Volvo-Penta isolates their marine diesels from ground, and only momentarily connects the block to ground during start/stop operations to actuate the starter or stop solenoid. They also isolate the prop shaft and tout that design as an anti-electrolysis measure.
Not on all Volvo Penta engines.
My OEM starter is
always grounded to Negative Battery terminal, thus the Engine body is too. The starter is bolted to the engine.
Thus, the need to use Shaft Zinc, connected to the engine, to prevent Galvanic Corrosion.
In addition, per ABYC standards, a #10 Green/ yellow stripe wire, grounds it per Lightning standards. [ABYC now includes that in E-11]
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@Borbeck you boat is normally
not grounded when on it trailer. Why is that important? Lightning strike reduction.
When in the water,
metal in the water needs to be grounded and galvanic corrosion protected.
The term
Grounded, for boats in water, means that your submerged metals are at the same electrical potential as the surrounding water.
We use Zero Volts DC of a Neg terminal, of the on board battery , normally.
Jim...