Carbon Fiber Eyestraps

I am contemplating using one eyestrap on the bow and one on the stern to put carrying toggles on my WD12H.  I am attaching to deck to the hull within the next couple days and have yet to apply any epoxy to the top of the deck.  I'm thinking that putting the eyestraps on sooner rather than later would make for a better bond.  Anyone with experience/trials/tribulations and willing to share them?  Heck, I don't even know it it's a good idea to use the eyestraps for carrying the 'yak. 

Thanks!


8 replies:

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RE: Carbon Fiber Eyestraps

If you are able to reach into the bow and stern to put nuts on some thru-bolts then I would say install some backing blocks now and go with that plan.  If you must hold the straps down with screws I would say no.  I may get some disagreement on this, but deck fittings subject to significant load should only be thru-bolted.

Paul

RE: Carbon Fiber Eyestraps

If you are able to reach into the bow and stern to put nuts on some thru-bolts then I would say install some backing blocks now and go with that plan.  If you must hold the straps down with screws I would say no.  I may get some disagreement on this, but deck fittings subject to significant load should only be thru-bolted.

Paul

RE: Carbon Fiber Eyestraps

If you are thinking of simply gluing on the eyestraps without screws or bolts, you will probably see them fail rather quickly. You don't need to through-bolt them, however. I have used a system on three kayaks over the past four years that works well. I use 1/4" nylon fairleads screwed through the deck with 1-1/4" screws, then bury the part of the screws that extends under the deck with my end pours of thickened epoxy. You can see photos on any of my kayak building pages at www.twofootartist.com Either way, finish the epoxy and varnish work before attaching the eyestraps. -Wes

RE: Carbon Fiber Eyestraps

If you glue them on now it would harder to work with the glass later. Also you might have a dry spot in the laminations of the plywood, resulting in a weak joint. If you wait until after the glass, you would be assured to have some additional strength in the area. I have used both blocks and end pours. If you use blocks make sure that they are recessed enough to allow the deck to fit properly. If you make end pours use alot of sawdust and microballoons to lighten them up. Good luck.

RE: Carbon Fiber Eyestraps

The weight of an end pour does not seem that significant, as investigated by this site:

http://www.guillemot-kayaks.com/guillemot/blog/nick/end_pour_vs_end_block

RE: Carbon Fiber Eyestraps

The forces that the fitting will encounter in this application are tension trying to pull the fitting off the deck, shear trying to slide the fitting off the deck and torsion trying to twist the fitting off the deck.

In a properly bonded joint the fitting will become the outer layer of the plywood. The forces will be transferred to the top layer of plywood, which will transfer them to the inner layers. So as long as the composite fitting can stand the forces, the wood can stand the forces and the glue in the bond line is at least as strong as the wood, there's no need to worry about failure.

Through-bolted fittings are not necesarily stronger than bonded ones. They are typically stronger than screwed-in fittings, which most people are familiar with. They accomplish this by using the compressive action of nuts and washers to turn a tensile force into compressive force that is distributed over a larger surface area. Wood is much stronger in compression than it is in tension so this makes a stronger attachment. Metal is also stronger in shear and torsion than wood so there's no danger of the bolt failing before the wood. Burying the bolt in an end pour further increases the bearing area for the bolt, resulting in a better distributed load.

However, a through-bolted attachment is a local stress concentrator. Because it's so much harder than wood, the stress from all the forces tends to get concentrated at the point where the metal bears on the wood. This can result in the wood being compressed (shredded if there are exposed threads bearing on the wood), the hole becoming oval instead of round, the bolt coming loose and water getting into the hole, the wood and the boat.

A properly bonded fitting distributes the load over a larger surface area on the top surface of the wood. There is a smoother transition between the fitting and the wood because their mechanical characteristics are closer than metal and wood's are. In addition, the wood's fibers are continuous, not broken up by having a hole drilled through it. A metal through-bolt, on the other hand, is a stress concentrator mounted right on top of a weakened area.

The bonding of the fitting and all subsequent layers has to be strong. That means that if it's going on top of glass, the glass has to be properly bonded to the wood - no bubbles, dirt, floating, etc. Also, the weave should not be filled before installing the fitting. Under no circumstances install the fitting over varnish or paint. Even if it manages to bond to the varnish, the varnish itself will fail and the fitting will pull loose.

One last consideration - fatigue failure. If the wood is very thin and flexible under the fitting, the bond may fail from fatigue (the same effect you get from repeatedly bending a paperclip). This can be avoided by putting a backing plate under the deck to locally stiffen it. This plate should have rounded corners (an oval plate would be the best) and a fillet around the edges to avoid being a stress concentrator itself.

Finally, as far as the original question goes, the wood and epoxy should be plenty strong to support this application. I don't know the characterisitics of CLC composite eyestraps so I'm not the one to say if they are strong enough. You might try the CLC customer service line for more info. Let us know the results.

Laszlo

 

RE: Carbon Fiber Eyestraps

If I was going to do that, I'd glass the boat, then set the eyestrap in epoxy with a fiberglass patch that fits around the eyestrap feet. Kindof an 'H' shape cut in the fiberglass. Then blend it in with subsequent coats of epoxy. Should be quite strong, I'd think. 

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