Cabin top deflection? Help please.

Discussion in 'Sailboats' started by Shawn W, Aug 30, 2010.

  1. Shawn W
    Joined: Aug 2010
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    Location: Placitas, NM US

    Shawn W New Member

    As any WWP-19 owner will tell you it sure would be nice to remove the mast compression post, at least while in port or at anchor. I read the "verticle down force stepped mast" thread with great interest, especially daiquiri's comments. It seems that if I install Johnson quick-release levers on the shrouds and stays it will relieve most of the verticle down force, and allow the compression post to be temporarily removed while in port.

    Here's my question: How much deflection can a fiberglass cabin-top take before the surface shows stress cracks? 1/8"? 1/4"? 1/2"! (Not being an engineer, I have no idea what other information you would need to answer this question, so please let me know.)

    My plan is to:
    1. Install the quick releases and ease the rigging
    2. Measure the down force at the mast foot
    3. Remove mast and standing rigging
    4. Remove the compression post
    5. Gradually load the unsupported cabin-top until the weight equals/exceeds the measured down force, keeping an eye on the deflection with each increase in weight.

    This should tell me how much, if any, re-enforcement I'll need to install. (My guess is that the arch of the cabin-top will easily withstand the down forces involved, but of course I don't want to ruin my cabin-top if I'm wrong!) Thanks for your help.

    Shawn

    PS: The compression post will be kept in place most of the time; only being removed when cooking/eating and sleeping. So long-term fiberglass fatigue shouldn't be a factor, right?
     
  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    this is a complicated question since the lay-up configuration and the shape of the beam/cabin roof, cabin walls, connections, etc. also can interact with the deflections and cause cracking or separation of the layers or boned joints, not to mention gel coat or surface finish cracking.

    Generally fiberglass/epoxy is pretty good for deflections presuming there are no joints or unusual geometry interactions. Fiberglass/polyester resin will not tolerate much deflection at all before cracking occurs.

    If you can limit the deflections to a ratio of about 1/240, you should be okay. That is max deflection to span ratio, so a 240 inch span has a max load limited deflection of 1 inch. This is very conservative and works for almost all configurations, this can be reduced to 1/180 if there are no interaction issues (secondary structures such as internal bulkheads, fixtures or fittings, etc. that could be affected by the deflection). Structural damage is a serious risk at larger deflections unless it was specifically designed for large deflections.

    Fiberglass is also pretty good with fatigue life if it was laid up properly (no voids or discontinuous structural elements within the lay-up).

    Do you have pictures of the cabin you are intending to alter in this way? Both outside and inside.

    That is an interesting idea you are proposing, but you really need to make sure you have NEVER forgotten to install the post before you get underway.
     
  3. Shawn W
    Joined: Aug 2010
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    Location: Placitas, NM US

    Shawn W New Member

    Petro thanks a lot for your excellent reply, it is exactly the information I was looking for! I'll call the manufacturer today and ask about their fiberglass materials and production techniques. I'll take pictures this weekend when we're up at the lake. In the meantime here's a link to a comprehensive series of photos showing the manufacturing of both a Potter 15 and 19.

    http://www.wwpotterowners.com/Construction.html

    The 19 is the boat I have and starts half-way down. In some photos the raised keel obscures the compression post, which is between the keel boot and the small v-berth seat. The mast mounts on the highest deck level at the farthest forward point, just before the cabin top slopes down to the forward hatch level.

    I definately will take your "NEVER forget" warning to heart. It's on my checklist right below "NEVER hoist sails before the keel is down and locked or you'll most definately turn turtle."

    Thanks again -- Shawn
     
  4. Petros
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    Petros Senior Member

    the potter is a pretty well built boat, they have a reputation for being very durable and well made (though if I recall, there were several differnent manufacturers of the WWP).

    I do not think you will have any issues, but if the deflections are too large for comfort you might consider installing ceiling reinforcement to support the weight of the mast, and than use the removable column for sailing.
     
  5. Shawn W
    Joined: Aug 2010
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    Location: Placitas, NM US

    Shawn W New Member

    Petros, you're right about the different manufacturers, the previous owners of IM had some problems toward the end, but the "new" owner (bought the company in the mid-90) have a great reputation.

    The P-19 beam is 7'6", but to be safe I'm using a 6' span, which produces a .33" deflection at 1/240, or a .4" at 1/180. When I do the deflection test I'll set up a string grid inside the cabin so I can measure deflection at several different points. The steep sides of the raised cabin top won't be a problem, but the almost 90 deg. corner where the sides transitions into the flat is unsupported, and is probably where the greatest deflection will show up.

    Thanks again for your help, I'll let you know how the tests turn out.

    --Shawn
     
  6. Brent Swain
    Joined: Mar 2002
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    Location: British Columbia

    Brent Swain Member

    I mount my mast on a steel arc , which carries the load from the mast to the cabin sides, where a couple of pipes carry it to the chines ( or turn of the bilges in a fibreglass boat) This eliminates the support post in the middle of the accommodation. The turn of the bilge can easily be greatly reinforced to take the load and spread it out.
    The beauty of the 19 footer is it is easy to do the inclining experiment alongside the dock with a halyard, and measure the deflection.
     
  7. philSweet
    Joined: May 2008
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    philSweet Senior Member

    I'm thinking a tripod outside with a lifting tackle to unweight the mast. It would be about the same size as the support strut, so one or the other would be stored wherever you store it. Might not have to loosen the stays at all. Loosening the stays could make the rig quite noisy. It would cost about 20 bucks and if nothing else might help with determining the mast load.
     
  8. Shawn W
    Joined: Aug 2010
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    Location: Placitas, NM US

    Shawn W New Member

    philSweet, I had similar thoughts, if the deflection turns out to be too great. Do you have a source for such a tripod and lifting tackle? As to the noise, I'm thinking a few bungy cords should solve the problem, tensioning the stays just enough to quiet them without putting much down force on the mast.

    Let me know about the tripod, thanks for the help.

    --Shawn
     

  9. david@boatsmith
    Joined: Aug 2008
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    Location: Jupiter Fl USA

    david@boatsmith Senior Member

    I would think that maybe installing a ring frame in place of the compression post might work depending upon the available space inside.
     
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