No prop runs at 100% efficiency — water always slips. Knowing your slip percentage tells you if your prop, pitch and gear ratio are matched to your boat, or leaving performance on the table.
Too much slip means wrong pitch or poor prop condition. Under 10% is excellent.
Accounts for your lower-unit ratio so results are true to your driveline.
High slip at cruise can mean venting, cavitation or an over-pitched prop.
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Tune the whole drivetrain.