In my area, there's not much need to motor.  I might motor anywhere from 2 - 15 minutes to get to the race area each Wednesday night.  So the hours on my engine (since I owned the boat anyway) are minimal.  I maintain it regularly though.

However last night we needed to do a 2.5 hour motor to get back to town from a race that ended elsewhere.  We motored at 2500 rpm the whole way, with me periodically visually inspecting the engine (admittedly from the sides, not the front) to make sure everything was OK.  I've got the two new temperature gauges monitoring cylinder temp and muffler temp as well.  I also have an inline water filter between the engine and the exhaust elbow to prevent blockages.

Just before getting to the dock, we heard a funny noise and a quick check showed that the alternator belt was in the process of shredding (my motor has an aftermarket alternator installed - see pics  https://goo.gl/photos/UJR7VdESqgAWXk1W8" rel="nofollow - https://goo.gl/photos/UJR7VdESqgAWXk1W8  ).  Since there was nothing that could be done, we proceeded to the dock and just before arriving heard the belt let go completely.

Now, I replaced that belt only 2 years ago and we actually just retightened it a couple of weeks ago, so I was surprised it broke but it was very late by then so I left that night and returned today, expecting to simply need to clean up the broken bits of belt and install a new one.

Much to my surprise, I found engine oil all over the compartment, especially directly above the engine.  NO, we were not motorsailing and the water last night was quite flat.  Then I realized the dipstick for the engine oil was missing!  I felt around and finally located it, just forward of the holding tank.  The dipstick was quite bent.  

I suspect the dipstick somehow wiggled it's way loose and got caught in the alternator belt, pulling it out the rest of the way, bending it and damaging the belt as well.  The engine then sprayed oil over the belt (and engine compartment) which combined with the initial damage caused the belt to fail.  In a way I'm lucky the belt broke otherwise we may not have noticed until it was too late.

Having said that, I'm unsure how the dipstick worked it's way loose in the first place.  Also annoying that even with me checking the engine periodically, this still happened.

I don't think my engine vibrates too much (I mean it's a fair bit, but it's a one banger 30 year old diesel right?).

Has anyone ever had this problem before?  

The engine was running to bend the dipstick and to spray oil above it. Belt failure often has collateral damage. Other owners are unlikely to have this problem because the standard engine didn't have an added alternator like you do, someone must have added that later on. The standard engine had no belt at all. 

It certainly is possible the belt failed first and a stray piece grabbed the dipstick.  No way to confirm this although it does make sense under the circumstances.

Yes, the pic of the dipstick shows black belt remnants.  The entire compartment was covered with what is best described as black powder (and oil).  There was little of the belt left.

I'm not sure how much oil I lost.... it looks messy but it was less than 400ml., and probably less than that  However if we go with the hypothesis that the belt failing is what pulled out the dipstick and the motor ran after this for only 5 minutes or so (at very low rpm) until we got back to the dock, then I can see how the engine can quickly starve itself of oil unless you are paying close attention.

My solution for future will be to shorten the dipstick handle by bending it into a loop instead of a hook at the end. This should eliminate the problem should I have another alternator belt fail.  Of course in future I now know that should my belt break, to immediately check that the dipstick is still in place.

Because I have an aftermarket alternator, this issue seems isolated to me and probably isn't relevant to others.

I agree re: the heeling issue.  The notion of having a sailboat that can't be motorsailed at all is a big problem, especially up here where I may need to do long deliveries of 100 miles or more and motorsailing can save time and fuel.  I may experiment a bit with motorsailing at various angles of heel and on port/starboard tack while someone continually watches the engine to see what my boat can tolerate as I hear it does vary from one L28 to the next.