How do I know if my boat is over-propped or under-propped?

Compare the rpm actually reached at wide open throttle with the engine rated rpm. If the engine never reaches its rated rpm the boat is over-propped, meaning the pitch is too long and the engine is lugging. If the engine hits the limiter without delivering full power the pitch is too short and the boat is under-propped.

What is propeller slip?

It is the difference between the distance the propeller would travel screwing into a solid body, one pitch per revolution, and the distance the boat actually covers. Apparent slip is obtained by comparing the geometric pitch speed with GPS speed. It is not a fault: it is the condition that lets the blade generate thrust.

Does a four-blade propeller really improve the hole shot?

At the same pitch and diameter a four-blade absorbs more torque, turns lower and gives up some top speed. The hole-shot advantage usually credited to it comes from the fact that it is normally fitted with a shorter pitch. The test bench lets you separate the two effects by changing one parameter at a time.

Propeller Test Bench, pitch A/B comparison

Q: How much difference does one inch of pitch make?

As a rule of thumb, one inch more of pitch lowers maximum engine rpm by roughly 150 to 200, and one inch less raises it by the same amount. Shorter pitch frees up revs and hole-shot thrust, longer pitch travels further per revolution and raises top speed. The test bench shows both sides of that trade at once.

Try the tool

Open the Propeller Test Bench →

The Propeller Test Bench compares two fixed-pitch propellers fitted to the same boat, with the same engine and the same load. Change the pitch of one of them and the whole trade-off appears at once: bollard thrust, thrust margin over the resistance hump, time to get on plane, the rpm the engine can actually pull at wide open throttle and the top speed reached.

Boat owner at the helm of a moored RIB, with a propeller resting on the table and a comparison chart on the laptop screen — Pitch is almost always chosen at the dock, comparing two propellers with the engine off.

It is the question every owner faces at the dealer's counter: shorter pitch for the hole shot, or longer pitch for top end? There is never a single answer, because it depends on how heavy the boat is, how much rpm the engine can deliver and how the boat is used. Here the whole compromise is visible at once, figure by figure.

This is the ninth project published in AI-LAB: the physical model, the animated canvas drawings and the interface were generated by conversing with Claude, Anthropic's AI model.

How it was built

The project started from a natural-language description of the problem: compare two fixed-pitch propellers while holding everything else about the boat constant. Claude implemented the propulsion model, meaning thrust and torque coefficients as a function of the pitch to diameter ratio, a hull resistance curve normalised on the volumetric Froude number, the search for equilibrium between the torque demanded by the propeller and the torque delivered by the engine, and the integration of acceleration from standstill. Every drawing, from the helical wake trace to the pitch triangle and the hulls in trim, is vector artwork on native canvas, in a single standalone file with no server-side dependencies.

What the tool shows

Side-by-side comparison of two propellers: pitch, diameter, blade count and disc area ratio set independently for propeller A and propeller B.
Hole shot and planing: bollard thrust in kgf, thrust margin over the resistance hump, time to get on plane and the minimum rpm needed to stay there.
Match diagnosis: the rpm reached at wide open throttle is compared with the engine rated rpm, to tell whether the propeller is over-propped, under-propped or correctly matched, together with the pitch that would bring the engine back into its useful window.
Slip and efficiency: apparent slip, the same figure you get by comparing the tachometer with GPS speed, and propeller efficiency in every condition.
Animated visualisations: the helical trace left in the water by the blade tip against the ideal screw, the pitch triangle, the rpm-versus-speed operating curve, the acceleration curve from standstill and the two hulls running in their respective trim.
Automatic warnings on likely tip cavitation, excessive slip and a boat that never breaks free of the hump.

The questions it answers

How much difference does one inch of pitch make? As a rule of thumb one inch more lowers maximum rpm by roughly 150 to 200, one inch less raises it by the same amount. The tool shows the effect on both fronts at once, hole shot and top end, instead of leaving it to guesswork.

Am I over-propped or under-propped? Look at the rpm reached at wide open throttle against the engine rated rpm. Below the rated figure the pitch is too long and the engine is lugging; on the limiter without delivering full power the pitch is too short.

Do four blades improve the hole shot? At the same pitch and diameter a four-blade absorbs more torque, turns lower and gives up some top end. The advantage usually credited to it comes from the shorter pitch it is normally fitted with: here the two effects can be separated by changing one parameter at a time.

Reading the three rpm figures

Three different rpm figures appear in the panel and should not be confused. The rated rpm is an engine specification, the speed at which the manufacturer declares power. The test rpm is simply the throttle setting at which you want to photograph the boat and has no effect on the physics. The rpm reached is a result: it is where the engine actually settles at wide open throttle, when the torque demanded by the propeller matches the torque delivered. Comparing that figure with the rated rpm is the diagnosis of the match.

Model limitations

The test bench is an indicative tool, designed to make the cause and effect relationships between pitch, rpm and speed visible. The propeller is described with coefficients linearised on wide-blade systematic series, and hull resistance uses a normalised curve rather than an estimate dedicated to the specific hull. The rotational inertia of the driveline is not modelled, so planing times come out slightly optimistic. A final choice calls for the real engine power curve, the diagrams of the propeller series adopted and the judgement of a professional.

Privacy note

The tool runs entirely in the browser: no data is ever sent over the internet. Every calculation happens locally via JavaScript, with no external server calls.

AI-LAB publishes exploratory projects: this tool is not part of Valuemate's commercial service portfolio.

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