solution to minimizing the gap between mast and partners would be to add an external sleeve to the mast, but I'm not keen on that idea. So perhaps softwood wedges might still be the simplest solution for a parallel mast.

    Just one more question, is it necessary or even desirable to plug the base of an aluminium mast. My 222mm dia, 5mm wall thickness, spun taper lighting pole section seems strong enough without an end plug, but what does everyone else do?

  • 12 May 2015 21:39
    Reply # 3341096 on 3341055

    Jerry,

    I've just taken delivery of some pourable polyurethane rubber for my mast partners - Polycraft FC8000, Shore A80 hardness, supplied by MB Fibreglass, here in the UK. It is much cheaper than the product you are intending to use: (£90 for 5.5 litres). 

    I don't know why there's such a price difference - perhaps you can shed light on this. Shore 80 should be hard enough, I believe. David Tyler is the expert in this and I hope we'll get a response from him. By the way, it's worth having a look at the Spartite website, which gives step by step instructions on using their product.

    Asmat

    Ah - David and I were both responding at the same time.

    Last modified: 12 May 2015 21:41 | Anonymous member
  • 12 May 2015 21:29
    Reply # 3341083 on 3341055

    Jerry,

    If you made the partners for softwood wedges, then I'd use them. The pourable urethane is very useful in cases where the gap is too small for wedges, the taper is uneven, the mast passes through at an angle, etc, but it isn't necessarily better than wedges if you have nice conical hole, concentric with the mast. 

    However, if you do use urethane, go for the minimum gap through which you can drop the mast, as there's no benefit in having the urethane too thick. Say a 5mm gap.

    The step should be as deep as the partners.

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