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Bolt Through Fiberglass, Shear Load - Design Help Needed

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sparky961:
In another thread, I posted a link to another forum where I'm doing some significant repair and refurbishing to my fiberglass sea kayak. 

http://madmodder.net/index.php/topic,12245.0.html

Those repairs are just about complete, but I have one more area yet unmentioned that needs fixed.  The problem is that I'm not sure the best way to handle this one, as restoring it to the original design doesn't seem like it will solve the problem.

Most kayaks have a seat "pan" with side posts.  These posts have holes through which bolts attach a back band.  The back band takes a significant amount of pressure when you're paddling and rolling like you mean it.

On my boat I've always had issues with the pivot points of the backband.  It started out with the nuts coming loose and "sawing" their way through the fiberglass, making an elongated hole.  I fixed this by adding two nylock nuts on the back side - not a typical use, but they never came loose after that.  The shear load continued to destroy the hole though, so I added stainless steel reinforcing plates on the back side with 5-minute epoxy.  It worked well for a while, then one side came loose and the damage continued.

My plan is to fix the damage using glass and resin, completely filling in the existing holes and generally reinforcing the seat pan structure where I see stress cracks.  This will involve grinding down the damage and rebuilding it from there.  It will also require more gel coating, which I have developed a dislike for, even though the results I've obtained have been quite satisfactory.

My question is this: How can I redesign this attachment point to pivot easily and accept a considerable shear load without sustaining damage over time?  I think the key is going to be to spread the load over a larger surface area, and I have some ideas that might work, but I'd rather something that's been tried and well tested already.

Some details:

- The fastener through the hole is stainless, probably around M6 or M8
- The load in shear is at least the amount you can apply to your lower back with your legs almost fully extended, feet on a firm surface.  So, quite a bit if you've got big leg muscles.
- Since this is used in water, everything must be corrosion proof
- Since I sit next to it, nothing can be sharp

chipenter:
I have repaired canoes with fiberglass bandage , its soft and the only sharp bits are on the ends , look for nut inserts for furniture they look like a washer with a pressed extrusion in the middle threded , my repairs were from white water rivers .

AdeV:
Could you embed the plate within the layers of fibreglass? I guess that might be difficult, now that the boat is complete...

How about having a second plate on the outside of the hull, and rivet the two together (as well as epoxying them)? With eveything rigid, and the rivets helping to hold the epoxy, it should last much longer. The only question in my mind is- would the aluminium rivets suffer galvanic corrosion against the s/s spreader plate? If rivets are no good, a selection of small S/S bolts & nyloc nuts perhaps, cut or made to length so they don't stick into you...

PK:
At our previous premises there was a racing dinghy company opposite, we did some work for them, they helped us out with some things, it was good.

The way they did hard points was to insert a perforated metal plate inside the layup. The perforations let it bond really well and you only need a very small plate.

Nb, it looks like you are using glass, but, if you use carbon, remember that it's a metal and you'll get bad galvanic corrosion if you don't wrap the aluminium in glass to insulate it from the carbon.

PK

awemawson:
"Carbon is a metal" PK  :scratch:

You have me truly puzzled - I'm sure it is probably a conductor but not a metal surely  :med:

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