A man joins the crew of a ship. After a few days he gets restless and asks "What does one do about sex around here?"
The others direct him to a large gun barrel with a hole in it. At first he does not like the idea much but, when he tries it, he finds it surprisingly enjoyable. He has another go the next day, and again the day after.
Then he asks, "Can I do this every day?".
"Yes, every day, except Wednesdays".
"Why not on Wednesdays?"
"Wednesdays is YOUR turn inside the barrel!"