That's somewhat of a loaded question. I happen to own a Yachtboy 400, and I can listen to Gander, New York, Shanwick, Santa Maria with just the whip antenna it comes with. If you want to listen to the weaker ones, like Dakar, or maybe some Pacific comms, then a simple wire from Radio Shack will do the trick. It consists of a fifty foot long copper wire, with an insulated lead in. You can solder an alligator clip onto the insulated wire and then clip it to the antenna. Be forewarned...once you do that, set your Local/DX switch to local, because the receiver will overload.
I hope this helps. Have fun!