I have a 15' Alaskan that I put a pair of Invader clamp-on 'riggers on. Seems to do okay with 8lb balls. Not too sure I would want to go too much bigger.

Incidentally, I got the Invaders for about $150 for the pair on eBay and they are pretty nice. They have a clutch, a counter, and bring in 18" per crank.