How calculate mast base strength rotating mast?

Discussion in 'Sailboats' started by Rinus, Aug 28, 2012.

  1. Rinus
    Joined: Aug 2012
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    Location: Netherlands

    Rinus New Member

    Dear all,

    Half a year ago my girlfriend and I bought a modified Fox 22. The thing hasn't been sailing since it was built and was never really completed.

    I am now trying to find out wether the mast base is strong enough to withstand the forces of the rig. Problem: the boat doesn't actually go by the rules, so I don't know which calculations I can use straight out of the books. Question: which calculations seem useful for my boat? I must admit that I have not finished the Principles of Yacht design yet, but after reading all rig-specific parts, I couldn't make it work yet.

    My boat: 22 foot aramid/glass vinylester/epoxy keelboat with carbon rotating mast (probably an old cat mast) and water ballast. Marine grade plywood bulkheads, hull solid laminate, deck foam sandwich, all laminate with aramid. All three shrouds on one front shackle. Light jib furler. Mast stepped on ball/rod/plate on deck. Mastsupport inside: thick stainless tube. Lead torpedo keel with full metal fin. Halyards fixed to clutches on mast.

    Estimated dimensions in meters (sorry not used to imperial)
    LOA: 6,60
    LWL: 6,00
    Bmax: 2,50
    Draft: 1,80
    Air draft: 11,10
    Mast step above waterline: 1,50
    Mast length: 9,60
    Mast weight: max 25 kg
    I: 8,10
    J: 2,20
    P: 9,30
    E: 3,60
    Displ.: 1000 kg
    Ballast weight torpedo: 300 kg
    Ballast tanks: 150 liters x2
    Sail surface: 95% HAjib 8,46+main 19,25=27,71 m2 say 28 m2 (maybe more, fully battened and lot of roach, used 115% of PxE/2)

    My concerns: will the mast base, consisting of a metal rod with delrin/teflon/whatever plastic ball be able to withstand the forces of the full rig, considering the weight of ballast and draft of the keel? Obviously, on catamarans the rig works fine. But will it also do so with the righting moment of all the ballast?

    It isn't an official mod, so the designer did not have any specific drawings or specs on it.

    What do I need to calculate the forces? Where do I start?

    For good measure: I don't intend to race it, but like it to be able to take it to sea/rough waters.
  2. capt vimes
    Joined: Apr 2009
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    capt vimes Senior Member

    eric mentioned in his papers linked here the load calculations for a free standing mast...

    the basics are the same - the maximum force a mast has to withstand is the maximum righting moment... give it a safety factor and work from there...

    what i recall, a stayed mast brings mainly compression loads into the maststep - all other loads are transferred via shrouds and stays which then brings compression into the deck structure and tensional forces into the keel.
    take the righting moment multiply it with the safety factor, calculate the forces in the shrouds (it is a simple load-triangle) and the compression along the mast, dimension them with an additional safety factor accordingly...

    i am no naval architect of any sort, got this from larsson and elliasons "principles of yacht design" and do not know if my memory serves me right here... :rolleyes:
    why not get a copy of this book or any other out there and go through it?
    or take some money and hire a professional? - which would be the smartest idea... ;)
  3. Rinus
    Joined: Aug 2012
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    Rinus New Member

    Thanks for your reply :)

    Is it really just the compression? No forces sideways? Calculating the righting moment should be doable.

    I got myself a copy of 'The Principles', but I was wondering whether with a rotating mast on a somewhat (the rod is about 10 cm) higher base point the same calculations would be applicable.

  4. johneck
    Joined: Nov 2011
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    Location: New England

    johneck Senior Member

    The righting moment is a reasonable way to start to get an idea of the bending load, but there is also additional loads due to rig tension used to hold the forestay tight and of course the aerodynamic loads on the sails. There are certainly side forces on the step and depending on rigging geometry possibly not insignificant axial forces as well. A free body diagram of the standing and running rigging should be able to give you an idea of how these forces go into the boat from the various attachment points.
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