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What this video covers

On a catamaran with Volvo Penta saildrives, one zinc anode corrodes normally while the other shows no corrosion — and Nigel Calder explains why the non-corroding anode may be less alarming than the fast-corroding one. The critical diagnostic step is checking the electrical isolation between each engine block and its saildrive transmission using a multimeter in ohms mode: an open circuit confirms isolation is intact; low resistance means it has failed.▶ 2:06 Volvo Penta saildrives use a plastic gasket to isolate the engine from the transmission, and that isolation must be verified, not assumed.▶ 1:40 If the port saildrive's anode is corroding rapidly, Calder suspects its engine-to-transmission isolation may be compromised, causing that anode to protect not just the saildrive leg but also the rudder shafts and other bonded metals.▶ 3:35 The starboard anode's slow consumption is not itself a problem, provided the saildrive leg shows no corrosion.▶ 2:25 Connecting both engines and rudder shafts to the boat's AC grounding plate is normal and correct.▶ 3:02 Saildrive anodes should last a full season under normal conditions; rapid consumption signals a broken isolation barrier.▶ 4:01

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