Your phone carries a genuinely good GNSS receiver. On open water, with a clear sky view, expect fixes accurate to a few meters, more than enough for a faithful track of your sail.
From fixes to a logbook entry
A raw GPS feed is one position per second. Sailing Log turns that stream into a logbook entry: distance accumulates in nautical miles, speed over ground is read from GPS doppler (much steadier than position deltas), heading is smoothed over a few seconds so it doesn't twitch, and the track is stored in compact chunks so a full season costs megabytes, not gigabytes.
What about accuracy on a heeled boat?
Heel barely matters; sky view does. A phone in a cockpit pocket sees plenty of satellites. Below decks under a metal deck is the one place fixes degrade, so wherever the phone lives, give it half a view of the sky.
Speed readings sailors can trust
GPS speed is speed over ground, the number that matters for passage timing. Knotmeter paddle wheels read through-water speed and lie whenever they're fouled, which is always. For most cruising, SOG from GPS is the more honest instrument.
Battery
Continuous GPS with the screen off costs a few percent per hour on a modern iPhone. Lock the phone and let the Lock Screen view show your numbers; the screen, not the GPS chip, is the real battery hog.