First, a practical matter, It is very hard to sand and fair Kevlar, it fuzzes. So you put it on the inside so you don't have to finish it. You can cover kevlar with glass or microballon epoxy to fair it but that is dead weight

Second, Components of a laminate do not take load individually. It is the strength of the whole. Once you exceed the strength of the whole, it starts delaminating, becomes about as strong as wet cardboard and and the whole thing falls apart.

Third a Carbon/wood/Kevlar can be made lighter and cheaper than a carbon/wood/carbon. Plus Kevlar has great impact resistance and tension strength. In case of a sharp impact, like a collision, there are a lot of cases where the carbon would break but the wood/Kevlar will flex or not break clean so the hull would hold long enough to go home and fix it. That is the theory plus panel testing plus hear say from white water kayaks anyway.

Fourth, for a homebuilder Kevlar is easier to laminate and a lot cheaper. I have bought it for as little as $2/yd on eBay. Carbon is ridiculous on eBay. To make carbon junk for their car, people are paying more than they could buy it from a real dealer like Fiberglass Supply Company.

Carl

Imagine if we could build our boats like this, 1 piece, the technology is already out there.
"The Premier I composite fuselage consists of 2, filament wound cabin sections (1/8th inch carbon fiber filament that is pre-impregnated with epoxy) that are bonded together."
Check out the pic.

I like the Kevlar on the outside idea. Makes good sense.

Darryn
Mosquito
1704
South Australia

Darryn, now you are getting closer to my crocheting idea, because I was thinking about how to make it as one piece, like a big sock, whether crocheted or knitted or woven, around the hull's internal structure and bulkheads. I'll bet a good basketweaver (or hatweaver) could make a REALLY light boat using palm-frond leaves (or whatever they use) -- and those are available in huge quantities for FREE.

Let me be blunt. Don't put Kevlar on the outside of a core composite. It is likely to fail.

In general, near worst case, for a catamaran the outter layer of a core composite is in compression and the inner layer is in tension. Kevlar is strong in tension and weak in compression. Look at what happened to the Kevlar/foam core Tornados. The Kevlar failed in compression. This also applies to similar materials like Specta/dynema/Vectran/etc

I hate to keep this thread alive BUT I don't want anyone to spend time building something that may fail.

I believe the Taipan 4.9 comes with optional Kevlar hulls, and I had assumed (although in complete ignorance) that it was on the outside. Anyone know?

I believe Boyer uses Kevlar on the inside and glass or Kevlar/glass on the outside. In any case Boyer has done suffient engineering to make sure their hulls hold up.

The real problem is a homebuilder substituting

Carl and Mary
Your crochetting idea is already in production, Forte RTS http://www.forterts.com/per.html uses a computerized braiding machine to make carbon parts. This technology is directly from the weaving industry. The carbon (or whatever fiber) is wound around a mandrel, so you need a mandrell in the shape of your part. As its being wound, it is impregnated with epoxy. It is then put in the autoclave, and cured at elevated heat and atmosphere. The result is strong, light and very reasonable priced parts. I bent 4 aluminum booms (falling on them) before I got a forte boom. I have beams, spin poles, booms and tiller extensions, and tiller connectors. The only thing is that the outside "skin" is an "uncontrolled surface" so its not smooth, it has texture and it needs to be UV coated. Tony will make you spars to your specs for very reasonable prices.

also, ebay carbon is a rip off. Go to carb.com for the best prices that I have found on the internet. 5.7 oz carbon weave 30 inches wide is $14 per linear yard at carb.com. 50in wide is like $20 and I have seen it at over $45 per linear yard other places. Ebay was even higher, its some guy selling his scaps, or selling sheets with epoxy already hardened to people for accent parts in their cars. Carb.com is in Hawaii. They told me prices were going up. Order now.

I also found good prices on nomex from http://www.plascore.com/ they were about 1/2 the price for 1/4,5lb,ox - a 3x8 sheet was $50. The lead time is 6 weeks.
I talked to Wade.

Wouter, Your 14kg figure for an A cat sounds low. 17kg-18kg target is what at least 6 new home built A cats are shooting for - check out the pictures http://intcanoe.us/mygallery/index.asp?offset=0 17-18kg is a total hull weight. I used 5.7oz carbon, 1/4 corcell foam and 2 gallonsmas epoxy with 1 gallon hardener. The hulls are 6 sq meters. I got 4 hull halves to weigh between 6kg, and 6.1kg, no paint, no gel coat, no dagger trunks, no sterns, no bulkheads. My all up hull target is 17kg. The trunks are super light high mod carbon Hall parts. I think they weigh 30 grams each. The biggest unknown is the finish - paint can weigh 3 kg per hull.

Any ideas on lightweight hull finishes? How about wood veneer? How about wood printed paper? How about a hawaiian print fabric? All white is boring.

Bill

The Taipan 4.9 can be ordered to have a glass/kevlar hull skin. If memory serves me right this means that the sandwich construction from outside to inside is :

Gelcoat
Glass
Kevlar
Foam
Glass

You don't want Kevlar on the outside of the outer layer as well as it forms a vary rugged fluffy surface when is touches something abbresive. Glass wears down alot more smoothly.

The kevlar is only in the hulls to give it extra impact resistance. If makes the hulls alot harder to dent. It is not in there to improvement tensile strength or stiffness of the whole hull.

Wouter

14 kg was taken from Marstroms webpage

http://www.sailcenter.se/administration/Boats/acat/aclass.asp

I know that homebuilder used to build A-cats for as little as 45-50 kg's before the 75 kg rule came into effect. However if you pitchpoled one of these than you would break the hulls. I think they used 3 and 2.5 mm marine ply for these hulls.

I'm often amazed of how light you can actually build when using marine grade ply. It is pretty good boat building material and it is very impact resistant.

Wouter

There is a very good description of the layup of the Shadow hulls in the following sail test under "Design and Construction"
http://www.shadowsailing.org.uk/documents/Yachts%20and%20Yachting%20Shadow%20test.pdf

Damn, my boats going to fall apart, its double Kevlar so it probably wont even make it to the water before the outer layer fails in compression and then, because its platform weight is 55kg it will break up when I pitch pole, dont know how it has survived the last 16 years
Darryn
Mosquito 1704
South Australia