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