Feasibility of a sailboard in space

[HRogge]

I am going to go for it.  What would be a reasonable top speed for a handwaved racing sailboard?  It's as light as practical, to get SPEED.  Naturally, the speed will be dependant on the angle relative to the solar wind, like any sailing craft--and a storm will really make her move--but also be risky.  I'm hoping that speed around 15-18% of lightspeed is reasonable, pushing close to 20% when there's a bad solar storm.
How fast could she go in atmosphere?  For example, zipping around the University of Ottawa...
Of course, that means that she carries no weapons or armor.
Have to be careful behind anything that shadows the solar wind--like a large planet, perhaps.  There may be unpredictable eddies and calm spots near them, as well as the expected calmer space.

Maybe 12%-14% normally and 16% in a bad storm? Speed is not only a matter of mass of the ship, but also a matter of "was this thing fast before we waved it" or even "would Fen think this thing is fast?"

Thats why the Blackbirds (especially the original one) are that fast, because it fits their original purpose and abilities.