Vinyl Mastic is the stuff you want.
[
www.foxelectricsupply.com]
I broke my Jefe in BC the night before flying in for an overnighter and was given a patch of this stuff for the foot long crack. The same boat has been going on the patches of this stuff for the last 9 months. The only problem is that if you want to weld the crack properly later, the stuff's a bitch to get off (takes hours).
Apply by cutting to proper length, heating slightly over the fire, and placing over crack on inside of the boat.