[C38] zinks, shore power, and electrolysis of the prop and key

[D. R. Strong]

Dear List:
   A note and invitation for comments about a recent unpleasant 
surprise. 1 month after we purchased Discreet Charm, she lost her 
expensive Flex-O-Fold prop while backing out of the slip when the key 
sheared. The prop simply unscrewed the nut and spun off into the mud of 
the fairway never to be recovered. We replaced the prop with another 
FoF.  In March of this year,  41/2 years later, she sheared another key, 
this time in forward gear, and the prop was not lost. We sailed home and 
into the slip listening with great anxiety to the "put, put, put" of the 
prop turning on the shaft.
    Before going into the water, the diver  doubted my diagnosis  of a 
sheared key and said that in 16 years of the diving business in SFB he 
had seen only 2 shearings keys. He went in and found a sheared key, and 
showed me a fairly subtle pitting on the prop that I had missed during 
haul out in December.   The zincs, replaced 8 months earlier during haul 
out were shot.  His opinion was that the pitting of the prop was due to 
electrolysis caused by exhausted zincs. We also found that the bronze in 
the sheared key had pitted, discolored into layers, and looked generally 
degraded. The diver's opinion was that the key degradation and shearing  
was part and parcel of lack of zinc protection.
    We replaced the key and now have the diver scheduled to change the 
zincs every three months. I will extend to every 4 months if the zincs 
look good after 3 months.

I also have changed my ways and plug in to shore power only while I'm on 
the boat.  The hour or so of charging before and after each sail is 
sufficient to charge the batteries.

Comments welcomed,
Regards, Don

Don and Karin Strong
Discreet Charm
Emery Cove, San Francisco Bay