A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Peter Monta
Date: 2017 Sep 21, 23:53 -0700
In watchmaking I believe "hack" means to stop the balance (the hairspring-and-wheel resonator that keeps time in mechanical watches) by some external means. A watch with a hack feature might include a small button on the case which, when pressed, would (gently!) force a braking pad against the balance wheel, stopping it. Releasing the button and giving the watch a flick (imparting some rotation to the balance to get it running again) can start the watch with sub-second precision. Care must be taken not to flick too hard to avoid damaging parts of the escapement.I don't know how the hack feature is implemented on a wristwatch when pulling out the stem. Does the balance actually stop, or is the seconds hand merely decoupled from the still-running train?
It sounds to me like the navigational use of "hack" in "hack watch" is unrelated. In particular, you probably don't want to hack a hack watch. Hacking is crude---generally you want the watch to run continuously for best performance. Chronometers would never be hacked, and even hacking a deck watch strikes me as poor practice.