NIGEL CALDER REPLIES

SAIL - - Under Sail -

Nor­mally speak­ing, the alu­minum saildrive leg is more vul­ner­a­ble to cor­ro­sion than the pro­pel­ler, so I sus­pect your pro­pel­ler cor­ro­sion had more to do with stray cur­rent (against which a sac­ri­fi­cial an­ode pro­vides no pro­tec­tion) than with gal­vanic cor­ro­sion (which is held at bay with sac­ri­fi­cial an­odes). The iso­la­tion trans­former may have fixed this prob­lem. With re­spect to an­ode choices, you are cor­rect, you should not mix an­ode ma­te­ri­als in the same ca­thodic pro­tec­tion sys­tem. The key thing here is de­ter­min­ing what con­sti­tutes the same sys­tem. If your pro­pel­ler is elec­tri­cally con­nected to the saildrive via metal-to-metal con­nec­tions, then the two are es­sen­tially the same sys­tem, and the mag­ne­sium an­odes will get eaten up by the alu­minum an­ode. If, how­ever, the pro­pel­ler and saildrive are elec­tri­cally iso­lated (by, say, some sort of plas­tic sleeve or a com­pos­ite pro­pel­ler hub) then the two can be con­sid­ered to be dif­fer­ent sys­tems, and there should be lit­tle to no in­ter­ac­tion be­tween the an­odes. My guess is you do not have elec­tri­cal iso­la­tion, in which case you are some­what be­tween a rock and a hard place. All but Volvo-Penta saildrives are not elec­tri­cally iso­lated from the rest of the boat, and as a re­sult cor­ro­sion— be used in salt wa­ter be­cause it be­comes too re­ac­tive.) How­ever, this may be re­sult­ing in over-pro­tec­tion and hence the blis­ter­ing of paint on the keel and the coat­ing on the pro­pel­ler. Alu­minum an­odes are rea­son­ably ef­fec­tive in fresh­wa­ter as well as salt­wa­ter, and may well do an ad­e­quate job for you with a greatly re­duced rate of an­ode con­sump­tion and no paint blis­ter­ing or pro­pel­ler coat­ing. I think

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