Re: Hemp Sails ?
Just to play devil's advocate...
I think size of the boat is key. I used cotton tarp sails on my little dinghy all last summer (I sense eyes rolling as I post this haha). After a daysail, I hang up the sail in the backyard to dry (though it doesn't seem to soak up much water at all, even in choppy conditions). Since I have a balance lug now, I just take the yard and boom off the mast and prop them up across the patio.
The only advantages of natural materials, it seems, are ease of building and low cost. The sail can be sewn flat, the "scoop" will manifest itself in use. So it is very fast/easy to make a basic sail. And cost is less than 1/3 of dacron.
However, you have no control (or as a previous poster noted here, limited control) over where exactly that "scoop" will be. The material is plenty strong, but needs extra stitching/reinforcement, since it doesn't hold stitches quite as well as dacron.
Cotton canvas seems to work well in squareish, unbattened sails, probably because it is very easy to manipulate with downhauls/vangs/etc., being stretchy. Most of all, it allows me to experiment with different sail designs very cheaply and very easily. (I rigged up an experimental kayak sail recently in < 2 hours and for < $10). In fact, I will be rigging up a lateen sail to experiment with on my latest build. Most likely the lug will prevail, but I can build an 80 sq ft lateen for about $75 total including the yard, so I have little to lose, and alot to learn. Most of all, I will have first hand experience with an ancient sail type, and will be able to make informed commentary on the type.
While performance has not been bad in my admittedly limited experience with the cotton, it is not as good as dacron, and I have added slightly more sail area than a comparable dacron sail would need to compensate for this difference. "High" performance in the sense of getting maybe a knot better speed on a reach or a few degrees better to windward just aren't as important to me as low cost and easy experimentation are, and I enjoy myself no matter what.
Of course, this thread is really more oriented towards the utility of natural sails on larger boats, and in that case I think several posters have confirmed from first hand experience that on larger vessels cotton/hemp just isn't practical. I certainly defer to them. But for smallish boats I don't think natural materials, if they can be found cheaply, should be just dismissed out of hand. There is a place for them, alongside polytarp and the like.
“The difference between an adventurer and anybody else is that the youthful embrace of discovery, of self or of the world, is not muted by the responsibilities or the safety-catches of maturity.” Jonathan Borgais