How this calculator works
Geometric pitch is the theoretical advance per propeller revolution. Actual advance is reduced by apparent slip. Gear reduction converts crankshaft RPM to propeller RPM. The calculation is a kinematic estimate: changing pitch also changes engine loading, RPM and slip, so it does not choose a finished propeller on its own. Use speed through water where possible; current can distort GPS-based slip.
Formula
v = pitch × engine RPM × (1-slip)/(60 × ratio); pitch = 60 v ratio/[RPM(1-slip)].
Worked example
A boat travelling 9 m/s at 3000 engine rpm, 2:1 reduction and 10% slip corresponds to 0.4 m pitch. A 0.5 m pitch propeller at 20% slip needs 3000 engine rpm with 2:1 reduction to travel 10 m/s.
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Frequently asked questions
What is the gear ratio convention?
A ratio of 2 means two engine revolutions per propeller revolution.
Can apparent slip be negative?
Yes. Current, pitch measurement or other assumptions can make measured advance exceed the nominal pitch. Negative slip is not negative hydrodynamic loss.
Will a pitch change preserve RPM?
Usually not. Check the engine maker's operating range and test the actual boat.