use of kevlar vs e-glass as outer skin

I'm about to start building a small skiff using foam-core construction and am considering using 5oz kevlar as the basis of the outer skin. From what I have read and seen 1 layer of this coupled with a layer of very thin CSM should suffice given the design and purpose of the boat (14' oa, 600# displacement, max speed 30mph, panel size 22" x 10".
Does this seem about right? (Reading the forum and going by rule of thumb my boat should require a 1/2" PVC core, a 2mm outer skin, and a 1.5mm inner skin)
why is CSM recommended between the foam and the skins (glass or Kevlar).
Wouldn't a layer of 4oz e-glass outside the Kevlar be stronger than the CSM ?
thanks
Tedmelrose

 

Anytime you have a cored construction you must have something in between the core and the skin that will adhere to both, else the skin will delaminate. With wood-core this is usually the layup resin; with honeycomb and in prepreg construction, it's a film adhesive. The manufacturer of your foam should specify something.
The stiffness of a cored laminate comes from the thickness between the skins, provided by the lightweight core. Its resistance to breakage, and its ability to take the impact of bouncing over waves (or, God forbid, rocks) is a result of the strength of the skins themselves and the bond to the core.
From what you describe, your layup looks kind of thin. It will be stiff enough, yes; however, it is quite rare to see a single layer on each side, or your thin double-layer setup. I'd forget about the math here and do your own tests. Try a test piece, about a foot or two long, and try to break it- see where it fails. If you can get the skin to tear when you load the piece, you need a thicker skin; if the skin comes off the core, you need a better bonding resin. When you're confident in your test sample, go ahead and build. You'll also then have a good idea how to lay up your hull as easily as possible.
Another note- when using cored construction, vacuum bagging is a Very Good Idea, as core and skin tend to separate while curing if they aren't under vacuum.

 

thanks Marshmat.
I take your point about the math. In order to go beyond the rule of thumb required calculus as far as I can see. Anyway, the various specs are pretty confusing. When I was looking at glass skins I was going to use the "rule" calculations which led me to 2mm outside & 1.5mm inside the core. The question about using kevlar comes from the fact that some builders actually do it - but with cores thicker than 1/2" (more like 1" for the bottom) The question about why use CSM on both sides of the core & under the fabric remains unresolved since all it is, as far as I can see, is a non-woven matrix to wet out with the epoxy or vinylester. I don't have any problem with using it; I just want to understand why it's commonly used rather than, say a layer of light cloth. Can it be simply that it wets out better, as I've been told. Interesting about the vacuum bagging, I'd thought that it was because the vacuum tends to drive the resin deeper into the foam & also results in a lighter layup by driving excess resin through the peelply. Live & learn, I guess
Thanks again
Tedmelrose

 

Ted,
CSM is used primarily with polyester resin to promote bonding-strength because it tends to become a lot more resin-rich than woven of knitted fabrics. Epoxies have a lot better bonding-strength to begin with, so we usually dispense with the CSM in those layups.

Kevlar's weak point is compressive strength, only about 28% of tensile strength, so it may not be all that well suited for an outside skin.

Yokebutt.

 

Another reason chopped-strand mat is so commonly seen- it's often used as the outermost layer as it does not transfer the pattern of the weave to the gelcoat when laid-up in a mold. For your skiff you are probably after low cost and light weight for the strength you get; CSM is rather heavy and weak compared to good cloth. Yoke's right, if you're using a good epoxy your layup doesn't have much use for CSM.
Kevlar can also chafe if exposed; nevertheless, I've seen it used as both skins in many parts. The quality of the skin-to-core bond is critical- if a part fails, this is usually where it breaks.
It's quite possible to mix different fibres in one layup if desired. If you're worried about Kevlar's compressive strength on the outside, put a layer of carbon or e-glass outside it. With a good epoxy and a good vacuum, your laminate should be fine.

 

thanks guys: You've been a big help.After reading your comments I guess I'll go with epoxy and 5oz kevlar overlain with 9oz e-glass for the outer skin. I bekleve that this will actually be heavier than currently used by a well-known mfg of pretty spendy boats -- although it may be prudent to wait for more comments on the weight of this proposed layup. I plan, pending more comments, to use 9oz e-glass for an inner skin (but remember that I also plan to core the bottom with about 2.5" of 2# foam which should beef things up.
I didn't realize that Kevlar floats in the resin & should therefore be vacuum bagged. How about rolling it into a previously applied primer coat that's been applied to the foam core? Seems like that'd stick it down so that the weave could be saturated without the float problem? (I'm sort of trying to avoid the vacuum bagging- at least partially because One of the DIAB manuals doesn't recommend it for male-mold construction (but they don't mention Kevlar either).
Again, Thanks. I'm sure glad that guys like you are around!
Ted

 

I have not read the manual you refer to,but I would be hesitant to omit any process that aids consolidation of any laminate.The problem is actually achieving a vacuum resistant male mould.What family of foam are you intending to use as 2.5" seems massively thick for a core?

 

wetfeet, I guess you had to have been there. The idea is that I intend to use a 1/2 or 3/4 inch core in the hull proper and then add about 2.5" of poured foam to the bottom to provide extra flotation and added strength.
But, while we're at it, ya'all, (I'm from Florida) I just read a very disquieting article in marinesurvey.com that basically says that ALL cored glass boats are disasters waiting to happen. What's everybody else's answer to that?
Back to my boat. I'm having serious difficulty finding a place to buy the foam. Seems that to get a decent price you have to buy a ton of the stuff. Too bad there isn't a one-off builders co-op or something.
Final question. No-oner's commented on the idea of using 1/2" 4-5# foam for a core. Soes that seem about right?
By the way, guess that if I go ahead with this I'll have to learn to vacuum bag. Sigh ..
Ted

 

Ted,
Get your hands on the last issue of Professional Boatbuilder, (proboat.com) it's got an article by Bruce Pfund explaining core installation quite well.

As for all cored boats being disasters waiting to happen, well, I suppose there will always be a certain number of backwards, simpleminded, redneck, luddites who'd happily spend more than twice the amount of money neccessary on fuel because of some imagined problem with a more efficient structure.

Yokebutt.

 

Ted

Yes, there are poorly built cored boats that are among the biggest disasters I've ever seen. That does not make me even a little inclined to say that cored boats are all disasters in the making.
In a discilpline like boat design, with a vast number of possible and desired end results, "broad brush" statements like that are completely meaningless.

So if I set out to break the human powered boat one minute distance record, and I made the boat out of rice paper and broke the record---only to have the boat dissolve at one minute and one second---would it (the boat) have been a disaster?

Hell ...tell Boston Whaler that cored boats are no good!

 

Core is bloody pricey (decent nomex is a hundred bucks a 4x8 sheet). So is resin. So is good cloth. So are boats.
If you lay it all up properly, there's nothing inherently wrong with cored construction. The problem is that it's easier to screw up the core-skin bond than it is to screw up a solid laminate, but this is not a problem if you use your brain a bit before pouring resin onto cloth.
Any book that says not to vacuum-bag is to be taken with a truckload of salt. It is virtually impossible to get a solid core-skin bond without vacuum. (Bagging's actually a lot easier than it looks or sounds.)