Prop Pitch Calculator

The prop pitch calculator takes engine/crankcase speed, gear ratio, propeller slip and boat speed and returns propeller pitch. It is built for quick answers, for checking somebody else’s arithmetic, and for the moments when a formula book is not within reach.

What Is the Prop Pitch Calculator?

This prop pitch calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns engine/crankcase speed, gear ratio, propeller slip and boat speed into propeller pitch.

Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.

How the Prop Pitch Physics Works

The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.

Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.

Units, Assumptions and the Limits of the Model

The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.

Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.

A Worked Example

Working through an example, with engine/crankcase speed set to 10, gear ratio set to 5, propeller slip set to 2, the calculator returns propeller pitch of -52,800. Substitute your own figures and the same steps produce the same kind of answer.