8 hp is ok, but
Joe, I have an 8 HP Honda 4 stroke long shaft which pushes Whiskey Girl along at hull speed when its opened up, so you can't ask more then that. But... it is mounted on an external swing type motor mount because the size of the motor head prevents it from going to a tilted position if it was in the cutout. Several people have commented on this before, search the archives, it seems that many 4 stroke engines have this problem. My swing mount brings the motor very close to the water, and I have a long shaft engine, but I still experience a little cavitation when I'm on a port tack <the mount is on the port side> because the prop is close enough to the surface to suck in air when a wave moves under us. My "feeling" is to go for as long a shaft motor as will clear the water when its in a tilted position. As an aside for safety reasons, consider getting an electric start, or make sure your first mate has enough upper body strength to get the motor into the water and start it. Mine doesn't, and it is a little nagging worry when I consider man-overboard conditions.