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Science
  General science
  Designs
Impeller engines use a rotating propeller in an air-tight housing to suck in water
Impeller engines use a rotating propeller in an air-tight housing to suck in water
The water is squeezed out through a narrow nozzle to greatly increase pressure. This draws in more water and the cycle continues. This is known as the Venturi effect
The water is squeezed out through a narrow nozzle to greatly increase pressure. This draws in more water and the cycle continues. This is known as the Venturi effect
The high-pressure water exiting the back of the boat propels it forwards
The high-pressure water exiting the back of the boat propels it forwards
Water enters the centre of a spinning chamber in a centrifugal engine
Water enters the centre of a spinning chamber in a centrifugal engine
As the water spins in the chamber, moving outwards, its pressure and velocity increase
As the water spins in the chamber, moving outwards, its pressure and velocity increase
 General science

Water-jet technology - how did it start?

Jet-boats were developed into a commercial proposition in New Zealand by Bill Hamilton, and the firm Hamilton Water Jets still makes jet boats today. Jet boating is a very popular leisure activity in New Zealand but technology is now used in all sorts of watercraft, from a tiny personal watercraft to car-carrying ferries.

The technology came out of a need to maneuver boats in rapid and shallow water where a conventional propeller outboard motor would strike the bottom of the river and the rudder would struggle against strong currents.

Water jets have very few moving parts compared to other propulsion systems and this means lower maintenance costs. They are also a lot more efficient than most conventional outboards when the jet drive and internal combustion engine are well matched, because you get more performance from a smaller engine and the water jet causes a lot less drag in the water than an outboard. For this reason, water jets are often built into the hull of the boat rather than on the outside.

How a water jet works

Jet boats work on the principle of Newton's third law of motion. By taking in and pumping out huge volumes of water very quickly the 'equal and opposite reaction' is to move the boat forwards. The water flows into an intake mounted on the bottom of the boat hull. This hole leads to the impeller pump, which speeds up the flow of water and throws it out of the nozzle. This creates forward motion. By moving this nozzle left and right you can control the direction of the boat.

The principal parts are an intake where the water enters the unit. The impeller (attached to the engine drive which turns and pumps the water), the stator, which controls the flow rate (increasing or decreasing the pressure and so speed of the boat) and the nozzle, which controls steering.

For a jet boat to accelerate, more water needs to be pumped through the jet drive. As the boat speeds up, even more water is needed until eventually you cannot pump any faster and top speed is reached. The Hairy Hogs' home-made impeller could pump more water faster, giving them the edge on test day.

To power jet-boat impeller pumps, you still need a combustion engine with a drive shaft, but the engine can often be of a smaller size than if it was powering a conventional boat-propulsion system as jet drives are more efficient.

Team Ragbo used a large centrifugal pump which would have been used to pump thick liquids like farm slurry. Centrifugal pumps are designed to put more pressure and speed into the flow of water – it is this increase in speed that can be utilised to propel a boat.

Water enters the pump and hits the spinning blades, the impeller, and centrifugal force pushes the water to the edge of the blades. As the blades spin, the accelerated water reaches the exhaust port where it is ejected.
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To spin their impeller, The Hogs connected a shaft through the housing, sealed it with grease and crimped it
To spin their impeller, The Hogs connected a shaft through the housing, sealed it with grease and crimped it
Team Ragbo used a heavy centrifugal engine which sat low in the boat
Team Ragbo used a heavy centrifugal engine which sat low in the boat
To stabilise the boat, they used a deep v-shaped hull
To stabilise the boat, they used a deep v-shaped hull
The deeper below the water-line that the hull is, the more draught it encounters and the more power is needed
The deeper below the water-line that the hull is, the more draught it encounters and the more power is needed
 Designs

The Hairy Hogs and Team Ragbo had a lot to think about if they wanted to create a viable jet boat:
  • Newton's third law of motion
  • Centrifugal force
  • Mechanical drive to impeller
  • Displacement of water
  • Hull shape
  • Weight vs power
  • Outboard vs inboard propulsion
  • Impeller design
  • Manoeuvrability
Hairy Hogs' impeller jet boat

The Hairy Hogs got the right balance of power over weight, manufactured a relatively efficient impeller and attained good manoeuvrability.

Their plan was to build a flat-bottomed jet boat by combining a 'grunty' engine and a bodge-job impeller to throw out a jet of water to propel them over the lake.

Key to their plans was finding a suitable bike or car engine that could be coupled with an appropriate shaft for the impeller. Failure to get this right could cripple their plans. Alan was confident he could bodge an impeller and housing. Due to the high revolutions, it required very careful planning, intricate fabrication and special balancing to avoid it falling apart. The blade was only half the story – the team also needed to devise the intake, ducting, shaft-bearing, shaft-seal and a nozzle they could steer.

Key to water-jet propulsion is the fine tolerances between the impeller and housing. To get maximum power, the Hogs and expert Alan ensured the gap between the impeller and housing was millimetre perfect. The shaft seal was made up of thick grease in which the shaft span, wrapped in tubing, which was in turn clamped with jubilee clips. Not an ideal solution, but it kept the water out for long enough. The steerable nozzle was a nice touch and is as used on all professional jet boats (minus the BMX handle bars). The team's design looked great – and it worked!

Pros
  • Conventional, proven technology
  • Good speed for a lightweight design
Cons
  • Engineering the impeller could backfire, it's an extremely fiddly job
  • Housing the impeller had to be perfect or losses in efficiency would reduce speed and could have created a non-mover – a large floating fountain!
  • Grease-packed sealant shouldn't work!
Team Ragbo's heavyweight pump boat

The team and expert Alex planned to use a diesel car engine matched with a giant, 180kg centrifugal sewage pump. The pump and engine weighed a massive amount, so they were hoping the power of the pump would outweigh the weight disadvantage. This turned out to be an error!

They used a drive shaft running from the engine to power the pump. The ideal is to combine use of the engine at low revs in first/second gear to obtain the correct mixture of torque and revolutions for the pump to operate at its best (2000rpm). The team would need to carefully assess the gearing ratio of their set-up to get the most out of it. At the front of the craft would be an inlet, at the rear an outlet. The ducting would be created from a high-pressure hose, irrigation hosing or another type of metal tubing. The key was to keep curves or elbows in the piping to a minimum to get the best efficiency and least frictional loss. The team would have to carefully assess their best hull for this heavier design as displacement would be an issue. As you probably saw on the show, the craft sat very low in the water and the extra drag didn't help their chances.

Pros
  • Simplicity of pump and robustness of pump – you can suck in leaves, twigs and children and it will keep pumping!
Cons
  • Heavy design, pump not designed for this usage
  • Poor performance, as this sort of pump is designed for a different purpose and has a different flow rate and volume vs speed bias
  • Displacement issues and speed handicap
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