Sailplane

Since a sail is a kind of airfoil,
why couldn’t a sail be devised
for a boat with sufficient lightness
such that lift could be added
to velocity by trimming the sail
to fly the boat as a plane? In the air
one should be able to reverse the trim
to descend back to the water.
With such a sail, one might need
to deploy an anchoring parachute
like the tail of a kite for stability,
and we’d need to work out how to steer,
for we know a boat can’t be controlled
without the resistance of water.