Hello David !
Thank you very much for your quick and detailed answer.
BTW, from Dec 14 there's Deborah Lake's "Smoke and Mirrors: Q-ships Against the U-boats in the First World War" on amazon.co.uk. Its synopsis is exciting, I'going to buy this book.
Well, it's quite amazing that I didn't think about finding informations I need in Q-ships books. A week or so ago I've read for the first time in my life about trawler-submarine scheme, in Harald Bendert's "U-Boote im Duell".
Regards -
Burt