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  1. #1
    Join Date
    Aug 2018
    Posts
    26

    Default How to fit a strong point if you have a GRP liner?

    Hi,

    My boat has a GRP liner as thick as a hull in places where I would like to put strong points for jackstays. Mounting a backing plate means cutting the liner out and I don't want to do that. Any ideas? How about well or riv nuts?

    Regards,

  2. #2
    Join Date
    Sep 2001
    Location
    Home - Sothampton, Boat - Gosport
    Posts
    9,765

    Default Re: How to fit a strong point if you have a GRP liner?

    When I brought my lines aft, I had to fit turning blocks in the saloon roof, which has a GRP lining. I drilled right through at the right size for the bolts, took a bit of tube, ideally stainless, of the right internal diameter to take the bolts and drilled from the outside to just touch the liner so the tube would just fit and prevent everything getting squashed together. The penny washers and domed nuts I used on the inside are plenty strong enough and, while not beautiful, look functional. In your case, presumably, you'll be using U bolts, so one of the double washers supplied could go on the inside; if your liner is as solid as it sounds, I think a bigger backing plate is probably superfluous.

    I don't think I'd trust my life to rivnuts.
    Steve
    Never believe anything until it has been officially denied.

  3. #3
    Join Date
    Dec 2012
    Location
    Gone cruising
    Posts
    2,195

    Default Re: How to fit a strong point if you have a GRP liner?

    Drill a pilot hole from the outside in, all the way through deck and interior liner. Now use this pilot hole on the inside for the center of a holesaw and cut a hole just big enough for your nuts, washers and matching socket. Once mounted, cover up the holes, either with a cover plate or some matching furniture plugs. If you need a backing plate, connect the holes with a saw.

  4. #4
    Join Date
    Jul 2006
    Posts
    4,697

    Default Re: How to fit a strong point if you have a GRP liner?

    I've posted before that I've seen a quarter inch eyebolt bent and distorted, and the nut behind almost pulled a penny washer through the GRP deck despite a ply backing pad. OK - so it (just) held two simultaneous violent MOBS, but the forces a jackstay can create are big.

    Rivnuts are quite certainly not the answer, and penny washers alone not necessarily enough either, unless into a very strong bit of GRP.

  5. #5
    Join Date
    Oct 2006
    Location
    Bradwell and Leigh-on-Sea
    Posts
    14,392

    Default Re: How to fit a strong point if you have a GRP liner?

    Could you attach them to cleats?
    Not all who wander are lost

  6. #6
    Join Date
    Aug 2018
    Posts
    26

    Default Re: How to fit a strong point if you have a GRP liner?

    Quote Originally Posted by MoodySabre View Post
    Could you attach them to cleats?
    At the bow - yes. But not on the cockpit side unless they go on the side of the boat which is very incorrect IMHO. Jackstays should go along the centre line of the boat as much as possible (IMHO).

  7. #7
    Join Date
    Dec 2013
    Location
    Deale, MD, USA
    Posts
    1,637

    Default Re: How to fit a strong point if you have a GRP liner?

    The problem with penny washer (fender washers in the US) is that they are typically only 0.042 inches thick in the sizes in question. You can easily bend them with pliers. They will bend before the WLL of the bolt is reached. They wre made for fitting fenders to cars and arn't good for much else. They do NOT distribute the load.

    However, you can order and "extra thick" version (0.125-inch thick) that is quite suitable. Many better boat builders use the extra thick version. It's a quality detail to look for.

    A person crusade against flimsy penny washers. Chandleries should sell something that can't actually work for the intended purpose.

  8. #8
    Join Date
    Jul 2003
    Location
    West Australia
    Posts
    11,515

    Default Re: How to fit a strong point if you have a GRP liner?

    The only option to the OP if he does not want to cut the liner in any way is to fit a large ss plate over the deck area.
    The attachment being firmly attached to the plate. This can be attached to the deck with riv nuts or screws tapped into the deck. You can then have around 10 or 12 screws attaching the plate over a relatively large area to give the required strength. What is the required strength and what is the strength of a screw multiplied by 10? That I can not answer. So not necessarily a method I would advocate but possible.
    Re Thinwater comments on penny washers. Yes obviously the thickness will dictate the strength added by the large washer. However even the thin ones will add to the distribution of pressure on the GRP. So again an engineering calculation on strength added but certainly better than none. ol'will

  9. #9
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    7,561

    Default Re: How to fit a strong point if you have a GRP liner?

    Quote Originally Posted by Stemar View Post
    When I brought my lines aft, I had to fit turning blocks in the saloon roof, which has a GRP lining. I drilled right through at the right size for the bolts, took a bit of tube, ideally stainless, of the right internal diameter to take the bolts and drilled from the outside to just touch the liner so the tube would just fit and prevent everything getting squashed together. The penny washers and domed nuts I used on the inside are plenty strong enough and, while not beautiful, look functional. In your case, presumably, you'll be using U bolts, so one of the double washers supplied could go on the inside; if your liner is as solid as it sounds, I think a bigger backing plate is probably superfluous.

    I don't think I'd trust my life to rivnuts.
    This is bad engineering, your turning block is only affixed to the liner and is standing on the stainless tubes. You don't have vast loads, so will likely get away with it. Post #3 is the way to go.
    Rainbow Marine.
    www.rainbowmarine.co.uk

  10. #10
    Join Date
    May 2016
    Location
    Suffolk
    Posts
    7,561

    Default Re: How to fit a strong point if you have a GRP liner?

    Quote Originally Posted by Yngmar View Post
    Drill a pilot hole from the outside in, all the way through deck and interior liner. Now use this pilot hole on the inside for the center of a holesaw and cut a hole just big enough for your nuts, washers and matching socket. Once mounted, cover up the holes, either with a cover plate or some matching furniture plugs. If you need a backing plate, connect the holes with a saw.
    This is the correct way to do it, the fitting is securely bolted to the deck, with no reliance on the liner and no need for spacers.

    This assumes that the deck isn't cored. If the deck is cored, it's a different procedure.
    Rainbow Marine.
    www.rainbowmarine.co.uk

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