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Anyone have experience with Kevlar?


mjshp
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As we gear up for the build of the Spindrift 10N, we are pouring over the plans and realized the hull is not covered in fiberglass. The new dink will primarily serve as teneder to our larger 36 foot cutter.

Beaches here in Alaska are noriously jagged and poky. Think barnacles on sharp rocks. To wit we are seriously conseidering covering the boat from keel to just just above chine with kevlar cloth. I realize this will add some weight, but being new to epoxy and glass, I have a few questions.

How do I estimate how much more weight the additon of 4 oz cloth will add to the boat?

What kind of weave should be used?

If you have worked with Kevlar, do you have any tips?

Thanks!

Michael

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I have worked with Carbon/Kevlar combination in 6oz. We did it in a canoe/kayak building class I took last summer at the WoodenBoat School. It is easier to work with than I thought it would be but is definitely different from fiberglass. There are two things that are different. First is that it is much harder to tell when it is wet out. It is not like fiberglass that completely changes from white to clear. It wets out with only a very suttle change in reflection which is hard to describe but is definitely discernable. You have to watch it carefully for this change to be sure it is thouroughly wet out. Second, it is more difficult to cut. You need to establish a specific set of sizzors and only use them for Carbon/Kevlar. It will ruin normal sizzors.

As to weight, I do not have an answer. However, it was interesting, in the class we built a 12' stripper canoe and used the carbon/kevlar on the inside. Apparently, C/K is most economical in the 6 oz size so that is what we used. The normal specs for the boat called for 4oz fiberglass on the inside and outside. So per the instructor, we had no weight savings using the 6oz C/K when compared to 4 oz fiberglass cloth. We did it mostly for the education and the 'cool' look. If you are interested further the instructor in the class has written a book you might want to check out at your local library. 'Building Strip-Planked Boats" by Nick Schade.

As a sidelight, I built my Core Sound 17 'Lively' about 4 years ago and did not fiberglass the hull or bottom. I did use polyester cloth on the keel and a small section along the chine where it flops over when grounded. Of course we have mostly sand here in Florida but I have never had any problems with wear and tear on the hull and I pull it up on the sand all the time.

You have to decide whether you want it for strength or abrasion resistance. Polester cloth is very good for abrasion resistance and a hell of a lot cheeper.

Dale

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How do I estimate how much more weight the additon of 4 oz cloth will add to the boat?

I'm not sure with kevlar but with fibreglass an estimation of weight is easy. With a hand layup it is usually not possible to get a resin to cloth ratio better than 50/50 ie the same weight of cloth and resin (vacuum bagging/resin infusion can achieve better ratios with much less resin which is why racing boats are built that way). This means you only have to estimate the area you are glassing, multiply by the weight of the cloth and double it to get a reasonably accurate figure. I imagine kevlar is about the same as glass. Some other cloths like dynel and Xynole apparently use a lot more resin.

A quick estimate of your proposal (in metric as I'm used to that) is:

Area maybe 3 sq metres

Cloth weight 130 gsm

Therefore total cloth weight is 396 gms- say 400gms

With resin - 2 by 400 equals 800gms

i.e. about 1.75 lbs

Not a lot of extra weight.

Cheers

Peter HK

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I have used 5 oz Kevler. Since it doesn't go transparent working the air out of the layup is a problem. I used fluted/ribbed rollers that are normally used to roll out chopper gun layups but I found the fluted/ribbed roller very handy to work ait out of fiberglass mat that I used to beef up production molds. And the roller did a fine job getting ait out of the kevlar layup as well. Keflar is good, but I think xynole (?) is better for abrasion resistance. Course both xynole and dynel (another abrasion resister) soak up resin like a sponge, so I would go with 3X or 4X cloth weight to estimate finish weight. Even with Kevlar and glass to get the surface filled and smooth the finish is closer to 2 1/2X cloth weight.

Epoxy by itself is fairly hard. In your case I might go with 4oz or 6oz FG on the bottom, say up to the design waterline. That would give more damage resistance and save a bit of overall weight by skipping FG on the sides. My daughter's wood/epoxy beach catamaran just has FG tape on the keel seam and after a year of sticking it on sandbars and dragging her up on broken shell littered beaches I need to scuff sand the keel area and recoat with varnish, but the hull and glass tape are fine.

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On a 42' sailboat at St. Marys shipyard I've seen someone do a kevlar cloth with a kevlar fiber epoxy mix, coat the inside of the hull with a thick layer. They had hit floating debrie at night and had damage to the hull below and at waterline. The idea was to make it harder to get a hole/crack in hull again. It was tested in an area with no coating, and coating using a sledge hammer. The kevlar fiber held up really well.

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mjshp,

I'm not sure a 4oz layer of kevlar on the bottom will provide as much protection or benefit for the long haul. On a dinghy, your not worried about hitting something at night and getting holed you just want scratch and dent protection for beaching, bumping and the occasional drag. I think a razor sharp mollusk against the bottom will be able to cut 4oz kevlar and thus dig into the bottom. Whats more, once the kevlar is disturbed by rocks or scraping, it turns to fuzz and cannot be sanded (just becomes more fuzzy) You can resin coat it and scrape it flat maybe but after a while you'll have a fuzzy bottom and you'll have to recoat or paint or something. I think I would go for a good 3 or 4 coats of epoxy on the bottom followed by a good solid coat of a hard epoxy paint like Awlgrip or Alexseal. Alternatively what about a 3 or 4 inch by 1/4" thick strip of some UHMW plastic along the keel to take the beating or just the standard SS half round protection.

-Alan

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Kevlar has it's place, but it wouldn't be my first choice for an abrasion sheathing, Xynole would be. It's also a lot cheaper and easier to work too. Place HDPE or other sacrificial strips on the hull bottom, in bedding (not glued), knowing you'll replace them every so often and sheath with Xynole. Dollar for dollar and pound for pound, this is a much better route.

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  • 2 months later...

A Spindrift is one of the lightest dinghies you could possibly own. It is really easy to get used to having a light dinghy over a heavy one. I suggest you consider not adding any cloth. A good full length chafe strip on the keel and a little care beaching and you really don't need anything more. It's all a matter of priorities, but in my case I really like light weight.

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