I don't know about skulls, but for crew shells there is sometimes just a little bit of a keel at the tail, and then the rudder. For most of the length it's sort of a semicircle. Here's a picture of a cross (very cross) section.
http://www.jrshells.com/pages/jrs_vespoli_repair.html
What I found intriguing is the risk that they took. The ice could have esily drifted closed and then they would have to walk home.