The bay where I'm sailing is a regular training area for military submarines. It happened to me last year that a half dived vessel was directly coming my way - only the last meter of the periscope was visible. I was downwinds with spi and saw nothing than the stern wash from the (admittingly perfectly camouflaged ...) periscope in the very last moment- I passed so close by that I feared for my boards. This was very, very frightening! What would have been the rules here?