A propulsion system for {TAIL ~ a watercraft} includes {HEAD ~ a concavity} formed in a hull of the watercraft 
A propulsion system for { TAIL ~ a watercraft } includes { HEAD ~ a concavity } formed in a hull of the watercraft
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH