Wouldn't magnifying or focusing (same thing) mean it would need to be gyro stabilized? Now we are talking serious
money. The beauty of
solar cells is that they still
work even when they are not directly pointed at the sun.
The average energy the Earth gets from the sun is 164
Watts per square meter over a 24 hour day. More so at the Equator and less so at the poles and of course, nothing at night. 164 watts/ 746 watts per horsepower is 0.21 horsepower per square meter. So if you want to develop one horsepower we are going to need about 5 square meters of surface area....and that's assuming the conversion of the suns heat energy to stored
battery energy is 100% efficient....which it is not even close to that.
Perhaps somehow 5 square meters of an energy collecting device could be applied to the sail somehow? Most small boats are not going to have the deckspace.
I could though imagine a large barge like
catamaran with nothing but solar cells on
deck that putts along on its own
power at a
knot or two. But then who would want to ride on that when the
wind is much more capable of propelling a
boat?