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

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Pikor

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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,
 
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.
 
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.
 
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.
 
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.
 
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
 
Stemar said:
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.
Click to expand...

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.
 
Yngmar said:
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.
Click to expand...

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.
 
William_H said:
...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
Click to expand...

In fact, no, not really. I've published several articles on the topic, and a standard bolting washer contributes far more strnegth than a penny washer, because it is thicker. In fact, most manufacturers us bolting washers rather than penny washer when bolting solid FRP.

Here's the thing. The deck is many, many times stiffer than the thin washer, so the washer can only carry that small fraction of the load.

Sometimes it is best to recognize shite for what it is than to hope it does something.
 
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