colinite hull cleaner will take that off no problem. it requires some elbow grease though. goes on and off like wax. after that 3 or 4 coats of wax on the transom. colinite fleetwax in the container is the best wax for season long protection. then you're going to have to clean after every trip or get a different motor
28 Carolina Classic, 34 Webbers Cove, 18 George Williams Skiff
I used this Fireplace Glass Cleaner on pretty much everything with soot and it worked pretty well. I had a Volvo TAMD41A so that makes me kind of an authority on soot.. You can get it on Amazon or Lowes..