Deck stepped mizzen without compression post

Discussion in 'Sailboats' started by FleurDeLys, Oct 17, 2014.

  1. FleurDeLys
    Joined: Oct 2014
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    Location: San Diego, CA

    FleurDeLys New Member

    Structural design question here concerning mizzen mast support.
    I have a 1975 Tiburon 36. A Crealock designed ketch that later evolved into the Cabo Rico 38. It has a slight depression in the deck below the mizzen mast on the aft cabin. The step is supported by the cabin roof alone. The deck is fiberglass with a 5/8" balsa core. In the immediate area around the step it has two layers of 3/4" ply between fiberglass, under the aluminum shoe. There is a bulkhead a couple feet fwd of the mast step. It is clear some moisture has invaded the ply and possibly a small area around the step. Either through the hole beneath the mast (cable run), through the shoe bolts, or the traveler fittings just forward of the mast. A slight flattening in the center of the aft cabin ceiling can be seen from below. Repairing this to the original core, or even an improved core should not be too difficult. I plan to open the deck up from above after removing the mizzen. Since this boat has made it nearly 40 years, should I worry about adding compression support? I really don't want to add a compression post as it would crowd an already small aft cabin. I have seen photos of another Tiburon with a wood beam across the ceiling directly under the step. Not sure how it is fastened or how it translates the loads. I assume it is through bolted through the cabin exterior walls. My boat is 36' LOD, 11'4" Beam, with a 29' aluminum mizzen mast with 4 x 1/4" shrouds. No fore or back stays, or triatic. Each shroud is tensioned between 10 - 15% breaking strength. Currently 10% (700 ft/lbs). I'm guessing the Mast weighs around 200 lbs at most. I'm not an engineer, but I'm guessing the deck needs to be able to support around 4.5k ft/lbs downward force (not including safety margin)? Safety margin = 3x base number? Is it reasonable to assume a deck alone could support this? My non-scientific options to improve the loading support have been, adding fiberglass layers (maybe G10) spread out further across the top, adding layers to the bottom, or adding a beam (size?) secured to the cabin sides. Any thoughts on if I actually need extra compression support after the deck repair, and if so what would be the least restrictive to the aft cabin.
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If the repair lasts for another 40 years, it should be good enough. The longevity of the boat as designed is a good indication that is was well done. The usual problem is with bad installations and lack of maintenance. Through deck fittings should be rebedded every few years. If your deck has balsa core, now is a good time to take off all fittings and rebed them
  3. Petros
    Joined: Oct 2007
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    Location: Arlington, WA-USA

    Petros Senior Member

    you actually have not suffered a failure of the structure, not bad after that much time. it sounds like there was moisture intrusion which allowed it to creep. So I would say it sounds plenty strong enough as it was designed.

    Here is what you I might consider if I was doing this repair and I was going to go beyond the original design: redesign the attachment installation to make it more resistant to moisture intrusion by relocating the fasteners to areas where moisture will not be trapped as easy, and perhaps enlarge the area on top that spreads the loads of the step out over the roof. If you spread the load over a larger area the roof will have less concentrated loading, and it is not likely to creep or deform over time as much.

    note that wood under pressure will change shape over time, it is called creep. if you keep the constant load below a certain limit it will creep much less and not noticeable. this is the design consideration for wood design for constant loads, transient and impact loads can be double or more the continuous loading since no creep will occur and it is only the ultimate loading that is the main consideration. So spread the step loads over a larger area on the roof, and it will be stronger, without having to add a beam or post under it.
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