1 December 2006: You can't get home from here.
Julie and I
set off for Avalon on Saturday. We arrived just after dark. There
wasn't much wind and we motored the last two-thirds there.
Well, we have two pictures taken long after that fateful day. The one armed prop shot shows where the blade broke off. I examined the remaining blade closely and discovered small cracks next to the hub. With the hub in a vise I put a few pounds on the blade and it broke off, just as the other one had. Well, perhaps I put 10 pounds on it, I don't remember. The important thing to realize is that this prop was waiting to fail. It really took very little force.
The second photo shows what the mating surfaces of the break looked like. The crack in the prop had corroded roughly a third of the way through the metal from all sides. Only a thin band of yellowish bronze was intact at the center of the blade. To make sure about the color of the material, I ground off a patch on the blade. This shows in the second photo on the right end of the break. Well, un-oxidized bronze is bright golden all right.
I put this down as a case of stress corrosion cracking. The blades are under the highest stress near the hub. If there is any prop vibration, that is where the blade bends. If this was the original prop on the boat, it went from 1976 to 2006, thirty years in salt water. The prop was "protected" from corrosion by a zinc covered prop nut. But no electric field can extend into a hairline crack in the metal. Exactly what chemical reactions occur I leave to the metallurgists out there. You can be pretty sure the fluid inside the crack was oxygen depleted. What oxygen might diffuse in would quickly oxidize the surface of any fresh exposed metal.
This may be a little like crevice corrosion with stainless steel. It is the chromium oxide layer which forms instantly on exposed stainless that protects the iron in the alloy. Bolt a piece of stainless to your hull without bedding it will give you an joint that weeps a rusty colored stain. Bedding keeps the water out. The diffusion of oxygen is very slow through a crack. Therefore not enough chromium oxide can form. The unprotected iron reacts with the salt water and slowly dissolves. At least that's what it looks like to me.
This prop has been well protected by the propeller shaft zincs. I install two shaft zinc anodes as a rule and direct the diver to replace one when they are half gone. From then on he waits until the remaining zinc has fallen off (or nearly so) and replaces it. That way I fully consume all of the zinc rather than replacing it when half is still left. I polished the stainless shaft so that new zincs will be sure to make good electrical contact with the shaft. This is essential for the zinc to provide any protection to the other metal. If your zinc is not corroding away, it is not doing any good. Their is no downside to having two zincs on the shaft. It is just that most people don't do it that way. They waste half of the zinc as the diver generally replaces it when it's half-consumed to be on the cautious side.
The strut is coated with marine growth, but a touch of the wire brush showed that the metal was in fine shape. To be perfectly consistent, I probably should have polished it as well to check for fatigue cracks. I should also have replaced all of the zincs, but I was very pressed for time and feel lucky I got everything done and the boat back in the water on schedule.
I can tell the new propeller is a good match to the boat and engine because the engine just reaches full rated RPM at wide-open throttle. No soot is evident in the exhaust. If the prop is undersized you will lack power and the engine will suffer from light loading. Diesels like to be worked, not idled. If the prop is oversized, the Diesel engine will not reach full RPM and will blow black soot out of the exhaust.
I first installed the
propeller it seemed like it might be pitched too high as the engine
reach full RPM. That problem worked itself out after a dozen
hours. I suspect the previous prop was slightly undersized and
was suffering as a result. It's a good rule to cruise at 80% of
engine RPM to prolong the life of the engine. You don't have to
to open it up when needed, though. Under-loading the engine
cylinders to glaze up (become polished too smoothly) and the piston
rings do not
seat as well. I suspect this causes a loss of compression as well
insufficient lubrication of the piston rings. But I'm not a
mechanic, so that's only a guess.
remind me to let the yard paint the boat next time. Friday
afternoon to Monday morning, I've never been so exhausted in my
life. What you will do when money is tight. . .