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  1. #1
    Join Date
    Jun 2007
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    Ireland, Carlingford
    Posts
    439

    Default Chamfering a backing pad

    Is it really necessary to chamfer a backing pad for, say, a U Bolt to which a lifeline would be attached? I ask because I am using some marine ply and I don't have the tools to properly chamfer it.
    Thanks

  2. #2
    Join Date
    Nov 2002
    Location
    West Mersea
    Posts
    6,419

    Default Re: Chamfering a backing pad

    Do you mean chamfering to shape the backing pad to match the underside of the deck which has some shape? If so yes. If the underside of the deck has some shape, camber or whatever a flat underside of the backing pad would try to straighten the deck and thus stress it. Lot depends on the size of the backing pad.
    www.crossingthethamesestuary.com

  3. #3
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    10,100

    Default Re: Chamfering a backing pad

    An alternative to shaping the backing pad would be to stick it on with a generous amount of thickened epoxy, which is probably a good idea anyway; I'd use microfibres. As for chamfering the edges, theoretically, it transfers the load at the edge over a larger area. In practice I doubt it would make any difference.
    Steve
    Never believe anything until it has been officially denied.

  4. #4
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    36,279

    Default Re: Chamfering a backing pad

    Quote Originally Posted by Stemar View Post
    An alternative to shaping the backing pad would be to stick it on with a generous amount of thickened epoxy, which is probably a good idea anyway;
    Yes, I'd stick it on with some epoxy filler. Don't worry about chamfering the edges.

  5. #5
    Join Date
    Nov 2015
    Location
    Portsmouth
    Posts
    226

    Default Re: Chamfering a backing pad

    I think it might be that the OP is asking about chamfering because she/he intends to glass over the backing pad. Glassing over a block with right-angled edges is tricky because chopped-strand glass fibre cloth doesn't like folding over the edge and won't properly encapsulate the block.
    For a lifeline U-bolt I guess it doesn't really need to be glassed over. Painting the block liberally with resin should be enough to protect the block from damp and rotting. A marine ply block of at least 1/2" thickness should be strong enough to spread the forces involved.

  6. #6
    Join Date
    Jul 2002
    Location
    Home: Kent. Boat: Chichester
    Posts
    43,553

    Default Re: Chamfering a backing pad

    Quote Originally Posted by Stemar View Post
    An alternative to shaping the backing pad would be to stick it on with a generous amount of thickened epoxy, which is probably a good idea anyway; I'd use microfibres. As for chamfering the edges, theoretically, it transfers the load at the edge over a larger area. In practice I doubt it would make any difference.
    +1 except that if it going on GRP I' probably use thickened polyester filler rather than epoxy eg https://www.tcschandlery.co.uk/plast...0-ml-tub/p8096 ,,,, or just a car body filler !

    In my experience worth applying some wood preservative to prevent mould and mildew after fixing

  7. #7
    Join Date
    Jun 2007
    Location
    Ireland, Carlingford
    Posts
    439

    Default Re: Chamfering a backing pad

    I wasn't thinking of using epoxy. I asked because in a book I read recently it suggested chamfering the edges. It didn't say why but I'm assuming the reason is the possibility of a "hard" edge creating a weak spot.

  8. #8
    Join Date
    May 2001
    Location
    UK East Coast
    Posts
    36,279

    Default Re: Chamfering a backing pad

    Quote Originally Posted by srah1953 View Post
    I wasn't thinking of using epoxy. I asked because in a book I read recently it suggested chamfering the edges. It didn't say why but I'm assuming the reason is the possibility of a "hard" edge creating a weak spot.
    Chamfering the edge which touches the fibreglass effectively reduces the contact area. If you stick it on with filler, it'll have an even loadbearing surface with no hard edges.

  9. #9
    Join Date
    Nov 2016
    Posts
    2,546

    Default Re: Chamfering a backing pad

    Quote Originally Posted by srah1953 View Post
    I'm assuming the reason is the possibility of a "hard" edge creating a weak spot.
    My understanding is that you're exactly right.

    I don't see why you can't take an iron file or a sander of some sorts, and chamfer your block that way. Takes but a couple of minutes...

    Also make sure you round the corners/cut or sand off any sharp angles, for otherwise you can get a concentration of load leading to cracks in the GRP..... or so I'm told.

  10. #10
    Join Date
    Sep 2001
    Location
    Me - Zumerzet Boat - Wareham
    Posts
    12,213

    Default Re: Chamfering a backing pad

    If you are talking about chamfering the top edges, I think it is a good idea because square edges splinter easier and splinters can snag on braised rope, webbing or clothing.

    PS: A Router bit on an electric drill would do a job.


    Bosch_Router_Bit_85298M_COMBO_(EN).png
    Last edited by LadyInBed; 17-08-19 at 15:15. Reason: PS
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