Harborless's problems with a "runaway diesel" is so odd that I really want to know more. How can this happen when the Governor sets the max amount of fuel unless the governor isnt working? Bristol suggested that overfilled oil was allowing oil to be burned instead leading to runaway. Yes, I can imagine an engine continuuing to run on oil but to run away? I woulda thought that in order to run fast that good properties would be necessary and with just the oil, it isnt being injected right.
I've always found that REALLY understanding symptoms solves 90% of problems so we have to get a solid handle on them.
frogwatch, your point that in order for an engine to run fast you must properly meter in the fuel and air, and plenty of it makes perfect sense.
But in this case there is an abundance of fuel, so much so that excess "fuel" is exhausted partially and fully unburnt (absolutely tons of smoke is emitted, and raw "fuel" pours out). In the case of a runaway there is plenty of fuel and air is the limiting factor. It just happens that there is enough air to take the engine to an unsafe RPM, and discard the extra fuel.
It is truly terrifying. I keep a piece of plywood near the air cleaner now so that I can use it in the event of a runaway. But I shudder at the thought of having to use it.
It happened to me on a previous boat, a Yanmar YSM15, I overfilled the after an oil change. tarted her up, revved a bit and it ran away. I had nothing handy to stuff in the air intake. I was lucky in that case, It stopped on its own after about 30 secs with no damage. I can tell you that those 30 secs felt like about an hour!