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How a jet pump moves the craft

A stand-up moves by throwing water out of the back. It takes water in under the hull, speeds it up, and steers by aiming that jet, which is why it steers poorly with the throttle shut.

What this rests on

Basis
Documents held in this archive, each listed under Where this comes from
Difficulty
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Water in, faster water out

A jet-driven craft moves by throwing water backwards, and it moves forward as the reaction. That push is called thrust. The pump takes water in from under the hull, speeds it up, and sends it out of the back as a jet. The sections below follow the water through the pump in the order it meets each part.

A jet pump in side section, from the intake grate to the jet Seen from the side, front to the left. A box at the left stands for the engine, and a thick shaft runs right from it to the hub of the impeller. Along the bottom a straight line gives way to a row of short slanted bars, and behind them a heavier line runs on under the pump. Dashed arrows rise from below through the bars, and a dashed curve climbs a sweeping passage into a straight tube. In the tube the hub carries slanted blades inside a raised band, then come upright fixed vanes, then the tube tapers, and a short straight-sided tube with a dotted upright line through it sits on the end. A dashed arrow leaves it to the right, and an arrow along the top points left. Each part carries its name. the craft moves forward engine drive shaft hull bottom intake grate water in ride plate pump housing pump stator wear ring steering nozzle pivot impeller tip gap nozzle narrows jet thrown back

The order the water meets them: intake grate, impeller turning in its wear ring, pump stator, nozzle, steering nozzle, and out as the jet.

Follow the water from the grate to the jet: the impeller gives it its push, the stator takes out the spin, and the two nozzles shape the stream and aim it. Throwing that jet backwards is what drives the craft forwards, and aiming it is what turns the craft, so when the throttle is shut and the jet fades to little or nothing, the steering fades with it.

How we know: the parts and the water's path through them are US patent 8,905,800 (lines 64 to 70 of the held copy) and US patent 5,833,501 (lines 67, 91 and 94); the engine ahead of the pump turning its shaft, US patent 5,520,558 (line 48); the craft moving as the jet's reaction, US patent 6,705,908 (line 36); the small gap at the blade tips, US patent 6,857,919 (line 194); the ride plate closing the pump from below, US patent 5,833,501 (line 93) and US patent 6,705,908 (line 89); the jet's flow turning the craft, and slowing or stopping off the throttle, US patent 6,523,489 (lines 185 to 187) and US patent 6,705,908 (line 39).

Not to scale.

The intake grate and the drive shaft

Water comes up into the pump through an inlet in the bottom of the hull, ahead of the impeller. Across that inlet is the intake grate, a set of bars that keeps rocks, weed and other debris out of the pump. The engine sits ahead of the pump, and a drive shaft runs back from the engine to the pump. The engine turns the drive shaft, and the drive shaft turns the impeller on its end.

The impeller turns inside the wear ring

The impeller is a hub with curved blades round it. It spins inside the pump housing, the casing of the pump that is fixed to the back of the hull, in a close-fitting ring called the wear ring. The gap between the tips of the blades and the wear ring is kept small, so the water the blades push has to go backwards rather than leaking round the tips. How far the impeller would move forward in one turn is called pitch, and it has an article of its own in this topic.

Why a worn wear ring makes a craft slow

Debris caught between the wear ring and the impeller wears the ring and opens up that gap. Water then slips back past the tips instead of being pushed out of the back, and the pump loses efficiency. On the pumps the rulebooks cover, the wear ring is a separate part that can be replaced, and two racing bodies name it as a part of its own: IHRA (the International Hot Rod Association) in its rulebook V.20260603 at J.18.3, and IJSBA (the International Jet Sports Boating Association) in its 2024 Ski Lites document at SKL.6.1. A worn wear ring is one reason a craft loses speed. It is not the only one, and this page does not diagnose.

The stator, the nozzle and the steering nozzle

Water leaving the impeller is spinning as well as moving backwards, and the spin does nothing to push the craft. Behind the impeller sits the stator, a ring of fixed vanes. This is the pump's stator, not the ignition stator on the engine. It straightens that swirl so the water's effort goes into moving backwards. Next the nozzle narrows, and water squeezed through a narrower opening leaves faster. A narrower nozzle is not simply better: it trades how much water leaves for how fast. Last on the pump comes the steering nozzle, a short tube on the end that pivots from side to side and is linked to the handlebars. It aims the jet.

It steers by aiming the jet, so no jet means little steering

Turn the bars and the steering nozzle swings to one side, aiming the jet, and the jet's push turns the craft. With the throttle shut the flow through the nozzle slows or stops, so there is little left to aim, and the craft steers poorly. At slow speed the jet gives very little steering at all. This is a safety point, not a detail: letting go of the throttle to avoid something can leave very little steering at the moment you need it. Systems called off-throttle steering were made to give some of it back, and the glossary's entry says what this archive holds about them.

The ride plate underneath

Under the pump, at the back of the hull, sits the ride plate. It closes the bottom of the pump area, so the pump is not left open underneath, and forms the last part of the hull bottom the craft runs on. IHRA's rulebook calls it the ride plate (V.20260603, J.18.1), and IJSBA's 2024 Ski Lites document calls it the pump cover plate (SKL.2.5). Changing it changes how the craft runs, and a plate that holds the nose down can cost top speed. Both rulebooks allow a replacement and limit how wide an extension may be and how far it may reach.

What this page leaves out

It names the parts in the order the water meets them and says what each one does. It does not describe any one machine's pump, give a size, gap or speed, or say how to measure wear on a wear ring or choose an impeller. What a class allows you to change in the pump is in the class-rules articles and the legality pages, not here.

Where this comes from

  1. Water enters through a grate over an inlet in the hull bottom that keeps out rocks, weeds and other debris, passes the impeller in its housing and the stator, and leaves through a narrowing nozzle and a steering nozzle as thrust. US patent 8,905,800, Inlet grate for a water jet propulsion system, lines 64 to 70 of the copy this archive holds.
  2. The craft ejects water rearward and moves as the reaction, the direction of the jet turns it, and the pump cover forms the bottom of the pump room. US patent 6,705,908, Jet-propulsion watercraft, lines 36 to 39 and 89 of the copy this archive holds.
  3. The engine sits forward of the pump and drives it through a shaft, and the pump is fixed to the transom. US patent 5,520,558, Jet propulsion unit for a watercraft, lines 48 and 72 of the copy this archive holds.
  4. Blades angled round a hub turn inside a wear ring, fixed stator vanes follow them, and a plate behind the inlet covers the pump from below. US patent 5,833,501, Cavitation control for marine propulsion system, lines 67 and 91 to 94 of the copy this archive holds.
  5. The gap between blade tips and ring is kept small to stop leakage, and the pump loses efficiency as it grows. US patent 6,857,919, Impeller for marine propulsion device, line 194 of the copy this archive holds.
  6. Narrowing the outlet trades the volume of water for its speed, and a close tip clearance keeps water from leaking back. US patent 7,241,193, Variable marine jet propulsion, lines 53 and 72 of the copy this archive holds.
  7. Debris deposited between ring and impeller wears the ring out, with a loss of efficiency, and the ring is a part of the housing that is replaced. US patent 10,399,653, Jet ski impeller, lines 51 and 82 of the copy this archive holds.
  8. Defines the term as the distance the impeller would advance in one turn. US patent 11,485,457, Hydrojet propulsion system, line 133 of the copy this archive holds.
  9. Off the throttle the flow through the nozzle slows or stops, steering becomes less effective and minimal at slow speed, and systems were built to restore some of it. US patent 6,523,489, Personal watercraft and off-power steering system, lines 21 and 185 to 192 of the copy this archive holds.
  10. J.18.1 names the intake grate and the ride plate and limits a ride plate extension's width and reach; J.18.3 lets wear rings be replaced within OEM specifications. IHRA rulebook V.20260603, lines 1513 to 1524 of the copy this archive holds.
  11. SKL.2.5 names the pump cover plate and limits an extension's width and reach; SKL.6.1 allows replacement wear rings within OEM inside diameter. IJSBA 2024 Ski Lites document, lines 58 to 60 and 192 to 193 of the copy this archive holds.
  12. Ride plates that cut porpoising do it by driving the nose down, at a cost in top speed. Between the Lines, 1997, Group K, line 294 of the copy this archive holds.
Cite this page

“How a jet pump moves the craft”, BaddJet archive, https://baddjetarchive.com/tech/pump/how-a-jet-pump-works/

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