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