>>102252576This is entirely physically possible.
Even though when the guy blows on the sail he generates an equal momentum in the opposite direction, when he sucks air IN he's actually pulling the raft forward.
So this would be more efficient if he sucked air in facing forward and blew it out facing back.
But even with the sail, the raft gets forward momentum when air is sucked in, and doesn't lose any momentum assuming the sail perfectly catches all the forward-blown air (it doesn't, but as long as it catches SOME of the forward-blown air the net momentum of the suck-blow would be forward)
