Raymarine Autohelm ST 30 - Offset Adjustment

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ST30 Depth Operation and hstallalion Handbook
Offset
The instrument measures the water depth below the transducer. To
display water depth below the keel or to the surface an offset must be set
up. A negative offset is used to reduce the displayed value and give depth
below the keel, a positive offset will give water depth to the surface.