Another mast problem

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  • 28 Aug 2019 00:13
    Reply # 7852225 on 7851186

    I am no engineer, but it seems to me that you could use solid timber at the lower end of the topmast, where it is buried into the aluminium tube, and for the same distance above the join, then use hollow sections above that.

  • 27 Aug 2019 23:46
    Reply # 7852199 on 7851186

    Hi Steven,

    one thing you can do is construct the bottom of the mast with a solid wood section sleeved inside the aluminium tube and extending just above the deck. That will allow you to raise the aluminium tube section which will shorten the top wooden section by half a meter or so and could be the difference you need to get the topmast within your stress parameters. It will also strengthen the aluminium tube at its maximum stress point where it passes through the deck and will allow you to taper the lowest section down to the mast step.

    All the best with the project.

    David.

  • 27 Aug 2019 20:52
    Reply # 7851879 on 7851186
    Anonymous member (Administrator)

    Is this the one you mean?

    Arne

  • 27 Aug 2019 10:59
    Message # 7851186

     

    Hi,

    Things have moved on a bit and it now looks fairly certain that I will be buying the Folkdancer hull for a nominal sum. I am still trying to contact the Folkdancer group to get a copy of the sail plan but till then I am assuming that the sail arrangement will be very similar to Arne’s Ingeborg. As I plan to build a cambered sail I will unashamedly base the design of the rig on Arne’s rig.

    The Folkdancer is a bit heavier than the IF at 2540kg so the righting moment goes up to just under 2500kpm using a strength factor of 2. This means that if I was building a hybrid mast of aluminium and timber I would be using an aluminium tube of 6” outside diameter and a wall thickness of ¼”. This is the best fit for the calculated bending moment as there are not a lot of wall thickness sizes available. Another disappointing thing is that the maximum length available is 5000mm. I intend to build the upper wooden section from Douglas Fir and this is where my problem starts. If I built the top section with a wall thickness of 1/5 diameter then the bending moment of the mast at the joint is slightly greater (due to the shorter aluminium tube) than the strength of the wood. Should I make the wall thickness greater or should I have a total rethink? Aluminium top section as well?

    Thanks, Steve

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