A slave ship is a slang term for the dutch rudder adaptation of a circle jerk. Involving two partners, the dutch rudder is one individual moving the other partners arm to help masturbation. Like some of the other masturbation adaptations, the idea is that the experience is more enjoyable when another person is involved and assisting in some way. When you put several of these pairs of "Dutch Rudderers" together in one area, this is called a slave ship. The word "tugboat" can also be used and carries the same meaning. Tugboat is oftentimes preferred over slave ship in case your friend "circle" is interracial.
"Lets go in the woods, snort some crystal, and have a slave ship around the campfire."
"Those guys bought a giant leather corner couch so they could sit around and have a slave ship party."
"I'm tired of doing the dutch rudder with just you Tom, and I'd like to have someone helping me at the same time, so let's invite over a few guys and do a slave ship in the basement."
"Make sure to send the slave ship Facebook invite as 'a tugboat' because I want Jamal to come. He's so strong looking."