Kevlar vs Fiber Glass

I recently learned you can use kevlar over fiber glass. As I understand kevlar is a lot stiffer then fiber glass and you will need less coats. So what is more important on a boat stiffness or flexibility? I plan on sailing on blue water at some point but I want to build the safest boat I can for rivers and blue water. So I want to start off on the right track. However, I've also heard kevlar is overkill? I'm no pro please post your thoughts.

 

When Kevlar first became available, there were lots of enthusiasts who believed it to be a wonder product that would revolutionize the industry.

It mostly failed to live up to the initial expectations.

Because it is very difficult to work with in composite construction, few will voluntarily use it in a second project.

 

Kevlar does not sand very easily in resin. It tends to make a hairy blob of fibers. For this reason, its uses are limited. It may also float in a laminate stack or so I'm told. I did not experience this because I buried it, so never saw any evidence.

I used it in two places in my build.

The beaching keels. I buried one piece of kevlar in a stack of 8 layers of 1708. This piece is designed as a failsafe abrasion measure. Should the beaching keel lose her sacrificial timber and either I don't see it or it happens due to grounding on a reef, I would have some extra protection for the hull.

On the helm, I used red aramid/carbon for a decorative addition.

 

Absolutely true.You could add that it is almost impossible to cut the cloth without the special shears to that list too.Its important to remember that there are two distinct families of Kevlar,I know of somebody that bought some "bargain" Kevlar a long time ago and the now abandoned hull might be getting to the point where it can easily be cut up and dumped due to the UV degradation.Ballistic Kevlar isn't quite the same as laminating Kevlar.....

 

Kevlar also has low compression strength so unless you use it in just the right place, it's not that great. Though it is low density. I would personally only bother with it for an abrasion strip along the keel of a dinghy or canoe, or other boat that gets dragged up on a beach.

 

Using kevlar in a lay-up is not easy, it's hard to wet out. It will add a lot of strength, but needs to be protected. If it is exposed and not fully wetted it will wick moister like crazy and delam. Leave it to the aircraft industry and ultralight canoe builders.

 

If buried in the laminate; it is hard to mess up. Not to disagree with you, but there are ways to mitigate the problems.

 

Not to disagree with you, but if buried in laminate, isn't it hard to know you messed up and have a wetout problem making a stress riser? That avoids the fairing/furry problem but compounds the adhesion problem.
My other point would be that to get the most out of a stiff, high strength fiber layer it would want to be the outer fiber and aligned with the strain.

Speaking of mitigating problems, I think of kevlar as a high performance that can be used to mitigate the catastrophic failure of carbon fiber parts in impact and overstrain.

 

Buried in a laminate fails to use its properties which are tensile, it is best used on the inside of a sandwich laminate.
While abrasive resistance is good the fluffing up when ground is a problem, the solution to the the fluff is to wet sand it with a course wet and dry paper which given its love of water is counter intuitive but some time with a fan heater or paint stripper gun will get the water out.