STL between systems

I was looking over my notes on the Sol-Corps colonization plan from C.J. Cherryh's books and the STL tech used to get from system A to system B is a combination slingshot, solar sail. Now based on my notes from the net (I do not have the books this tech appears in) the ships might be reaching speeds near C. I did the math using the timeline notes I have and it seems that the time of Travel in Earth Years is X LY between systems rounded (distance) + 1 additional year (probably representing part of the acceleration &amp; deceleration time is my guess).<div><br /></div><div>I am not physics savvy but time dilation thoughts keep nagging me.  I have next to 0 education in physics (it is just what I have absorbed over the years)... so input on this welcome.  How fast would the average speed of a ship be for say 7 Earth years to pass between departure and arrival traveling 6 LY in distance?</div><div><br /></div><div>Though storyline game time wise her formula is slow but not too slow to make the travel, trade &amp; colonization between star systems not impossible. </div><div><br /></div><div>I have not found any descriptions of her ideas about the solar-sail ships (yet), but I would like to work on how this might work in SF.</div><div><br /></div><div>I would also like to work up ship ideas that are based on other STL tech as well.</div><div><br /></div><div><br /></div><div><br /></div><div><br /></div><div><br /></div>