Thirded...
Only thing I've ever found that takes it off quickly is 16 grit norton grinding disks on a 7 inch grinder tipped up at an angle and rubbed down to the
gelcoat so it slats both
gelcoat and tar off. That's if you get lucky and it's the purple based stuff.
If you are unlucky and it's the black VC tar they use on the barges... A torch and a metal scraper can
work too, it depends how close to tar the stuff still is...
Nasty.
Zach