Swordships of Scorpio — Alan Burt Akers

The fourth book of the Dray Prescot series.

What does a man do when fate makes him the protector of the royal head of the land of his enemies? If it is Dray Prescot, Earthman on Antares, he sets aside his quest to do his duty. His duty was to reach Vallia and his princess Delia and help her claim her throne. His duty was to defend Vallia's ancient foe and place its rightful heir on its throne -- sworn to attack Vallia. So when the third force, the pirate fleets known as the swordships came between the two contending demands, Dray sees that only by following his own personal star could the contradiction be resolved...