The tunnel thruster system 7, wherein the one of {HEAD ~ the pair} of {TAIL ~ opposed propellers} and the pair of contra rotating propellers are reversible 
The tunnel thruster system 7 , wherein the one of { HEAD ~ the pair } of { TAIL ~ opposed propellers } and the pair of contra rotating propellers are reversible
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH