I am not impressed that much by advertisements on things that you see for only a short time.
I have put the name of some of my websites on my truck using magnetic signs, and the results are not impressive. I have talked to other people using magnetic signs on vehicles, and the results were not encouraging.
I don't think anyone would pay you very much for putting a sign on your sail unless it was up eight or ten hours a day in a high traffic area. If you did that, your sail would be destroyed by UV light. Sail rot
would be more expensive than the pay you recieved for the advertisement.
Now if you are a round-the-world racing
yacht, the results would be different because you would get media coverage that went into magazines and television.