Be very very carefull if you are going to put a razor blade against your hull. It will work well for really old crusty graphics if you only try to remove the top vinyl layer on the first go leaving the crusty adhesive underneath. Then apply an acetone soaked rag overtop of the adhesive and leave there for a few minutes (it's useful to place it ahead on the next area you will be working on). Then take the razor blade and lightly scrape away the adhesive - it will come up like butter. Beware that when the acetone evaporates (which happens quickly) the adhesive will go back to a nasty gummy state...so clean your razorblade very often on a paper towel and use many towels. If you try and rub away the adhesive left on the hull, it will smear and dry and smear and dry and smear and dry and will be very difficult to get it all removed in one sitting.
I would not recommend trying to remove everything on old crusty vinyl in one pass or even touch newer graphics with a razor blade. The new graphics - especially if using a premium grade vinyl, will remove cleanly by picking a corner with your fingernail (or using a hard plastic scraper that can be found at a local hardware store) and peeling away. Premium vinyl will tend to tear and come away in strips but will come up cleanly. Intermediate grade vinyl will come up in slightly larger pieces but will leave a good deal of adhesive behind. Waxing the hull prior to application (with a synthetic wax like Nufinish) will aid in clean removal of the grahpic and not adversley afect it's adhesion to the hull...although I do recommend doing a test before letting everyone wax their hulls first.
I recommend doing your sailors a second favor and supplying the graphics all on one solid piece of high grade white vinyl. They can either be printed on the white vinyl or built up in colored vinyl layers (using the intermediate grade now to save a little $$). This way, at the end of the event, the sailor only needs to peel away the single layer of white vinyl and all the graphics come up with it cleanly.
