Graeme is right about the Norwegian term «seilduk». Btw. “duk” used alone is often used about table-cloth. As for “duck”; I frequently find it in books written by fellow Americans, Howard Chappelle and Bill Atkins (written about 100 years ago).
Another thing from that Chapter 5:
I write about the barrel cut method as if it were ‘meat and potatoes’. In fact, the method has sometimes been debated as to if or how it works. After all, asking a perfectly flat piece of canvas to take a 3-dimensional, baggy shape, is quite something. That is why I nailed up a test panel before making my first cambered panel sail (NL 30).
For many years it was thought (by me as well) that it was the softness of the fabric which saved the day. Only a couple of years ago did it slowly dawn to me that this probably was not the case.
The fact is that the warp of the fabric is running parallel with the battens, so will not permit much horizontal stretch. Then, by looking more closely at a number of photos of Ingeborg’s sail, taken at right angle, I could spot a small but noticeable hollow at luff and leech. I had not reckoned with the extra length needed along the middle of the panel to make that camber. The sail sorted this out, all by itself.
This both gave a nice and even camber, plus that it prevented any hooked leech to develop - just like that - by accident! If I had made a gaff sail of that size, with that webbing at the leech, it surely would have ended with a hooked leech.
As it is, I feel no need to add Bermudan-style battens between the JR battens.
Arne
PS: In the diagram below I have incorrectly used the term 'stretch' along the battens. I should have used the term 'tension'...
(Full size diagram at Member’s album, Arne’s sketches, section 7)