here and here.
Each completed round trip to Antares makes an average thirty per cent profit, although the Borthudians currently capture fifteen percent of the ships that travel that route. Nicholas van Rijn reinforces his ship, the Mercury, and gives it a super-powerful engine with pressor and tractor beams able to deflect, then capture, the Borthudian ship that attacks it. This refitting of the ship costs three times as much as the ship can expect to earn. However, van Rijn will redesign only every fourth ship.
Thus, he will still lose three of every four ships attacked. However, the Borthudians will gain twelve slaves from three out of four encounters whereas van Rijn's company will capture twenty Borthudians from each fourth encounter so that the Borthudian press gangs, never knowing which League ship will be able to fight back, will soon be whittled down.
The Spacefarers had voted not to take ships through the Kossaluth of Borthu but will now volunteer because, when the Kossaluth loses so many press gangs that it has to negotiate, the indemnities exacted will be large enough to enable each freed slave to retire for life whereas, if the Kossaluth negotiates immediately, then it will be able to "'...send students to our academies for not much more than the usual fees.'" (The Van Rijn Method, p. 172, my emphasis)
If the Borthudians simply stop attacking, then their precarious empire will not last long, especially since the League will import subversionists and guns. Van Rijn, of course, has covered every angle.