Stui,
The folks at West Marine must have been hung over. They are called "drogues". West Marine carries a few different kinds. I've had great results with drogues made by Hart & Hart. They are more expensive than the others but are built to last. They come in two sizes, 32" and 46". I tried to insert a link but screwed up somehow. Just do a search for "drogues" on West Marine's website.
I use one 32" with my 18ft glasspar. It trolls at 2.7 mph without it. If you have a big boat, I recommend the 46". You can always increase speed. One big one is better than two smaller ones on each side of the boat. With one, there is a constant pull to that vs two where they dig in at different times making it more difficult to steer.
You want the leading edge about mid-ship. Tie it off with a strong rope. Then tie off the back end with smaller rope. This is used to keep the drogue close to the hull and also makes it easy to remove from the water.
Hope this helps out somewhat.
Let me know if you have more questions.
Rick