it all depends on how much you want to spend, and that will determine how fast you can go..
a 47" hull needs a good amount of power to get good speeds. a stock zehn will get the boat to mid 40s and a RCMK will push it to mid 50s +
on electric you would need a NEU 1521 min on 8s to rock it good.
KB45 motors are amp hogs, they wast alot of energy in heat, but you could try the 700kv on 10s
http://hobbycity.com/hobbycity/store...hless_Inrunner
or the 1000kv motor on 8s?
http://hobbycity.com/hobbycity/store...hless_Inrunner
the key with big boats is low kv with high volts. that is the only way to get good speeds without burning things up