Um, the mast I have is hollow aluminum, I think with a slit running down most of it? Hmmmm
yeah, the masts are teardrop shaped aluminum extrusions. If you shine a flashlight into the slit (the is the slot that the front of the mainsail slides up in), you should see that it doesn't go all the way into the mast. Something like this:
When people are talking about sealing the mast, its the main hollow inside the extrusion. If you capsize and the mast isn't sealed, it will fill up with water, the boat will turn all the way upside down, and it will be much harder to get back upright...
As for pictures, see this FAQ:http://www.hobiecat.com/forums/viewtopic.php?f=18&t=8574
There's a lot of good info in the "Hobie Sailing - FAQ" forum.