Oh no they don't! Our yacht had one of the best radar reflectors (fender type) permanently fitted and we still had our share of near-misses. The only thing that really, really works is in fact the transponder, which actively sends a radar return when painted by another set. That's why aircraft carry them.

No, the sails idea was a thought aimed at improving the passive return from a sailing yacht. I had no idea that it had already bee tried - would be grateful for any references to this you might have.