It is probably the app from Jack's thread.
Could be. There are tons of apps out there that will read the GPS and show the speed on the screen in knots. Thats rather easy to code and not a big deal.
The big deal is to find an app that matches your needs. On a boat that lacks basic instrumentation, maybe a simple GPS speed readout would be fun to have. If you already have that, you can look to the app for more. The app that
@Ron20324 is talking about is RaceQs, which indeed tracks and logs many aspects of boat/crew performance, including speed, heel angle, tacking angle, sideslip, and VMG. The phone does need to be locked down for this to happen. Most useful for reviewing after the fact as part of a post race debrief.