Bolt Through Fiberglass, Shear Load - Design Help Needed

[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 

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

[seadog]

Even though the Carbon group in the periodic table contains Tin and lead - 'Carbon is classified as an element in the 'non-metals' section which can be located in groups 14,15 and 16 of the Periodic Table. Non-metallic elements exist, at room temperature, in two of the three states of matter: gases (Oxygen, Hydrogen & Nitrogen) and solids (Carbon, Phosphorus, Sulfur and Selenium).'

Thank you Wikipedia 

[Lew_Merrick_PE]

Sparky --  In the aerospace industries it is common to "lay-up in" various bushings and nutplates to take critical shear loads in "fiberglass lay-ups.  These are (usually) 400 series CRES (read: stainless steel) components heat treated to (at least) Rc45.  If a through bolt is your desire, then merely a T-Bushing set should do the trick.  The main thing to remember is that you want to maximize the OD and thickness of your T-bushings.

[sparky961]

Thanks for all the ideas. Initially I had been thinking about embedding a SS plate in the layup. But I have great concerns about water migrating in between the materials over time and causing damage that won't be detected before it is extensive. The different thermal expansion rates also cause me to worry about burying a plate in there.

Lew may have hit on one of my own candidate ideas, if I understand properly what a T-Bushing would be.

I'm thinking to create a pocket on the back side, in which the bushing would sit with some play. On the other side a washer would complete the sandwich arrangement and allow it ro pivot.

I'll have to include some sketches later when at my computer. It's too difficult on a phone.

[Lew_Merrick_PE]

Sparky -- The T-Bushings I am familiar with have a knurled cylinder for the thickness of the lay-ip, a T-head on one side (call it 2.5X the diameter of the knurled body), and a light slip fit to a washer on the other side.  The knurled body is a very lighy finger-press to the hole.  The knurls are coated with epoxy for installation.  Does this help?

[Charles]

You can buy a thing called a "big head" fasteners, it's basically a bit of perforated stainless with a nut welded to it, bond this to the existing grip with epoxy, then laminate over it creating a captive nut.

Alternatively, bond on a piece of tufnol, whale or 10G40 would be my grade of choice here, the drill and tap it once the adhesive has set. Tufnol will give a better bond to epoxy than s/so, and is non conducting, so no galvanic corrosion

[Fergus OMore]

I'm sort of sitting with a pair of very useful suture pliers which my late wife gave me and showed me how to properly use copper wire to hold ply sheets prior to using glass tape in making a Mirror dinghy.

Again, there is fiberglass mast for my son's sail board cluttering  my garage and  my carpenter was using a fiberglass hafted maul  as he is making me a new workshop. I did an accelerated gel to repair an underground drain - which was cracked. It was an old technique previously used to replace the rotten wooden keelson around the centre board housing on an old Enterprise dinghy( E 748)

On a sort of light hearted diversion, the body of the Reliant Robin out of Trotter Trading in the TV soap 'Only Fools and Horses' was fiberglass.  I had a 4 wheeled version called the Reliant Rebel. The hinges , locks etc were mild steel.  All our yesterdays but my son is still running about in a Lotus Elise SE- also a glassed job.

Today's carbon fibre technology seems available to the DIY person.

Regards

Norm

[Nicolas Hulme]

It's an old thread but no one mentioned Rawl-Nuts anywhere in it, they are very useful for fixing to materials which have adequate strength but which tend to wear away, Rawl Nuts are very good for fixing wings to fibreglass bodyshells on kit cars where vibration would cause other fasteners to expand the holes.