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.

On the way to our mooring, I ran over a line in the water. It wrapped in the propeller and stopped us dead. OK, I've become a little night-blind.  Hired a diver who freed us and told me I broke a blade off the propeller. Maybe he could get a prop by helicopter on Monday. Sounded very expensive and at the mercy of strangers far from home.

We spent Saturday night on the mooring.  The next morning I decided to sail back while we still had plenty of provisions and most of our money. I figured that if the wind died and we got stuck between Catalina and home we could call for a tow.  "Home" was about 23 nautical miles away. The forecast called for 10-15 knot winds in the evening. I remembered that the island tends to block westerly winds and we would have a chance after we were out of the island's shadow.

We got a few miles out and the wind died completely. We bobbed around until late afternoon. With two hours of daylight remaining, the wind returned. 20 miles to go. The wind suddenly increased to 15 knots and the boat shot along at between 5 and 6 knots. It was a somewhat wild ride until I got the sails adjusted for the increased wind.

After dark the wind began to drop. 5 miles out, 5.5 knots. 4 miles out, 4.5 knots, three miles out, 3.5 knots. At this rate we would always be nearly an hour from home. Thankfully the wind returned just enough to keep us moving well. We crossed into the harbor around eight and managed to avoid the large freighter going out as soon as we got inside. Remember I have no engine to push the boat, at least not more than at a very slow speed.  I tacked across the harbor and got to the entrance to our marina when the wind completely died. We were able to slowly motor into our slip on a propeller with one blade.  We stayed here and partied instead. We plan to return to the house this afternoon (Tuesday). Its been fun.

You can tell how much fun it was because we have no pictures.  None.

2008 Update:  Wait, the prop was ready to break.

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.

May 1, 2009: The new propeller.

I took this photo after polishing the prop and propshaft when I hauled the boat out to re-paint the bottom.  I originally polished this used propeller before I had the diver install it.  I think keeping it clean and polished is probably more efficient, although that's only a guess.  Polishing does make any surface pitting or corrosion more evident.  There is also no evidence of cavitation corrosion which could be caused by over-speeding the prop or operating it with a ding in the blade.

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.  

When I first installed the propeller it seemed like it might be pitched too high as the engine would not reach full RPM.  That problem worked itself out after a dozen operating hours.  I suspect the previous prop was slightly undersized and the engine was suffering as a result.  It's a good rule to cruise at 80% of maximum engine RPM to prolong the life of the engine.  You don't have to be afraid to open it up when needed, though.  Under-loading the engine causes the 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 as insufficient lubrication of the piston rings.  But I'm not a Diesel mechanic, so that's only a guess.

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