Mast Candidates

  • 14 Sep 2021 20:31
    Reply # 11088715 on 11057048

    Thank you everyone for your insights. I think we'll be getting our hands on the 32" spar for this project. Once it's in the yard I'll be able to start fashioning the receiving cup for the keel step and properly build up the partners. 

  • 14 Sep 2021 09:43
    Reply # 11086689 on 11057048
    Anonymous member (Administrator)

    Adam,
    I am sure David W.’s calculations are good. Anyway, the C&C 25 is less than two tons, so a mast with 8’ diameter and 4-5mm walls should be plenty stiff and strong  -  and then a bit more.

    Have you checked the weight of the mast? My only worry is that it might be on the heavy side for the C&C 25. The Nonsuch 26 appears to have twice the displacement of the C&C.
    Even the Nonsuch 22 is a bit heavier than the C&C 25, so it could well be that the smallest mast you are considering would be a better choice.

    Arne


    Last modified: 14 Sep 2021 09:45 | Anonymous member (Administrator)
  • 14 Sep 2021 05:35
    Reply # 11084654 on 11057048

    Hi Adam,

    assuming a sail area of about 300 square feet with a center of area about 18 feet above the water then the righting moment for the rig at 20 miles per hour wind is about 5400 foot pounds. The strength of the smaller of the masts, using a strength for the aluminium of 20,000 pounds per square inch, will resist a moment of over 20,000 foot pounds. The longer mast is not quite as strong but both would give a safety factor of above 3 which is more than adequate for most purposes.

    Arne has a different calculation method, but we usually come up with pretty close results.

    All the best, David.

  • 14 Sep 2021 03:26
    Reply # 11083587 on 11057048

    Re: the diameters and which Nonsuches 

    32' belonged to a nonsuch 22, her diameter is 8.5" at base to 4.5" at mast head. 

    36' belonged to a nonsuch 26, her diameter is 8" at base to 5.5 inches at mast head. 

    Re: to the comment about nonsuch masts being unstiff. I wondered about that too, however, part of the customary bend has to do with the pressure exerted by the wishbone on the clew of the sail. There's a lot of pressure used to keep that sail flat. I'm not sure i'd be facing the same kinds of forces. And the diameters and thicknesses are no less than what I'd be getting as a lamp post. 

    Also - at $800, I'm unlikely to lose much sleep if I get it wrong :)

    Re: Arne - thank you for responding. I suspect you are correct but a sober and disinterested second opinion is always welcome :D 

  • 12 Sep 2021 21:55
    Reply # 11068306 on 11057048

    The most critical dimension for strength consideration is the diameter of the mast. What are the diameters of the masts under consideration? This will allow some rough calculations to be made as to strength and hence suitability for your purpose.

  • 12 Sep 2021 16:46
    Reply # 11065555 on 11057048

    I'd think a tapped hole with nothing in it would be an opportune place for a stress fracture to start.

  • 12 Sep 2021 09:15
    Reply # 11061773 on 11057048

    Adam,

    Do you know what size of Nonsuch these were fitted to? I can't find a luff length for the Nonsuch 22, but for the 26, it was 41ft, and it seems unlikely that either of the masts under consideration was originally fitted to a boat of similar size and righting moment to yours. A mast for the Nonsuch 22 would not be stiff and strong enough, I fear.

  • 12 Sep 2021 08:56
    Reply # 11061638 on 11057048
    Anonymous member (Administrator)

    Adam,

    the choice of mast depends of the diameter of the two candidates. If the diameter is big enough, I would go for the one with the thinnest wall thickness.

    I would not worry about that row of small rivet holes from the sail track. After all the Nonsuch masts have held up well, as far as I know.

    As for that bolt; your own suggested way of dealing with it makes sense to me.

    Good luck!
    Arne


    Last modified: 12 Sep 2021 15:39 | Anonymous member (Administrator)
  • 12 Sep 2021 00:00
    Message # 11057048

     

    Alright - mast question for the subject matter experts - especially Arne but anyone else with build experience.

    As part of my conversion of a C&C 25 I've been hunting for a mast.

    Two masts: One 32' the other 36' have been found. I only need 31 feet.

    They are aluminum extrusions. Both formerly from Nonsuch's.

    Both are sleeved.

    32' thickness tapers from 7mm to 5m

    36' thickness tapers from 5mm to 4mm

    The challenges:

    One extrusion has had holes tapped into it for a sail track. Does this pose a structural issue?

    The other - where the top and bottom sections meet has a bolt that protrudes. I could ... in theory ensure that the bolt that protrudes has its profile reduced, and that a small fairing be glassed over it to ensure that the parrels or battens slide over it. As it is - there's a real risk of it snagging either or both.

    These are the best two candidates to come along in over a year's search. Everyone seems to be squirreling away their lamp posts. OR in the case of the Southern Ontario aluminum extrusion industry, they will not produce extrusions unless you are ordering in batches of 10,000.

    And before the suggestions flow about a wooden mast, that option is my very very last resort as I'm fully aware of my limits as a carpenter.


       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

                                                              Site contents © the Junk Rig Association and/or individual authors

Powered by Wild Apricot Membership Software