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  skid plates and polyetthylene
  Posted by: pblanc on Oct-05-12 8:40 AM (EST)
 

-- Last Updated: Oct-05-12 8:45 AM EST --

First, Kevlar felt, which is used in most of the commercially available skid plates, is not the best, far from it. The Kevlar felt material is not very strong, results in a thick plate that creates lots of drag, and the material chips and abrades fairly easily. It is used because it is convenient and fairly cheap.

You are better off buying some cloth and cutting your own plates. I have used both Dynel and fiberglass for this purpose. S fiberglass is a bit stronger than E 'glass but the widely available E glass would be strong enough. Dynel soaks up more resin and results in a thicker plate (but not nearly as thick as Kevlar felt) so one layer is usually enough. With either E or S 'glass I would use 2 or 3 layers, of slightly different sizes cutting at least one layer 'on the bias' so that its fibers run at a 45 degree angle to those of the layer above or below it.

Either Dynel or fiberglass can be easily cut with a good pair of scissors. I would first cut out a template for your skid plate out of heavy packing paper. Fold your template down the long axis to make sure it is symmetrical. You can make the template any length and width you like. For most usages, a skid plate that starts about 6 inches below the top tip of the stem will be fine. Cut it in a tear drop shape (if that is what you want) and long enough to overlap the area of wear on the hull bottom by a couple of inches. You can use that paper template to mark out your fabric and draw a line on your hull to mask it.

The second consideration is getting a good bond to polyethylene. Polyethylene is a rather chemically inert polyolefin and adhesives have a very hard time getting a grip on it. Sometimes the old urethane adhesives (which stink to high heaven) sold with skid plate kits would yield an acceptable bond and sometimes not. I would use G Flex epoxy made by West Systems, however.

Even with G Flex, bonding to poly is a bit tricky. What I would do is temporarily tape your paper template to the hull and make sure it is centered, then mark along the edge with a Sharpie. Sand the poly inside your mark with something like 80 grit paper to rough up the surface a bit, then very extensively wash the hull with water and a degreasing agent (like Dawn dish soap), rinse well, then wipe down the area with isopropyl or denatured alcohol. After the alcohol has thoroughly evaporated you need to pretreat the surface of the hull by flaming it with an inexpensive propane torch. Don't omit this step. If you do, the epoxy will not bond.

Flame pretreatment is not intended to heat the plastic. What it does is temporarily 'etch' the polyolefin surface by oxidizing it. To effectively oxidize the poly you must contact the surface with the very tip of the inner blue cone of the flame and that little tip must contact every square centimeter that you want the adhesive to bond to, so it is best to do this in an area dimly lit enough that you can see the blue cone. You can pass the flame over the surface fairly quickly as you don't want to melt the plastic. Go over the surface in a very systematic fashion so you are sure you don't miss any.

Once you have pretreated the surface I would quickly mask off the perimeter of the area with painter's tape to contain the epoxy and provide an easy visual marker of where you want the edge of the cloth to wind up. Dynel and especially fiberglass cloth is woven and easily changes its shape after it is cut so as you wet it out with epoxy you want to make sure that it lays on the hull in its original cut shape. You want to start wetting out your cloth within 30 minutes of flaming the hull surface, if possible.

G Flex epoxy is easy to use. You mix it in any quantity you wish in a one-to-one ratio by eye. It is somewhat more viscous than traditional epoxy (not quite as thick as honey) so it takes a little more time to work it into the cloth, but it wets out fabric just fine and it has a little longer pot life than some other epoxies.

If you are using multiple layers of fabric it is not necessary to flame the surface of the wetted cloth before applying the next layer. I usually like to allow the first layer of cloth to cure to a 'green' state before applying the next, but some folks like to wet out multiple layers of cloth simultaneously. I wouldn't with the G Flex because of its viscosity. If you wait until the epoxy is completely cured before applying more, you should first wash the surface to remove any 'amine blush' which can interfere with getting a good bond. This is not necessary if you apply the next coat of epoxy while the first is still green.

After your last layer of fabric is on, you will need to apply another thin coat or two of epoxy to completely fill the weave of the cloth. After the epoxy is fully cured you can feather the edge of the patch (if you wish) with sandpaper. You can also paint the skid plate with spray paint. Krylon Fusion paints are popular for this purpose as they are allegedly formulated to achieve a better bond to plastics. The paint will scratch off, but it is very quick and easy to just mask off the plate and touch it up by spraying on a bit more. Before painting, again wash the surface of your patch to remove the amine blush.

You can buy 4 ounces of G Flex resin and 4 ounces of hardener in a kit for about 20 bucks:

http://www.jamestowndistributors.com/userportal/show_product.do?pid=8694&familyName=West+System+G%2FFlex+Liquid+Epoxy+Kits

I think that would be enough to apply a couple of skid plates.

Jamestown Distributors also sells fiberglass and Dynel cloth.

In the future, learn to approach concrete boat ramps abeam (sideways). Don't ground the stem of your boat on a concrete ramp. Contrary to the notion that a lot of folks have that Royalex and polyethylene are 'industructable' materials, neither are all that abrasion resistant and can wear through within a season of this type of repeated abuse. with practice you can carefully ground the boat sideways on the slanting ramp and step out of it amidships, sometimes without getting your feet wet.


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