Chainplate attachment.

Discussion in 'Boatbuilding' started by Mychael, Nov 7, 2008.

  1. Mychael
    Joined: Apr 2006
    Posts: 479
    Likes: 14, Points: 0, Legacy Rep: 125
    Location: Melbourne/Victoria/Australia.

    Mychael Mychael

    Firstly my boat is a "plastic" one and about 22yo and as far as I know never had any structural issues in it's lifetime. It's designer is well regarded over here.

    Anyway I was sitting below the other day and looking at the chainplate attachments for the rigging.
    Nice long stainless plates on one side only of thick teak ply bulkhead (guessing around 10mm-12mm thick)and secured with 5 bolts (per side) of about 8mm size.

    On the opposite side of the bulkhead there is only the heads of the bolts with matching washers under the heads, these washers and to varying degrees the heads of the bolts have pulled into the grain of the wood.

    Now I am not qualified in construction techniques so this is why I ask things on here but it seems to me that were there also a matching plate or at the very least large washers on the head side of the bolt then it would be a stronger attachment by virtue of supporting the bulkhead timber in compression from the bolt tension.
    It would appear to me that the design as it is now puts the loads upwards in the holes where the bolts go through. Would it not also be better to have the holes oversize then sleeved to take some of the load off the wood?

    Mychael
     
  2. Guest625101138

    Guest625101138 Previous Member

    The joint should rely on the friction between the wood and the stainless steel. So the steel is compressed onto the wood by putting the bolts in tension. The joint will operate most effectively with the bolts in pure tension rather than shear.

    So this means that if the bolts are pulling at an angle and experiencing some shear force they are not loading the chainplate adequately. Using a backing plate will certainly help as this will increase the compression on the chainplate resulting in higher friction force to stop the chainplate sliding. Using spacers on the screws is not a good idea because you are now diminishing the compressive force and run the risk of relying on bolts in shear.

    Over time the wood will shrink and the bolts lose tension. Once the chainplate starts to move it will wear the timber so if movement is evident you should redo the connection. Check the bolts to see that they are not bent. If they are then replace them.

    Backing plate with holes matched to chainplate is the best. Heavy washers properly sized for the bolts is next best. If the load is correctly spread into the timber you should break the bolts before the timber fails in compression.
     
  3. Mychael
    Joined: Apr 2006
    Posts: 479
    Likes: 14, Points: 0, Legacy Rep: 125
    Location: Melbourne/Victoria/Australia.

    Mychael Mychael

    Thanks Rick. I'll have to go for plates in pairs with two holes per plate..
    I'm thinking that should allow me to safely remove one bolt at a time, put the plate in position but rotated to the side, replace that bolt, get if reasonably firm then pull the 2nd bolt and spin the plate into place the tighten both bolts up again.. Repeating the process till I get all the backing plates fitted.
    How does that sound? I know the ideal would be to take all the tension off the rigging and fit a single one piece backing plate but that is a much more involved job.

    Mychael
     

  4. Guest625101138

    Guest625101138 Previous Member

    It does not take much force to hold an unloaded mast upright. If you back off the turnbuckles you should be able to reduce the chainplate load to very low level. Your idea of two hole backing plate should work fine.

    The things to keep in mind is that you want the bolts in tension, you do not want to crush the wood and friction does the job.

    If you think about it you have a very large area of wood in compression compared with the very small area of the chainplate in tension that ultimately carries the load so the wood does not have to be heavily stressed. Splice plates in steel structures work on the same principle and the steel/steel coefficient of friction is lower than steel/wood.
     
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