deck stepped mast; design question

Discussion in 'Sailboats' started by Westwind2, May 5, 2008.

  1. Westwind2
    Joined: May 2008
    Posts: 7
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    Location: Ontario, Canada

    Westwind2 Junior Member

    I have an Ajax28 (same hull as a CandC27) whose shrouds lose tension as the boat lies at dock and whose bulkhead pinches the top of the salon door. I would like to design a stronger mast step by installing an Olson30 Beam of Destiny/Jock Strap lateral brace between chainplates to prevent the hull from collapsing inward and 2 jack posts on either side of the salon door to transfer the vertical load to the keel. Question1:Should the tabernacle be fixed-point-bolted: through the cabin top; through the support beam; to the jack posts; and the jackposts to the keel? Question 2:Should all these be fixed point attachments or should they be pinned enabling the deck to deform relative to the support beam; the support beam to move laterally relative to the jack posts; and the keel to deform below? Question 3:Should I also bolt the beam and the jackposts to the bulkhead in order to minimize fore/aft beam and post deflection or would this overload the half inch thick bulkhead causing it to deform or causing its tabbing to break free from the hull when sailing close haul in a seaway?
     
  2. alan white
    Joined: Mar 2007
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    Location: maine

    alan white Senior Member

    Your problem is, I think, complicated by the fact that you are only talking about bolting a product in place.
    A better fix would be to concentrate efforts into the span between the two existing support posts, which is not stiff enough to carry its load.
    That chunk needs to be replaced with something stiffer.
    There are a lot of ways to stiffen the beam. One way would be to embed aluminum or stainless vertically oriented plates into the beam (which otherwise could be epoxy and plywood).
    Another question is, did water ingress soften the original beam, possibly through either the tabernacle bolt holes or the wiring hole?
    Maybe all you need to do is replace the old beam with a new one just like it.
    It may not be inherently weak. Usually such areas are overbuilt.


    Alan
     
  3. Westwind2
    Joined: May 2008
    Posts: 7
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    Location: Ontario, Canada

    Westwind2 Junior Member

    Alan, thank you for your advice. I investigated the span across the salon doorway. Under the tabernacle the cabin top is gently arched ~3/4" thick f. gl. wood core composite. The ~3/8" wood core appears to be dry. At the apex of the arch an inverted, 3-sided aluminum channel (~2" wide by 2 5/8" high) contains a smaller c.s., probably solid, 4-sided aluminum ground into an arch that supports a ~1 1/4" by 19" strip of cabin roof arch spanning the door that is offset to the starboard. The aluminun is not deformed but the longitudinal centre of the mast as it sits in the 11"by 5" tabernacle is ~1/4" aft of this aluminum channel support! There is a bulge in the salon roof liner below the aft edge of the unsupported half of the tabernacle. The lateral roof arch causes the aluminum channel to taper to near zero thickness over the starboard doorpost. Although the door posts are bolted to the bulkheads, they are not attached to the aluminum channel nor the floor beneath the door sill. The floor, the only glassed-in transverse timber int the hull, is ~3" by 4" at the keel but it tapers to near zero beneath the doorposts. This floor has been drilled at its max. thickness to access the leading keelbolt and appears to be broken. Design Question1: If I sister the bulkhead with two 2"X2" aluminum Beams of Destiny to provide fore and aft support to the tabernacle teeter totter (after replacing the woodcore maststep with solid epoxy/glass and sistering the floor with two 1/4" stainless steel plates), should the points of attachment from tabernacle to floor be fixed or pivoting? As you know, I am concerned about the asymetrical loading of the shrouds while sailing close hauled in a seaway. Design Question2:Should the Beam of Destiny be bolted to roof as the bulkhead is not tabbed to the roof? Question 3:Since the door is offset to the starboard side of centre, should I attach the Jock Straps (asymetrically)from the chainplates to the base of the door posts or should I extend the stainless steel floor ends equally as measured from the keel line in order to attach the jock straps to the steel ends (symetrically)?
     
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