Outlander siad it best. Keep it narrow---and sharp. Possibly a two balde although three blades will probably work OK. Weight really isn't an issue as far as I'm concerned. You shoot and arrow spined correctly and get good flight. That puts all the momentum behind the head and improves penetration.

Kinetic energy is just a number. Jeesh, what did we do before mathematics and computer programs? Guess we just went and shot our bows and killed animals.