Here's the solution.
After
cleaning and inspecting everything, I went to the
cockpit to try starting the
engine. The
engine compartment was still wide open, and I asked my 16 year-old son to watch the engine. It started instantly. I looked over the stern – no water coming out of the
exhaust. I dashed back to the instrument panel and hit the stop button just as my son yelled that there was water squirting out of the engine. I jumped down below and saw water coming out of the base of the exhaust riser, underneath the
fiberglass insulation packing. I wiggled the exhaust riser and found that it was broken clean through! I unwrapped the
insulation, and saw that the pipe was broken just above the 90-degree bend that connects the riser to the exhaust manifold. On the Beta, the exhaust manifold opening is at the back of the engine, near the
heat exchanger. The exhaust riser pipe is
stainless steel, about 2 inches in diameter. There is a 90-degree bend close to the attachment flange, then a straight vertical run of about a foot to the mixing
elbow. The mixing elbow is then joined to a length of exhaust hose that leads down to the water
lift muffler.
There is a factory weld that joins the vertical exhaust riser to the 90-degree fitting at the flange. The factory weld held just fine: the break occurred about an inch above it. The break was rather jagged – not straight across. Fortunately, my local welding shop says they can
repair the pipe. I’ll also order a spare from Beta.
So here’s my diagnosis. The exhaust pipe cracked during my recent long
passage. On the day of the incident, it broke right through. The break was concealed by the
fiberglass insulation wrapping, so I didn't notice it. The smoke was not from a fire – it was black
diesel exhaust. One of you suggested that when I backed away from the
dock, the
dinghy blocked the exhaust at the stern. Not sure the relative geometries would allow that, but it's possible. If it did block the exhaust, then that would force
diesel exhaust through the broken riser and into the engine compartment. The engine air intake is about 4 inches away from the base of the exhaust risher, pointing right at the place where the break occurred. The escaping exhaust went straight back into the engine, producing the black smoke. With no exhaust pressure going up the pipe, the sea water flowing to the mixing elbow was not being forced out to the
muffler. The easiest path for it was back down the exhaust riser. I didn’t notice the water on the day of the incident because I thought it was a fire. By the time the smoke had cleared, of course, the water was no longer dripping. It was only the next day when my son saw the water gushing out. The exhaust riser/elbow is at the welding shop at the moment, but as I think of it, I think the sea water exit hose attaches to the elbow pretty much at its highest point, which is a bad design/installation. It should be attached on the downhill slope of the elbow to prevent water from flowing back down the exhaust riser. I won't know for sure till I get the part back on Monday.
I am still puzzled by a couple of things. First – why did the riser break? The exhaust riser and mixing elbow is held in place by a 1.25 x 3/16”
steel strut bolted to the engine and attached with a
steel strap looped around the insulation near the mixing elbow. Engine vibration should therefore be transmitted to the flexible exhaust hose: the engine and exhaust riser/mixing elbow would be vibrating together, so there should not be any stress on the exhaust pipe. I noticed, however, that the strap had worn away much of the insulation on the pipe. This may have allowed the pipe to vibrate independently of the engine, probably on a harmonic, leading to the fracture.
Second – why was there black smoke? Before I saw your excellent responses, I couldn't figure this out. It's obvious, however, that the escapting exhaust fed straight into the air intake pretty much eliminating combustion oxygen. When you look at the air intake breather, it almost looks like it's designed to suck in exhaust fumes!
Third – why didn’t water get into at least one cylinder and cause a hydro lock? When I started up the engine post-cleaning, there must have been water in there from the incident. I count myself very lucky that there was no hydro lock, which could have caused terrible damage (connecting rods, bearings, etc.). If I had known what the problem was, I would have removed the injectors before cranking with the starter. That’s what I will do before I start the engine again.
Further comments and suggestions welcome.