You should have positive righting moment with the keel up and sails rigged, otherwise it's totally unsafe. When you're sailing downwind you have the keel up for speed. If you get knocked down in this situation--a broach, for instance--the boat needs to be able to right itself otherwise you probably die. This boat sank when it got knocked down, and it doesn't seem the keel had anything to do with it.
Do you know that raising the keel when sailing downwind was standard procedure on ships like this, or are you extrapolating from experience on dinghies and small keelboats? There are many yachts where keels are retractable, but left down when sailing downwind (the Hobie 33 comes to mind).
