The Boat Mill

Prop Pitch / RPM Rule-of-Thumb Calculator

Estimate how wide-open-throttle (WOT) RPM will change if you change propeller pitch, or which pitch should bring the engine into its rated RPM range. It uses the common rule that each inch of pitch is worth roughly 150 to 200 RPM.

When to use it

An engine should reach the top of its maker’s rated WOT range with a normal load aboard. If it cannot (over-propped), it lugs, runs hot and wears faster. If it overshoots (under-propped), it hits the rev limiter and wastes top speed. The usual fix is a pitch change, and the question is always: by how much?

Run the boat at WOT with a typical load in calm water, trim set for best speed, and read the tachometer. That is the “current WOT RPM”.

The rule of thumb

new RPM ≈ current RPM + (current pitch − new pitch) × RPM per inch

pitch for a target ≈ current pitch + (current RPM − target RPM) ÷ RPM per inch

Less pitch lets the engine turn faster; more pitch slows it. The calculator keeps RPM per inch between 150 and 200, which covers most outboards and sterndrives. Use the lower figure for heavy, slow boats and the higher for light, fast ones, or calibrate it from a previous prop change on your own boat.

Worked examples

Over-propped outboard. Reaches 5,200 RPM on a 17 in prop; rated range is 5,500 to 6,000. Dropping to 15 in: 5,200 + 2 × 175 = 5,550 RPM, inside the range.

Choosing a pitch for 5,600 RPM. (5,200 − 5,600) ÷ 175 = −2.29 in, so try about 14.7 in. Props come in whole-inch steps, so 15 in is the practical choice.

Under-propped. A boat hits 6,300 RPM on a 19 in prop with a 6,000 RPM ceiling. 19 + (6,300 − 6,000) ÷ 175 = 20.7 in, so try 21 in.

Estimated WOT RPM from 5,200 on a 17 in prop

New pitch	150 RPM/in	175 RPM/in	200 RPM/in
14 in	5,650	5,725	5,800
15 in	5,500	5,550	5,600
16 in	5,350	5,375	5,400
17 in	5,200	5,200	5,200
18 in	5,050	5,025	5,000
19 in	4,900	4,850	4,800

When the rule breaks down

Changing diameter, blade count, cup or prop material at the same time as pitch. A four-blade prop of the same pitch usually turns fewer RPM than a three-blade.
Large changes. Beyond about two inches the estimate drifts; test, then step again.
Displacement boats and inboard diesels, which are usually propped by a calculation based on shaft horsepower and hull speed rather than this rule.
A dirty bottom or a heavy load during the test run. Re-test in normal trim before buying a prop.

Safety

Never run an engine above its rated maximum RPM to test a prop. If WOT exceeds the maker’s range, back off and change pitch. Tighten the prop nut and fit a new cotter pin or tab washer every time a prop goes on, with the engine stopped and the key out.

FAQ

Why is RPM per inch limited to 150–200?

Outside that band the rule is not reliable. The calculator clamps values you type to that range.

Does a pitch change affect top speed?

Usually slightly: the right pitch lets the engine make its rated power, which often helps more than any theoretical speed loss.

My engine maker gives a different range.

Use theirs as the target. It is the authority for your engine.