Building a mast from aluminium and wood

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  • 07 Nov 2013 11:23
    Reply # 1429624 on 1144363
    Thank you David, you have been most helpful. It shouldn't be too long and I will find out. In any case I think it's now a calculated risk. I was quoted $22000 for two prof. made aluminium masts 250mm OD and 6mm WTH at 240kg each. If this doesn't work out I will build two staved mast and still be ahead.  
  • 05 Nov 2013 16:16
    Reply # 1428176 on 1144363
    I don't think that will help, Dieter. Only more wall thickness will help. But I think the diameter of your tubes are such that they will be strong enough.
  • 05 Nov 2013 13:43
    Reply # 1428079 on 1144363

    This is what I’m concerned about but will take that risk unless I find a sensible way to stiffen it. I don’t know if this is one or just a crazy idea.

    Looking at bamboo and how it is grown would it help if I could insert a number of bulkheads like discs in foam/glass sandwich (left over from cut outs) and fix them with expanding foam in to the tube?

  • 03 Nov 2013 20:55
    Reply # 1427043 on 1144363
    I'm a little uneasy about the wide difference in wall thicknesses, but I guess it will be OK.
  • 31 Oct 2013 23:23
    Reply # 1425786 on 1144363
    Thinking this through again I'm sure you'r right. I will follow this advise, abandon the stiffening  and make the collar to accommodate spartite and prime the tube to lessen the risk of corrosion in this area. As it happened the yard manager spotted the tubes and did take an interest on how I intend to use them. His concerns where much along the same line as yours in this regard.
    Thanks again. I'm feeling comfortable about the first 6 meters of the mast.
    I hope you don't mind me asking more questions.
    What are your thoughts on the next 7.5m. I have 203.2 OD x 3.0 WT x 6.000 and 4 of them. Does the next section  203.2 OD x 3.0 WT need stiffening before fitting it in to the lower section? I have given up on the idea of squeezing sections in to each other. 
     
  • 30 Oct 2013 23:11
    Reply # 1424976 on 1144363
    I would want the collar-to-mast to be very slightly flexible, to accommodate slight flexing of the tube, heat expansion and contraction, and suchlike things. I'm not convinced that a two-piece split tube over the mast tube will add anything beneficial, and I doubt whether a glass/epoxy collar would adhere to alloy in the long term. That's why the poured polyurethane joins (Spartite, etc.) between mast and collar are preferred.
  • 30 Oct 2013 22:12
    Reply # 1424930 on 1144363
    David,
    You'r right, about 450 square feet is planed. I have been watching your posts over the years and your masts seem to be the least  of your concerns.
    I think I will go without a doubler inside the tube and take a chance.
    What are your thoughts on two half shells around the outside at the partners glued with Fixtech polyurethane adhesive sealer prior to glassing on the collar with epoxy. If not for strength would it protect the mast tube from corrosion around the collar?
  • 30 Oct 2013 21:25
    Reply # 1424909 on 1144363
    Dieter,
    My mast is 220mm dia x 5mm wall, with a 5mm doubler of dubious effectiveness. It is carrying a sail of 630 sq ft. I would say that your masts of 219mm dia x 8.2mm wall should be able to carry the sail area necessary for a 40ft cat, which I would estimate as two sails of up to 500 sq ft. Would that be a fair guess?
  • 28 Oct 2013 22:14
    Reply # 1423272 on 1422950
    David Tyler wrote:Dieter, I think the answer is "very difficult!", and probably too difficult for anyone who has little experience of alloy fabrication. A big "come-long" with a strong wire was used, and a tab was welded to the doubler for hooking on the wire, in such a way that the wire could be un-hooked after the job was done. The doubler could be greased, or lubricated with slow-setting epoxy. The only penalty for failing to get it up into the required position is that the excess would have to be cut off and the idea abandoned, so it is worth trying on a small mast, but too difficult on a large mast. 

    In any case, it is debatable how effective the doubler is, since a tube that has been cut longitudinally loses many of its properties. I would say, just choose a tube that has the required properties without needing a doubler.
    The closer I look at this the more difficult this could be. The question is now did I choose the the right tube in the first place at 219.0 OD X 8.20 WT and do without a doubler or stiffen the tube around the outside with glass. Since I h ave a glass collar at the partners I could extend it to 1meter above and below.
    Thinking if the benefits of a split inside sleeve are doubtful is the stiffening on the outside the way to go. Since I have the tubes could I cut them in halves, glue them around and glass and fair it over. Like all jobs on the boat things change and evolve as it gets from the thinking stage in to the doing of it.
    Tank you again.
     


  • 28 Oct 2013 15:54
    Reply # 1422950 on 1144363
    Dieter, I think the answer is "very difficult!", and probably too difficult for anyone who has little experience of alloy fabrication. A big "come-long" with a strong wire was used, and a tab was welded to the doubler for hooking on the wire, in such a way that the wire could be un-hooked after the job was done. The doubler could be greased, or lubricated with slow-setting epoxy. The only penalty for failing to get it up into the required position is that the excess would have to be cut off and the idea abandoned, so it is worth trying on a small mast, but too difficult on a large mast. 

    In any case, it is debatable how effective the doubler is, since a tube that has been cut longitudinally loses many of its properties. I would say, just choose a tube that has the required properties without needing a doubler.
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