Builders' Forum Archives
Posted by LeeG on Oct 21, 2006
Jim, does the hull of have three layers of 4oz? that's mighty tough, tougher than any graphite. Like JimE says most of the wear is on the stern for dragging. Think about it,,an extra 6oz of some kind of goop all over the bottom a few mils thick or one oz of something right where the wear occurs.
You could cover the hull in 18oz of glass and the stern would wear to the wood if its narrow and dragged often.
I put those dynel strips on the original demo Chesapeaks because they were worn through the 6oz hull glass to the wood. But even with those strips 75% of the material isn't where the wear occurs. Highlighting the strip with graphite or black paint was one way to get around fairing in a thick cloth and repainting it to match the hull paint or leaving a t-shirt like weave on varnished hulls. With a few drags the epoxy is worn away and you're into the cloth,whatever it is, after that the resistance to wear is a function of how THICK the material is. Even with a layer of dynel it would wear away to the wood with a few drags over ashphalt/rocks.
Check out how the A.Hawks are finished using a putty of 80% cabosil/20% wood flour on the ends. Ideally you'd file a flat recess on the ends before the hull is glassed with the putty fairing out the indent and giving a THICK wear strip.
The high wear area is all of 8" on the stern and and 6" on the bow. If you really wanted to protect the bottom of the hull for dragging put a short strip of half oval 3/8 brass on the stern.
I went through lots of permutations of materials for the high wear areas and it became obvious that layering on strips of flat cloth required a lot of labor and fairing compared to applying an 1/8" of something directly over the narrow ends where it hits the beach. A 1/8"x3/8"x8" strip will last a LOT longer in that high wear area than a few mils of graphite epoxy or a few layers of 6oz cloth requiring fairing.
This isn't to say graphite won't make sliding over rocks easier, it really will. It'll also make it easier to slip off of racks or back down rocks if you let go. It'll make it slippery also because it's abrading away.
Think of this, if you were dragging a graphite/4oz glass hull flat to a rough surface,,and if you dropped it the damage to the wood would be protected by 4oz glass, not a few mills of graphite epoxy. If you dragged a hull with 6oz glass and varnish/paint it wouldn't be as slippery but if you dropped it the 6oz glass would be protecting it not the varnish or paint.
Ok, so now you lift both boats and drag,,it's all on a couple square inches on the stern and the pressure is 100X greater than if you dragged if flat on seaweed covered rocks. Now it doesn't matter what's back there as long as it's thick.
As long as you drag the hull flat to the surface and not by lifting from the bow the graphite can make a big difference, once you lift from the bow it really doesn't matter, and if you drop the bow it will matter how much glass is on either side of the ply.
I'm going from the experience of applying and fairing multiple strips of 3" 9oz glass tape,multiple nesting strips of bias cut 6oz e-glasscloth, multiple strips of bias cut 4oz e-glass cloth, 4oz s-glass cloth, 6oz s-glass cloth,strips of dynel, dynel cord, dynel cord stuffed with 1 1/2" kevlar tape, thickened cabosil/wood flour epoxy putty, 3/8" brass half oval, 2"unidirectional 11oz carbon, 5oz kevlar cloth, 5oz polyester cloth, 1" tubular webbing soaked in epoxy, 3/16" polyester rope soaked in epoxy.
If you want something that's tough for paddling up to a shore then getting out and dragging a few feet before lifting the kayak over rocks, or dragging a few yards to get the kayak away from waves to empty the kayak and lift it over rocks, then what's tough is 6oz of glass on the bottom and a thick strip of material in the ends about 1/8-3/16" thick for 6".
If you paddle up to shore the bow hits, if you paddle up to a rocky area the slick graphite bottom will allow you to slide forward but if you find yourself balancing one a rock it'll be the thickness of the glass, not the fill coats or coating that will keep the wood from making a crunch sound. Once you lift the bow and drag it's what's on that couple square inches of the stern and not the many square feet of the bottom that will determine how fast it wears away.
In Response to: Grapjite/epoxy by Jim L on Oct 20, 2006