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A synthetic flexible shaft coupling between a transmission and propeller shaft likely breaks the electrical path used for grounding, Nigel Calder confirms.▶ 0:43 The fix is straightforward: run a wire strap across the coupling to restore continuity, or establish a separate grounding path through another underwater metal fitting on the boat.▶ 1:09 Calder also notes a reason to keep the drivetrain isolated — the propeller shaft and propeller can be managed as a separate corrosion zone with their own anodes.▶ 1:25 The more critical warning concerns Volvo Penta sail drives, which are deliberately isolated from the engine via a plastic gasket and plastic bolt inserts.▶ 1:38 Bridging that isolation on a Volvo Penta sail drive significantly accelerates corrosion on the unit.▶ 1:52 Calder cites workshop examples where broken isolation caused exactly that damage.▶ 2:08 Anyone experiencing unexplained sail drive corrosion should check whether that factory isolation has been compromised.▶ 2:18

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