A log kept hourly is a navigation backup: lose the electronics and your last position, course and speed lets you dead-reckon. It is also the only way you will ever know your real fuel burn.
Position and progress
How The app takes it from the phone's GPS with one tap. If there is no fix, enter it from the plotter — and if there is no plotter either, write down the bearing and distance to something you can see. A rough position beats a blank.
Engines
How You are looking for a change from the number you wrote an hour ago, not for a magic value. A climb of ten degrees with everything else the same is the start of a problem.
Watch for A steady climb means flow is dropping: impeller, strainer, or a plastic bag on the intake.
How Vibration that was not there, a note that has changed. Write it down even if it turns out to be nothing — the log is where a pattern becomes visible.
Weather observed
How What you SEE, not what was forecast. Whitecaps start around 12 kt; spindrift and streaks start around 20. Recording observed against forecast is how the forecast becomes useful to you specifically.
How Height trough to crest, and period. Short and steep is worse than tall and long.
How The number matters less than the direction and the rate. A fall of 3 mb in 3 hours is worth acting on; 5 mb is a get-home-now signal.
How Estimate by what you can and cannot see — a known landmark at a known distance. If it is dropping, slow down, sound signals, radar on, and get everyone visible.
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Emergencies — Mayday script and six procedures · All checklists