Kevlar reinforcement: fact or fiction?

[rognvald]

I have finished rebuilding my rudder and am ready for the final layup of 1708 biaxial cloth. There is a mixed bag of consensus online in regards to reinforcing the leading and aft edges of the rudder with 6" Kevlar tape. One school says two layers of 1708 is more than sufficient for impact resistance while the other lauds the layup of Kevlar and glass as superior. The problem with the Kevlar after wet out and cure (as I have experienced in building a Kevlar/Glass shoe on the bottom of the rudder) is that it doesn't sand but unravels on the edges and would need an additional step to fair those edges into the rudder before the layup of the 1708. For those who have direct experience working with these materials, what do you think? Is the Kevlar worth the extra step?

[a64pilot]

Kevlar is great when weight is an overriding concern like aircraft or motorcycle helmets. But my opinion is when weight isn't the major issue you may be better off with an additional layer of glass. I'm not saying glass is as strong as Kevlar but additional layers of glass may be as strong or stronger than fewer layers of Kevlar
Be interesting to see what the experts say as I'm not


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[Roy M]

Kevlar/glass on my centerboard leading edge, those rocks better watch out this time.

[rognvald]

Originally Posted by Roy M

Kevlar/glass on my centerboard leading edge, those rocks better watch out this time.

Roy,
Did you do the layup of the Kevlar? If so, how did you handle the edge to attain fairness on the board before laying the glass?

[model 10]

You are not really using it in the right place in the layup. There should not be any finish sanding of Kevlar and even to cut the fabric, the right kind of scissors is almost a must. If you are bent on using it, plan on more glass on top of it and then finish that.

[rognvald]

Originally Posted by Guy

You are not really using it in the right place in the layup. There should not be any finish sanding of Kevlar and even to cut the fabric, the right kind of scissors is almost a must. If you are bent on using it, plan on more glass on top of it and then finish that.

Guy,
That is correct. But, on the shoe I built on the bottom of the rudder, there were small areas of overlap on the edges where the Kevlar "bled through" when sanding the glass overlay. This, of course, can be sealed with epoxy and finish sanded but the concern is a roughly 48" seam to deal with if the Kevlar is used on the leading and aft edges of the rudder.

[Cheechako]

I think by the time you need the strength of Kevlar for a strike, your rudder shaft will be bent anyway!

[s/v Jedi]

I don't think Kevlar gives a stronger laminate, but it does provide superior abrasion resistance. I have it around the boat in the turn of the bilge, to help during a hard grounding on rocks or a reef. It was added for just that, with fiberglass layers for 100% of required strength.

[Roy M]

The easiest way is to mix up a fairing putty of filler PLUS graphite powder, then when applying the glass layer use graphite powder in the epoxy to greatly increase the abrasion resistance. I used the graphite powder in epoxy when I built my hulls, thirty-five years ago befor Kevlar, only below the waterline. Be sure to sand it before it fully cures and hardens, otherwise a 16 grit wheel will only make light circular whirl marks. I used this mix in the old days for lifeguard surf dorys which got hauled regularly over rocks and sand. Gougeon Brothers turned me on to this technique and I've use a lot of it over the years.

[rognvald]

Thanks to everyone for the constructive feedback. I am going to forego the Kevlar tape on the leading and aft edges of the rudder and continue with the layup of the 1708 biaxial glass. The Kevlar shoe I added provided a good experience working with the product and perhaps will add some additional impact resistance and ,at least, some imagined or real peace of mind. Good luck and good sailing to all.

[colemj]

I doubt the kevlar shoe will add much impact resistance, but it will add a lot of abrasion resistance. I think it is a good application for a shoe, where the bottom of the rudder may end up rubbing a lot in a grounding. I don't think the leading/trailing edges will benefit much from kevlar.

Mark

[Cavalier MK2]

I'd suggest polypropylene/ vectra etc... for abrasion resistance. I think Guy has the right of it when using kevlar in a exterior application as the fibers can wick in a lot of water if abrasion exposes the cloth so it should have something on top of it.. Kevlar is great stuff that works in tension, its compression strength is relativly low. These characteristics make it good to use on the Inside of a hull skin on bows for example to reduce the chance of hull perforation, this also helps keep the hull skin from fracturing inward which a exterior lamination wouldn't help with. Other places to use it should be areas where the load is in tension. It can be tricky to apply as the cloth tends to float in the resin making it a good candidate for vacuum bagging, a grinder tends to leave less fuzz on the edges but plan on finishing them with a fillet for fairing.

[Weyalan]

My boat hull and deck are built with several alternating layers of fiberglass and Kevlar. I always assumed it was merely to make the boat bulletproof...

[onestepcsy37]

when i bought my boat (37' full keel) and hauled it i discovered that the bottom of the keel, for a length of about six feet, was badly abraded. was probably run over an oyster bed a few times.

i sanded the bottom smooth and then, on the advice of a local fiberglass shop, bought chopped kevlar. i mixed this into epoxy and troweled it on the bottom and the lower end of the leading edge of the keel. i was told to smooth it as best as i could before it hardened as it's a bitch to sand once it's set.

it's been ten years, i've hauled the boat three times since, and it still looks good and doesn't have a scratch. i can't say it's better or worse than just plain fiberglass but i do feel safer....

[Cavalier MK2]

Chopped strand will only wick the length of a strand, not into the laminate so it sounds like a good idea. Roy's suggestion has merit too as it toughens up the epoxy itself. While cloth types can have differing abrasion qualities the epoxy coating it usually doesn't and will wear down to the cloth underneath which will then start abrading.