In 05 I had a wreck with some road tar. It took about 5 hours of elbow grease and tar/bug remover to get it off, and that didn't include the bottom of the boat that you couldn't see. So I bought a bow buddy in town here and its worked pretty good
http://www.trailer-rockguard.com/ and I think it was only 300 bucks. Its now on its 4th boat and on thursday I drove from G.P. to Slave with about a hundred kms of fresh tar/oil and found 1 tar ball on the boat. I also have mud flaps accross the back of my tow vehicle attached to the hitch which helps keep the rocks off the boat.
tight lines