Boat Propeller Calculator

A propeller with 19 inches of pitch would carry a boat 19 inches every turn if water were solid, and the gap between that and the speed actually reached is slip. Enter the pitch, the engine rpm and the gear ratio for the theoretical speed, then add a GPS speed and you get the slip with a reading of what it means. Two further actions work out the pitch change needed to bring wide open throttle rpm into the range the engine is rated for, and the pitch a target speed would take. Slip is the figure worth watching, because one far from what the hull should give is the clearest sign the propeller is wrong.

Calculator Type
Speed Units
The second number stamped on the hub or blade. A 14 x 19 propeller has 19 inches of pitch.
The rpm the speed was measured at, usually wide open throttle.
Just the first number of a ratio like 2.00:1. It is on the gearcase data plate, not in the engine name.
GPS speed at that rpm, in the units chosen above. Leave blank for the theoretical figure alone.
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The second number stamped on the hub or blade. A 14 x 19 propeller has 19 inches of pitch.
With the boat loaded as you normally run it and the trim set for best speed.
From the engine manual, not from the tachometer.
The workshop rule is 150 to 200. Two hundred is used unless you change it.
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Speed Units
The rpm the speed was measured at, usually wide open throttle.
Just the first number of a ratio like 2.00:1. It is on the gearcase data plate, not in the engine name.
Twelve percent suits most planing hulls. Use the slip your own boat measured if you know it.
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