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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.

What this rests on

Basis
General principle. No specific figures, no source needed
Difficulty
Bolt-on

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.

Impeller pitch as one turn of a screw thread, laid flat Two right-angled triangles stand side by side on bases of the same length, each base marked as one turn around. On the left the sloping side rises only a little and is marked lower number; on the right it climbs steeply and is marked higher number. A dashed upright with an arrowhead at the top of each triangle shows how high its slope reaches. One turn of the blade, laid flat the rise is how far forward one turn moves it one turn around lower number moves less per turn one turn around higher number moves further per turn A progressive blade, whose slope grows along it A single curved line rises from left to right over a dashed base, starting almost flat and bending up more and more steeply. A small circle marks each end. The left end is marked leading edge, with a gentler slope; the right end is marked trailing edge, with a steeper slope. One blade, a slope that grows a progressive blade steepens along it leading edge gentler slope trailing edge steeper slope around the hub

Unwrap a screw thread and each turn becomes a slope, and the steeper the slope, the further one turn would carry the blade. Makers measure and mark that distance in their own ways, so one maker's number does not translate to another's.

How we know: the distance per turn is how US patent 11,485,457 defines it (paragraph 56), the slope that grows towards the trailing edge is US patent 9,758,226 (paragraph 24), and the makers' differing numbers are Group K's 1996 Between the Lines.

Not to scale.

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.

Spec tables

Parts this is about

  • Solas impeller, Yamaha SuperJet 701

    Solas · SOL-SJ701

    Data not confirmed

    IHRA Ski GP3 (legal with conditions) · IHRA Ski Lites (legal with conditions) · IJSBA Ski Lites (legal with conditions)

    Jet pump, impeller marked

  • Skat-Trak impeller, Kawasaki SX-R 800

    Skat-Trak · SKAT-SXR800

    Data not confirmed

    IHRA Ski GP3 (legal with conditions) · IHRA Ski Lites (legal with conditions) · IJSBA Ski Lites (legal with conditions)

    Jet pump, impeller marked

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.

Cite this page

“What an impeller pitch number actually means”, BaddJet archive, https://baddjetarchive.com/tech/pump/impeller-pitch-explained/

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