Wooden Boat People

By McKenzieDriftBoat.com

Technique question on applying 5oz plain weave Kevlar to bottom of boat.

Hi,

I've purchased 5 oz plain weave Kevlar to put on the bottom of the boat I'm building. I've enough for 2 layers.

Since it is plain weave I can't see any reason to change the direction of one layer to another. In other words I plan to cut 2 sheets long enough to cover from bow to stern. It seems to me that if I laid on layer 90 degrees I would be just creating more work for myself in cutting and lining up the sheets for that layer.

I checked "The Gougeon Brothers on Boat Construction" book and only found a reference to testing Kevlar impact tests that showed no real difference in the direction between layers.

As this is my first attempt at this I would sure appreciate feedback from anyone who can affirm my plans or give me direction and reasons why another layout would be superior.

Thanks,

Jeff Levin

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Replies to This Discussion

Permalink Reply by Eric Loudenslager on October 9, 2013 at 6:14pm

If using plain weave it would be best to apply it at about a 45 degree angle to the center line. That will give threads perpendicular to both the center line and the width of the boat. One layer should go one-way, and the second cross it, since you have the cloth laying on a diagonal. Plain weave turned 90 degrees would have fibers running the same direction in both layers.

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Permalink Reply by chris towles on October 9, 2013 at 6:53pm

As eric said, protected the 45 bias is never a bad idea. I usually do it on the bottom layers, then lay the one piece of 0 90 layer as the top. This is reLly no more work, you still have to cut that chine line, hope you have some sharp scissors!

Only thing i can add is that kevlar tends to float in a laminate, so its best to cap with something to hold it down like regular glass. I have been vacuum bagging my kevlar laminates but that adds steps to the preocess, one way to get a nice finish is to use a peel ply or a release cloth like visqueen so smoosh everything down. In addition, dont put kevlar is an impact area where it can get damaged. Kevlar is almost impossible to get thru, but since it tends to float in a laminate it is very easy for an impact to smash the bias of the cloth and you end up with a soft spot in your laminate. Kevalr is impossible to grind, sand, cut, etc. you will end up with a fuzzy mess. Always put glass over the top for a finishing layer where it will be in contact with rocks. Additionally the old mantra is that kevlar goes inside. Reason is that kevlars biggest strength is its strength in tension. It is commonly used as a last layer inside, but if you put it outside the boat, make sure it is at the bottom of the laminate next to the substrate with glass to top. If it were my boat i might do a layer inside and a layer outside for max protection. 2 layers inside would likely yield a close to impenetrable wood floor, but you would need an outside skin of 20 oz glass minimum. A floor like this would not allow impacts to propogate up thru the wood and explode the layers on the inside of the floor which is the common damage scenerio especially with a wood floor that is only glassed on the outside. The kevlar would hold the tension of the hit rather than letting the wood flex to its breakng point.

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Permalink Reply by lhedrick on October 10, 2013 at 11:31am

I have a boat with 17 ounce stitched triaxial kevlar over a honeycomb bottom.

One of my friends used the boat one time and cracked the bottom in about 4 places, rock impacts. Once it is encased in the epoxy matrix it becomes stiff and with regard to impact it's not as strong as you might think. I doesn't cut but is can break on impact.

I also find it's not as difficult to work with as people say it is. You need a good scissors with a serrated edge and never cut glass with that scissors, kevlar only. While you can't cut through it with a sander, a rotary grinder with a 40 grit disk has no trouble cutting it down. I use the type of disk used on steel for welding prep. You won't have the fuzz ball problem with a grinder.

Kevlar is a very strong fiber when being compared to glass fiber but, it is almost 5 times the cost. Difficult to know if it's worth the cost. If money is not an issue I would use more of it.

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Permalink Reply by Phillip Westendorf on October 9, 2013 at 7:40pm

Jeff,

There is a lot of discussion on a layup schedule for Kevlar. Here's a discussion on the use of Kevlar. It's a lot of facts/opinions but if you read it all it'll give you a better feel for the use of Kevlar for your boiat.

http://www.thehulltruth.com/boating-forum/265423-sell-me-kevlar-vs-...

Here's another discussion in Wooden Boat Forum, similar to your question. Take a look.

http://forum.woodenboat.com/showthread.php?159382-Sufficient-glass-...

Also there is another Layup Question regarding Kevlar on a Canoe Build. It also has some interesting information. It's also worth reading.

http://www.myccr.com/phpbbforum/viewtopic.php?f=49&t=40399&...

I read all three and the Readers Digest Version would be to Layup the Kevlar on the wooden bottom with an aditional covering of Fiberglass if you are intent on using the Kevlar. I don't think the weave direction of the Kevlar would add a whole heck of a lot, the second layer would just add more stiffness. Maybe the design of the boat does or doesn't need Kevlar? Is it a S & G or Framed, Bottom/Side thickness and type of wood bottom, etc....