As a reader of the series [more than once] I wondered on occasion what "Catharpins" were. According to "Steel's Elements of Mastmaking, Sailmaking and Rigging," 1794 edition, page 117, they are "Short ropes, to keep the lower shrouds in tight, after they are braced in by swifters, and to afford room to brace the yards sharp." Swifters (p. 131) are "The after shrouds on each side of the main and fore masts; they are above all the other shrouds, and are used as an additional security to the masts..."

By the way, try the Nathaniel Drinkwater series. Much better IM[NS]HO and there is no effort to stretch the series for a few extra bucks.