Solar sail and Ship sail difference in function Solar sails seem to operate more like parachutes than proper sails. Any mariner knows that following the wind is a fairly slow means of movement compared to moving perpendicular to the wind'speed direction. Resistance from the sea itself is necessary for perpendicular movement. 
My question: can a small degree of thrust, counter to the direction of solar winds, act as the resistant medium necessary for sailing more than downwind? Or, do the differences between the solar and terrestrial winds make this totally ridiculous?
 A: Solar sails can deflect solar wind with a side vector, in fact some NASA missions have angled their solar panels to nudge the spacecraft toward a subtly new vector to save fuel and because it is a very slight change. More than 60 degrees loses all thrust though, so it is a more complex calculation than typical vector calculations.
http://news.nationalgeographic.com/2016/02/160202-solar-sail-space-nasa-exploration/
A: Sailing boats derive their resistance to sideways drift from the so-called blade, inserted into the water near the CoG of the boat. The blade greatly increases the resistance to flow in the direction perpendicular to the sailing course (but without affecting it in the direction of the course). This allows the boat to sail perpendicularly to the wind or even to a smaller angle to the wind (as little as about 45 degrees), with little or no sideways drift.
Solar sailing boats (assuming they exist) would not be able to use a blade because in space there's no significant resistance to flow. These boats can basically only sail down wind.
Wind-based sailing boats also use their sails in a different way to solar sails: the thrust provided by wind sails is a consequence of the Bernoulli Principle. The flow of wind around the sail (in essence an air foil, like the wing of an air-plane) causes a pressure differential between the two sides of the sail, causing thrust.
