Most boat owners know their engine needs servicing. Fewer know exactly when, because the schedule in the manual is written in engine hours and the boat spends most of the year doing nothing at all.
Hours, not months
Oil changes, impellers, gear lube and fuel filters are all set against running hours in the engine manual. A boat that did three long trips in August may have used more of its interval than in the whole of the previous season.
Write the hours down after every outing
The hour meter is only useful if someone reads it. Note it when you come back in, and working out what is due next becomes simple subtraction.
Twin engines drift apart
On a twin rig the two engines rarely run the same hours. One gets used more for manoeuvring, one had a fault and was shut down for a day. Each needs its own count or one of them gets serviced late.
The impeller is the one that bites
A worn raw water impeller is cheap to replace and very expensive to ignore, because an overheating engine is often the first sign it has failed.
BoatLog is an Android app that tracks oil, impellers, gear lube and filters against the intervals from your engine manual. Enter the hours after each outing and it shows what is due, engine by engine.