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Mast dimensions

  • 11 May 2016 22:19
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    Reply # 4014185 on 3996301
    Anonymous member

    Go for the 5". An unstayed mast that flexes is an unpleasant creature to live with. The less flex the better.
  • 11 May 2016 22:14
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    Reply # 4014182 on 3996301
    Anonymous member

    No worries Mark, seems like mast dimensions are a hot topic. I have another question for the experts out there..

    I'm embarking on a junk conversion of a Pearson Packet Overnighter, 18ft. The book says base diameter 6.5" at the base for my proposed 200sq. ft sail (but this is for a wooden mast I assume).

    I've checked the JRA spreadsheet on other designs for boats near my size and for an aluminum pole base diameter is from 4-5.9" and length is 20-23ft, I calculate top diameter to be 2-2.5"

    So I've found two flagpoles locally at a reasonable price. 1/8" wall thickness.

    One 23ft 3.5" base and 2.5" top and one 34ft 5" base 3" top

    I'm thinking the 5" but will that give me enough flex at the top?

    Appreciate your help:)
  • 11 May 2016 13:23
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    Reply # 4013462 on 3996301
    Anonymous member

    Mark,

    sorry to hear that the IW30 project has faltered, she is a lovely boat, but as you say, rather a lot of work.

    I did think the wooden mast looked superb, though slightly odd on a modern plastic boat, probably more so with the new one.  It did occur to me that the mast would be far better on an old gaffer, I suspect you may be able to sell it if you are lucky, and end up with a lighter & more suitable mast.

    Best of luck,

    Mark

    ps, my daughter has just gained a place for a year of her Engineering course at Trondheim University, so I should be making another visit to Norway this summer. (not in your area I realise).

    Last modified: 11 May 2016 13:24 | Anonymous member
  • 11 May 2016 12:50
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    Reply # 4013431 on 3996301
    Anonymous member

    Then go for it. Check out this album for hints on cutting the shoulder, and assembly.

    Further thoughts: Since you are using a tube of such great wall thickness, I think the shouldered portion is going to need to be almost solid, with just a small conduit up the centreline. If the mast is solid, no problem, but if it is of one of the more sophisticated staved constructions, you'll need to make a tapered plug to glue inside, extending above the shoulder. At least the fitting process will be easy - apart from the likelihood of gobbets of set glue on the inside of the cavity.
  • 11 May 2016 11:44
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    Reply # 4013342 on 3996301
    Anonymous member

    Hi David,

    It would be possible, but I baulked at doing the work myself, particularly at the bottom, where loads are high and mistakes could be poorly repaid.  The same company that quoted for a replacement also quoted for an extension, but with high Norwegian labour prices, it would have cost almost the same...

    Mark
  • 11 May 2016 11:05
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    Reply # 4013283 on 3996301
    Anonymous member

    Another thought - is it not possible to scarph on an extra metre, either top or bottom, if the rest of the mast is good?
  • 11 May 2016 09:50
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    Reply # 4013231 on 3996301
    Anonymous member

    Thank you David.  I will go for it then, although the thought of cutting off the bottom half of the mast is rather scary!

    Mark
  • 09 May 2016 20:48
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    Reply # 4010419 on 3996301
    Anonymous member

    Sounds feasible to me, Mark. At this size, I have Tystie's mainmast to compare with (before I lengthened it). It was 12m long, with a base diameter of 220mm and wall thickness of 5mm, and made of T5 alloy. There was a doubler inside the bottom, parallel part, made of the same tube, but because it was split longitudinally to reduce its diameter, I didn't reckon that it contributed as much as a complete inner tube would have done. Let's say that the assembly was equivalent to a tube 220mm x 7.5mm wall. It was carrying sails up to 58 sq m, on a boat of 8 tonnes displacement. Now let's look at your tube. 200mm diameter x 10mm wall, T6 alloy is about equivalent to my 220mm diameter x 7.5mm wall, and it's on a boat that's half the displacement, with a smaller sail - I'd say you have a good margin of safety there. As to the topmast, as long as you obey the rule for bury, and put 600mm inside the tube, and as long as the wood is sound, and the wall thickness is great enough to be able to cut the shoulder to go into the tube without weakening it unduly - I think it all looks good.
  • 09 May 2016 20:25
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    Reply # 4010407 on 3996301
    Anonymous member

    Sorry to hijack this thread.

    I have taken the rig off my IW 31, reckoning it needed too much work, and bought a Maxi 95 (also in need of work, but easier) to put the rig on.

    However, the mast is too short. The Maxi has greater bury and I would like to add a bit of extra length at the top to reduce the twist in the halyard at full hoist.  A metre of extra length, giving total dimensions for a wooden mast of 12.1 metres, 1.6 metres bury and 26 cm diameter at partners.  Sail area is 48 sq m.

    A local mast making firm will nock me out another wooden one for about 20000NOK (roughly 2000GBP).  A stepped aluminium affair would cost a little more, although the firm that would make it are very cautious about committing to sizing and the such like, insisting that I write them detailed sizing instructions and they will offer no guarantees.

    The question I would like to throw out there is whether I could consider a hybrid mast at this size?  I have a perfectly good wooden pole that is 20 cm at 6.1 m from the top.  A six metre length of 6082 T6 at 200 mm and a wall thickness of 10mm would cost half of a new wooden mast, so the financial incentive is great. But is it sensible/feasible/safe on such a big rig? Is my guess at 10mm wall thickness about right?  All comments welcomed...

    Mark
  • 01 May 2016 18:34
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    Reply # 3997285 on 3996301
    Anonymous member

    1/8" or 3mm would be the norm, and optimal, for a mast for a sub-20ft boat.

    Last modified: 01 May 2016 18:34 | Anonymous member
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