: I'm far enough into my current project to start thinking about non-wood
: materials. Like everyone, I want to maximize strength-to-weight and I'd
: like to use some carbon cloth in places for the weight savings, and that
: leads me to my first question. Carbon cloth is rated just like glass,
: isn't it? So if I use 5.7oz. carbon it will still weigh 5.7oz/sq yard?
: That means I'll only see a negligible weight reduction over 6 oz glass,
: but what is the increase in strength?

There will be an increase in strength, but the carbon is also a bit more brittle. Since e-glass is already strong enough for our purposes, going to S-glass or carbon fiber cloth which weigh the same, would give you surplus strength, and not save you a bit of weight.

: Second issue is- on a day boat is the 25-30% strength increase in S-glass
: sufficient so that those of you who use this stuff on a regular basis
: (This is my first non-SOF kayak) would think it's o.k. to go with 4-oz
: S-glass instead of 6-oz E-glass?

Depends on what design you are building. If the design you've chosen works fine with 4 ounce e-glass, then you would need to find some lighter weight s-glass or carbonfiber to get the weight savings you seek.

So, what design are we talking about? Chances are that someone on this board has already built a version of htat design, and can give you firsthand knowledge of whether you need to beef up the reinforcement in any area--or not.

: Assuming that what I've read that glass is stronger in tension than
: compression, I'm thinking of using a full layer of 4-oz S-glass on the
: exterior with a second layer in the hull's "damage" areas, with
: a layer of 4-oz. S-glass on the interior, and the cockpit area reinforced
: with carbon fiber cloth.

Assume you hit a rock. Hard. The outside layer is going to be compressed. The inside layer is going to be put under tension. You want your greatest strength on the inside.

The outside layer is there basically to protect against scratches and scrapes. Think of it as a very thick coat of reinforced paint. When all you really need is paint, adding any reinforcement is overkill. Adding even stronger reinforcement would be what? overkill of overkill? I don't think we have a term for this. Use your cheapest materials (e-glass) for the outside. You need to think about thickness, not structural strength for this layer.

If you hit something pointy it might puncture the hull. In that case you patch the hole with duct tape and continue your trip. Or, you can spend a lot of money on layers of kevlar (or armor plate) to prevent the puncture. Practically, this just costs money and adds weight. A 4-ounce layer of E-glass is plenty strong. By the way, this is probably the easiest palce to add an additional layer of glass after ther boat is built. If you build it with a single layer of glass and find that after a year or two of use you have created a lot of damage, then add a second layer of 4 ounce glass, or even a third layer. that would give you 12 ounces of glass, the same as two layers of 6 ounce glass. But only add this much weight and cost if you find that you REALLY need it.

: I'm sure that I'm not being innovative and someone's had the same idea, so if
: you can let me know how it worked I'd appreciate it. I'm only going to
: build this boat once, so I'd rather spend a few extra bucks to do it
: right.

"Right" is a relative term. To the best of my knowledge, there are no plans for canoes or kayaks under 20 feet (neither strip-built, nor plywood S&G) which call for using carbon fiber in sheathing the boat. I don't even know of any that specifically call for the use of S-glass. Everything I've seen calls for layers of e-glass. Older designs called for 6 ounce glass. 40 and 50 years ago this was lightweight glass cloth. A lot of the newer designs are calling for 4 ounce glass cloth. They may use doubled layers for greater strength in some areas, or not.

If you have a supplier for 2, 3 or 4 ounce carbon fiber, or s-glass then you might be able to substitute some for the less expensive e-glass, but your savings in weight may be slight. Carbon fiber was in short supply last year, so prices on it are rather high. It hides the look of the wood, and gets hot when exposed to direct sunlight. Other reinforcements will soak up resin like a sponge, or be difficult to sand. Consider limiting your building choices to e-glass or s-glass.

Hope this helps

PGJ