this is completely wrong and i have no idea why people are thoughtlessly agreeing. "change of tact" is not a set phrase. it's a mistaken, ungrammatical bastardization of "change of tack", which is the correct, common set phrase and indeed has nautical, metaphorical origins. "change of tack" is a nice turn of phrase but it's utterly unremarkable. nobody says "change of tact" to refer to a "change of tactics" unless they're dumb. they would just say "change of tactics" or "tactical change". "tact" means "skill or sensitivity in dealing with others".