Beam construction for compression post removal

Discussion in 'Fiberglass and Composite Boat Building' started by Restoring Merganser, Dec 30, 2019.

  1. Restoring Merganser
    Joined: Dec 2019
    Posts: 4
    Likes: 0, Points: 1
    Location: Bristol, UK

    Restoring Merganser New Member

    Hi Folks, I'm in the middle of doing a full bare-hull refit on my Arden Clansman 24, the long and short of it is the keel needed re-ballasting due to water ingress and 10+yrs of freeze thaw cycles so I decided to re-fit the lot as the interior layout was awful. I plan to base my designs on the Arden Gypsy which uses exactly the same hull mould but is set-up more as a cruiser for 2 rather than a daysailor for 4.
    A major part of this is replacing the compression post of the Clansman with the Bulkhead/Beam arrangement of the Gypsy. I'm at that point now and wondered if for the beam it would be ok to use marine ply panels (6x 19mm) with some glass in between them and glass encapsulated for some extra stiffness and protection, my wood working isn't expert level but I can CNC cut the panels. The plan always was to fair and paint the interior so the visual of the wood isn't on the priority list.
    Any thoughts from the experts gladly received :)
     

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  2. fishwics
    Joined: Mar 2004
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    Location: UK

    fishwics Quiet member

    The strength/stiffness in a beam is given by the top and bottom; the bit in the middle is there only to hold the top and bottom together. If you are going to reinforce then put it (in this case) mainly at the bottom as that is the side in tension. The critical bit of the bulkhead (which I suspect takes most of the load of the beam) will be the edges around the hole. If those buckle, the whole bulkhead will fail. I'd make sure they were well reinforced with solid timber (as in your photo) stepped onto a frame or whatever is under the cabin sole.

    In essence you'd be replacing one compression post with two - one each side of the access-way - with a beam connecting the tops.

    NB I'm no longer an expert.
    Simon
     
  3. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Glass in between offers little xtra.

    It is really hard to understand the plan.

    6x19mm is 104mm, but yu offer no third dimension

    And as fishwics points out; you offer no information on what will shoulder the loads.

    for example, if your bulkhead is 5/8" thick; you would be creating a 104mm deep beam and resting it on the bulkhead?

    Anyhow, noone can really comment as your information is lacking.

    Also, what is the loading from above? We have no idea if this is carrying a deck and walking loads or something more like a mastfoot, etc.

    In order to design a beam; you need to know the loading planned..

    then the length of the beam is also relevant
     
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  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If you are copying the exact design and construction of the Gypsy, and it is proven to work, there is no problem. Is there anything you are planning on modifying?
     
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  5. Restoring Merganser
    Joined: Dec 2019
    Posts: 4
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    Location: Bristol, UK

    Restoring Merganser New Member

    Thanks for the replies :)
    My plan was to copy the design of the Gypsy and if anything go a bit bigger, the only references I have are a couple of photos, the designers have long gone into the ether so airing on the side of caution seems sensible.

    Due to my obvious ignorance on the subject of foot loads on the mast I've been searching around and come up with a static load number that seems a bit overkill for a 24ft sail cruiser
    roughly I got 3.16t at the mast foot?? (workings below)

    Masthead sloop, chainplates outside the gunwales, Double lower shrouds, Single spreader cap shrouds, forestay, backstay
    LOA 24ft
    Displacement: 2.1t
    Beam: 2209 mm
    Draft: 1066 mm
    Ballast: 998 Kg
    Righting moment @30deg of 7.6Kn - Approx (780Kg)

    4x fore/aft 6mm SS lower shrouds @20% Breaking strain = 432Kg*4 = 1728KG /1.15 for 30deg shroud angle = 1502Kg
    2x single spreader 6mm SS cap shrouds, @20% Breaking strain = 432Kg*2 = 864Kg (90deg to spreaders)
    1x 8mm Forestay (pretty sure this is for the convenience of the roller furler not the breaking strain so we'll assume 6mm tension of 432Kg/1.103 for 15deg angle = 391 Kg ,
    1x 6mm back stay 432Kg/1.064 for 20 deg angle = 406Kg

    that's come out as a static load of 3.16t??
     
  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The breaking strain is not an indication of the forces. Make a diagram of the mast and the rigging. The forces can be calculated with basic trigonometry by simplifying and ignoring mast deflection. However, it should be much easier to look at an existing boat. By looking at the photo, is that small pipe on the forward end of the table you are trying to remove? Seems like a lot of work for whatever space you may gain. Look at this website, they have an example of calculating the compression force.


    You can also calculate the force to buckle the mast and used that as the maximum force Column Buckling Calculator | MechaniCalc https://mechanicalc.com/calculators/column-buckling/
     

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  7. Restoring Merganser
    Joined: Dec 2019
    Posts: 4
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    Location: Bristol, UK

    Restoring Merganser New Member

    ahh ok, I read somewhere it was a good ballpark figure, seems to be a lot of finger in the air guestimations around mast loadings.

    RE Pipe, no that appears to be the flue for a small heater, it's the bulkhead/beam/dual post arrangement of that picture that I want to emulate, the boat it's going in is currently a mostly bare hull (other picture with bracing and supports temporarily removed), the initial question was would a ply/composite beam be a suitable replacement for a solid beam of the same size but it seams there is much more to consider. i'm at work at the moment i'll get a rigging diagram drawn up in Fusion360 this evening (another rock 'n' roll friday!)
     
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