Ask SAIL: Prop Zinc Deterioration

By Nigel Calder

October 16, 2015

PROP ZINC DETERIORATION

Q: My 2014 Dehler 38 has a Volvo saildrive 130S-F and a two-blade Flexofold prop. After six months in the Annapolis area, the two “small” zincs on the prop showed dramatic deterioration. In contrast, all other zincs (including the ring zinc on the saildrive) had minimal deterioration. Prior to launching the boat for the new season I did replace the two small zincs. This year I plan to keep the boat in the water for about eight months. Given the longer amount of time in the water (but also considering the fact that the saildrive zinc is virtually intact) do you think I need to replace the small zincs mid-season? If they were to completely deteriorate by season’s end, what is the worst that could happen?

Akis Goutzoulis, Annapolis, MD

NIGEL CALDER REPLIES

Volvo-Penta requires all metal propellers to be electrically isolated from their saildrive propeller shafts, which is almost certainly the case with your installation. If this isolation is maintained (you can check with an ohmmeter from the propeller to the saildrive leg; you should get a very high resistance) then the anodes on the propeller are only protecting the propeller, and the anode on the saildrive is protecting the saildrive. The propeller is a bronze alloy, which will inevitably eat up its anodes rapidly and probably does not need protecting in the first place. (Although Flexofold still supplies its propellers standard with anodes, it also supplies them without.) So I do not believe you need to replace the propeller anodes in mid-season, and in fact I don’t think you need them at all! That having been said, according to Flexofold you can actually replace them with the boat still in the water, as the mounting screws supplied with the zincs are pre-applied with a special Loctite compound. The critical anode from your perspective is the saildrive anode, which should always be replaced once it is half consumed. (Its effectiveness is directly related to the remaining surface area.)

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Nigel Calder

SAIL Contributing Editor Nigel Calder is probably best known as the author of the definitive Boatowner’s Mechanical and Electrical Manual and Marine Diesel Engines. A longtime member of the American Boat and Yacht Council, his boathowto.com provides dependable technical information for sailors. A prolific sailing writer, he and his wife, Terrie, have sailed a series of boats from the Faroe Islands to Portugal, the Bahamas, the U.S. East Coast, and Caribbean, and he’s written guides including Nigel Calder’s Cruising Handbook: A Compendium for Coastal and Offshore Sailors. He also teaches electrical and diesel classes at TrawlerFest in Baltimore.