First thing you need to do is drill a small hole at each end of that rip - it will keep it from traveling anymore.
Go inside your cracked area - that's where you need to make the strength of the repair happen. Then the exterior can be patched with either Marine Tex
( the color won't match ) or you can try and fade some gel coat patch for hopefully a closer color match.
And salty87 is right on the sanding part - it is not a walk in the park. Plus you're working with a corner which can be a pain - if you're not careful.
Keep us up to speed on your repair.