The thing about engine hours, at least in my mind, is that most electronic boxes that keep track of hours do it by time. Yes, I know that 60 minutes equals one hour, but an "engine hour", to be more meaningful, used to be 100,000 RPM. Aircraft engines that have to be overhauled every so many hours have hour meters that meter off the tach. Those hours are more meaningful when compared along side 60 minute hours. For example: an engine run at 5,000 rpm for 60 minutes turns 300,000 revolutions, or 3 engine hours......but by most hour meters , would only show 1 hour.
Anyway, I realize hardly any boat engine hr meters measure anything but time. Larger offshore vessels with diesels might be the exception.