Learning from 120 years of diesel experience. What were the two takeaways?

I was lucky enough to spend a few hours learning from the wisdom of two men who, between them, have well over 120 years of experience in marine diesels, at the Dennett Boatyard on the upper Thames. It's in aid of my next article for Classic Boat, aiming to cover all the diesel knowledge a 'lay' mariner should have, and I'll post it here in the fullness of time. The two guys in question, Michael Dennett (a boatbuilder for 66 years continuously and still working full time at 82) and engineer Steve Bill of Tom Jones Boatyard in Windsor ("Tom Jones is 99 - I'm only 78!"), shared their wealth of knowledge and good stories, including the macabre: sleeping bags wrapped around propellers, melted waterlocks letting the raw water intake straight into the bilges, and owners who don't understand the concept of filling the tanks with diesel! Never was learning so enjoyable. You could imagine these two on TV, doing something that might fall between Mortimer and Whitehouse Gone Fishing and Repair Shop. Of all their wisdom, I think it's fair to say that the two big messages were these: get to know your engine in port, on a calm day. Practice things like changing a seawater impeller, so the first time you do it, you're not tossing around in a chop with no power. The other mantra was to treat your owner's manual like a Bible.