If you are going to go night sailing on a regular basis, you should get running lights as well as doing the flashlight-on-the-sails thing. Like the other poster said about the drunk powerboater going 50 mph at night. Other sailboaters might recognize the reflection of a light on a sail, but most powerboaters are going to be looking for running lights and are just going to think they are hallucinating if they see some hazy white image. 50 mph sounds conservative compared to some of the speeds at which I see powerboats going across our sound late at night. Running lights are designed to not affect your night vision. So be safe and be prepared to light yourself up like a birthday cake, if you hear a powerboat coming. Night sailing is a wonderful experience.