When we raced as youngsters, we often thought it great fun to sail by other boats and advise them (with great concern) that their rubbblebars were loose. Leaving it to our competitor to determine exactly what a rubblebar was, and how to tighten it. Suggesting the use of rubber washers for keelbolts would seem to fall under the same situation: a total fallacy. Rubber is called for when compressible or elastic qualities are needed because things are moving. A keel, by definition, is something you do NOT want to move - the rest of the boat is built upon it! If it is moving, it is going to leak - this is the problem the Volvo RTW boats are having right now; their keels are moving and water is getting in. Perhaps a rubber GASKET, which would compress and keep water out, would be indicated in a hull/keel joint, but this is not the question you posed. Individual washers on a standard keel would not perform the same way and would probably serve to create leaks because of the differences in the tightened spaces and how they would allow the keel to work, because of their compressibility. I would suggest going back to your professor and asking for clarification of this issue. Sounds like he's sending you to look for the head keelson spanner- a standard joke for the newest crew on a submarine.