They are ultra light, lifting keel boats. From what I read the boat capsized but did not sink. The two people that died were clipped in and held under by their harness. Sounds like the boat is very stable upside down. These boats have been around for a long time and are fine in most conditions but I suspect they have a fairly low range of stability and once past they don't come back. The kid in the article is lucky his buddy was there to unhook him or there could have been a third fatality.

http://www.chicagotribune.com/news/local/breaking/chi-sailboat-capsizes-2-from-mackinac-race-missing-20110718,0,3611815.story