Every diesel maintenance schedule is denominated in engine hours, and almost every sailor's engine-hours record is fiction. The hour meter helps if you remember to read it; the logbook helps if you remember to write it. Usually, neither.
Computing instead of remembering
Sailing Log already knows your trip's duration down to the minute. So each trip ends with one question a human can actually answer: roughly what share of this trip ran under engine? Slide between Sail and Engine, 20%? 40%?, and the motoring time computes from the real duration. Prefer precision, type the minutes directly; slider, percentage, and minutes stay in sync.
Motorboats skip the question
Mark the boat as a motorboat or jetski and the whole trip counts as engine time automatically. There's nothing to answer; the kind of boat answers it.
Where the hours go
Per-trip engine time lives on the trip. The maintenance log records meter readings when you service, oil at 412 hours, with notes and receipts attached. Between the two, "when is the next oil change?" becomes arithmetic instead of archaeology.
The honest accounting also settles the eternal question: yes, your "sailboat" did 31% of last season under diesel. The slider remembers, even if you'd rather not.