Pump

What an impeller pitch number actually means

Pitch is the bite the impeller takes out of the water, and it is the single cheapest change to how a ski feels. It is also the one most often copied off somebody else's setup, where it does not transfer.

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Pitch is a distance, not a speed

The number on an impeller describes how far it would screw itself forward through the water in one rotation if the water were solid and nothing slipped. Two numbers usually appear, one for the leading edge and one for the trailing edge, because the blade is twisted along its length. Lower numbers take a smaller bite. Higher numbers take a bigger one.

Lower pitch spins up, higher pitch holds on

A lower pitch lets the engine reach its power band sooner, so the ski leaves the line harder and recovers faster after a turn, at the cost of top speed. A higher pitch loads the engine more, holds a longer top gear and pulls better on flat water, but it will feel lazy off the bottom and it can hold the engine below the revs where it makes power. Neither is better. They are the same trade every geared machine makes.

The engine tells you, not the internet

Pitch is the gearing between your engine and the water, so the correct choice is whatever puts the engine at the wide-open-throttle RPM its manual specifies, with you on board, in the conditions you actually ride. Too little pitch and it will exceed that figure and gain nothing. Too much and it will never reach it. That is why a number that works on one ski does not carry to another with a different pipe, different porting, a different pump or a heavier rider.

Condition changes the answer

Rough water unloads the pump every time the hull leaves the surface, so a setup that is perfect on glass can over-rev badly in chop. Altitude reduces engine output, which usually wants less pitch to keep the revs up. Rider and fuel weight move it again. This is why a setup sheet states the venue and the water condition rather than just the part, and why a pitch without that context is only half the information.

Before you act on this. This explains the principle and gives no pitch recommendation on purpose. The right pitch depends on your engine, pump, altitude and weight, and the only way to know is to measure engine RPM at wide open throttle against the figure in your manual. Copying a number off a forum is how people end up slower.

Spec tables

• Solas impeller pitch, two-stroke stand-ups

Parts this is about

• Solas impeller, Yamaha SuperJet 701

  Solas · SOL-SJ701

  Data not confirmed

• Skat-Trak impeller, Kawasaki SX-R 800

  Skat-Trak · SKAT-SXR800

  Data not confirmed

Symptoms this addresses: Bogs off the line · Revs out but will not reach top speed · Over-revs and loses drive in chop · Engine screams, ski does not move. All symptoms.