Propeller Slip Calculator (Boat Prop Pitch, RPM and Speed)

What this tool does

Pitch is the distance a propeller would advance in one turn if it screwed through a solid, like a bolt. In water it never does, and the difference between that theoretical speed and your real speed is slip. A well-matched propeller on a planing boat commonly shows somewhere around 10–20 % slip; much higher can mean the pitch is too low or the hull is overloaded or fouled, and a negative figure means the speed reading is wrong or a current is helping, so use speed through the water, not GPS speed. 72,913 is the number of inches in a nautical mile.

Formula

theoretical speed = prop rpm × pitch × 60 ÷ 72,913 (inches in a nautical mile) · slip = (1 − actual speed ÷ theoretical speed) × 100

Variables

Worked example

Limitations

• The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
• Mixing units is the most common source of error. Convert every input to the units shown next to each field before calculating.