1 · Speed prediction

Crouch's formula — V = C × √(HP / weight)

Total propshaft horsepower hp

All engines combined, at the shaft. Not crank horsepower.

Gross weight, wet and loaded lb

Everything aboard on the day — fuel, gear, crew. Not brochure dry weight.

Hull family

Sets the constant C. This single choice moves the answer more than anything else on the page.

The ladder runs your inputs against every constant. If two families are both arguable for your boat, your answer is that range, not one number.

Estimated top speed

—mph

Weight per horsepower—

Hull constant C—

2 · Propeller

Pitch and speed, either direction

Target speed mph

Carried from tool 1 unless you type over it.

Rated WOT rpm (top of window) rpm

Gear ratio : 1

Enter 1.75 for a 1.75:1 lower unit. Never 1.00.

Slip %

10–15% is a sorted planing rig. 20%+ means load or a rigging fault. Replace this with slip you measured in tool 3.

Rpm window — low / high rpm

high rpm

From the engine's operator manual. The band the engine is certified to turn at full throttle.

Required pitch

—in

Nearest stock pitch

— in

Rpm at that pitch—

Change vs. rated—

Theoretical speed, zero slip

— mph

Where the engine ends up in its rated window

——

Enter a rated window to see where this prop puts the engine.

3 · Slip log

What the boat actually did

Pitch run in

Gear ratio : 1

Run	Rpm	Observed mph	Theoretical	Slip

Enter three or more runs and the trend in slip gets read for you.

Run every rpm step on reciprocal headings and average the two speeds before entering. A one-direction run measures the current, not the boat.

What these calculators cannot tell you

Nothing about blade style: Cleaver against round-ear, blade count, cup, rake, progressive pitch — none of it appears in either formula. Two propellers of identical diameter and pitch can differ by several mph.
Nothing about rigging: Engine height, setback, jackplate position, trim and tab condition are absent. On a surfacing setup these dominate the result.
Submerged propellers assumed: Crouch's formula was correlated on conventional planing hulls with fully submerged props. On a surfacing setup treat its output as a lower bound only.
One pitch at a time: The slip figure assumes a single representative pitch and ratio. On twins, triples and quads the inboard engines work in disturbed water and rarely want what the outers want. Log each position on its own.
Empirical, not physics: These are curve fits from observed fleets. They bracket a result. They are not a tank test, a CFD run, or a day on the water.

These calculators return engineering estimates produced by published empirical correlations, offered as a planning and screening aid only. They are not a survey, not a specification, and not a recommendation of any propeller, engine or vessel for any purpose. Results depend entirely on inputs supplied by the user and on judgement calls the user makes, and they do not account for propeller geometry, rigging, hull condition, sea state or load distribution. Confirm every figure by instrumented sea trial before purchasing hardware or committing to a specification. No engagement, professional relationship or duty of care arises from use of this page.

Marine Performance Calculators