mast design

Discussion in 'Sailboats' started by flipster, Aug 7, 2014.

  1. flipster
    Joined: Aug 2014
    Posts: 1
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    Location: wales

    flipster New Member

    Dear guys and girls - please see attachment - I could do with some advice.

    Last w/e, my 10m, aluminium, twin spreader, 7/8th, aft-swept mast for my 26 foot cruiser racer (approx displacement 2500kg) failed whilst sailing in only a F4. Mast and all rigging less than 14 months old.

    15 -18kts wind, medium chop
    full main, no 3 genny, close hauled, (doing very well!!)
    backstay - half to third of 'max'
    kicker/vang - 'snug'
    main sheet - tight with traveller centered - playing it down-track in gusts
    crew 4 (3 on rail)
    Rig set up - in accordance with manufacturer's guidelines - about 20% of Break Strain of s/s 1x 19 wire shrouds (backstay and sails off) - approx 400-450kg/KN.

    Unfortunately, as we sailed, a block in backstay cascade failed with a loud 'twang and plop' noise. Eased sheets and then noticed pronounced bend in mast to windward. sails down. On return to berth, eased rig tension but bend got worse and had to 'splint it' and lower it before it broke entirely.


    1. what sort of tension could one expect in the shrouds/backstay?
    2. Would the backstay release (and any subsequent oscillations) really cause 'stress fractures'.
    3.Even if the set up was grossly wrong, or I had mis-set controls (over/under tension/over canvassed) should a mast fail so spectacularly in only 15kts of wind, even if backstay went?
    4. Should the presence of halyard exit slots really weaken the mast so much?
    5. Could this happen again or to someone else?


    Attached Files:

  2. NavalSArtichoke
    Joined: Oct 2013
    Posts: 431
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    Location: GulfCoast

    NavalSArtichoke Senior Member

    From the image provided, it looks like this mast is some sort of extruded piece with a roughly rectangular cross section.

    IMO, the long slot for the halyards does provide a location where the mast will be relatively weaker than a location where no such openings are located. It's not clear from the image how high above the deck this slot is located.

    Is this slot located only on one side of the mast or both? If you must penetrate the mast, keep the size of the openings to a minimum. If possible, the openings should be reinforced, either with a ring insert or with a doubler plate welded to the outside around the periphery of the opening.

    At the bottom of the existing slot, it looks like someone had second thoughts either about the size of the opening or its location. It looks as if a small piece of aluminum has been inserted and welded to the mast. Was this slot provided as is from the manufacturer of the mast, or has some modification been done?

    If you rigging isn't properly tensioned, I can see where oscillations due to variable winds could cause fatigue to be a problem. It has certainly started in the right location: a point where the strength of the mast is reduced, a location where there is a stress concentration, etc.
  3. Eric Sponberg
    Joined: Dec 2001
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    Location: On board Corroboree

    Eric Sponberg Senior Member

    Flipster, we don't have a complete diagram of your rig, so this is not a complete analysis. However, I think I can confirm a couple of your suspicions. Yes, the failing block in the backstay cascade precipitated an unloading of the rig on the aft side, which overloaded the rig from the headstay that was balancing the backstay. The mast, therefore, bent, and it failed at the weakest point, where the slot is for the halyard. That slot is huge, and there does not look to be any other reinforcing on the mast in way of the slot--i.e. extra wall thickness either side of the slot. It looks like there my have been a fairlead fitting over the slot that has popped off, but this does not constitute adequate stiffening of the mast wall in way of the slot.

    If the manufacturer of the boat called for 20% of the breaking strain on the rigging, that does not sound too unusual.

    But the real question is, why did the backstay block break? Is it undersized? Is it faulty? Yes, if that block breaks and the backstay becomes immediately unloaded, then the mast is not supported properly, and it is doomed if you don't react quickly enough. Yes, that halyard slot really can weaken the mast that much--in the photograph, the mast side wall is buckled (primary failure) and then cracked (secondary failures).

    Can this happen to someone else--if the design on other boats is the same, then yes, it is likely. The manufacturer may want to review the circumstances of your failure and determine why the backstay block broke. Do you still have the pieces? Should the blocks be made bigger? Should the instructions for sailing the boat include additional descriptions for setting up the rig, using the rig, and cautioning against faulty parts?

    I hope that helps.

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