How a Waterjet Works
A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. It works in relation to Newton’s Third Law of Motion – “every action has an equal and opposite reaction”. A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful firehose.
Put simply, the discharge of a high velocity jet stream generates a reaction force in the opposite direction, which is transferred through the body of the jet unit to the craft’s hull, propelling it forward (see diagram below).
In a boat hull the jet unit is mounted inboard in the aft section. Water enters the jet unit intake on the bottom of the boat, at boat speed, and is accelerated through the jet unit and discharged through the transom at a high velocity.
The picture below shows where water enters the jet unit via the Intake (A). The pumping unit, which includes the Impeller (B) and Stator ©, increases the pressure, or “head”, of the flow. This high pressure flow is discharged at the nozzle (D) as a high velocity jet stream. The driveshaft attaches at the coupling (F) to turn the impeller.
Steering is achieved by changing the direction of the stream of water as it leaves the jet unit. Pointing the jet stream one way forces the stern of the boat in the opposite direction which puts the vessel into a turn.
Reverse is achieved by lowering an astern deflector (E) into the jetstream after it leaves the nozzle. This reverses the direction of the force generated by the jet stream, forward and down, to keep the boat stationary or propel it in the astern direction.
Waterjet propulsion has many advantages over other forms of marine propulsion, such as stern drives, outboard motors, shafted propellers and surface drives. These advantages include:
HamiltonJet waterjets have fewer moving parts and most major components inboard, minimizing the chance accidental damage and increasing the life of components.
- Precise steering control at all boat speeds
- “Zero Speed” steering effect provides 360° thrusting ability for docking and holding stationary.
- Sideways movement possible with multiple jet installations.
- High efficiency astern thrust with “power-braking” ability at speed
HamiltonJet waterjets are designed to be optimized for vessel speeds from 25-50 knots, dependent on application’s engineering variables including hull resistance and type, engine power rating and rpm, impeller rating, etc.
- Propulsive coefficients as good or higher than the best propeller systems achievable at medium to high planing speeds
- Flexibility when using multiple waterjets may allow operators to continue to operate efficiently on fewer drives
Low Drag and Shallow Draught
- Absence of underwater appendages reduces hull resistance
- Shallow draught – the waterjet intake is flush with hull bottom to allow access to shallow water areas and beach landings with no risk of damage to the drive
- No protruding propulsion gear eliminates impact damage or snags
- Minimum downtime and simple maintenance routines
Smooth and Quiet
- No hull vibration, no torque effect and no high speed cavitation gives maximum comfort levels on board
- Low underwater acoustic signature
No exposed propeller for complete safety around people in the water and marine life
Maximum Engine Life
- Jet unit impeller is finely matched to engine power
- Power absorption is the same regardless of boat speed
- No possibility of engine overload under any conditions
- Single packaged module
- No heavy and expensive gearbox required for many installations. Simple driveline from engine to jet coupling
- Complete factory tested package, ready to bolt in
- No difficult engine alignment problems
- Highly efficient design
- Strong, lightweight, cast aluminum housing
- Wide choice of stainless steel impellers for optimum engine matching, strength and cavitation resistance
- Standard left-hand rotation (counter-clockwise looking at the engine flywheel)
- Integrated intake and mounting base for ease of installation
- Cathodic (Anode) protection
- Flush-mounted intake screen
- Power operated, split-duct reverse bucket for maximum operational control (except HJ212 model)
- Standard clean-out port with optional extension collar
- Wide range of hydraulic and electronic control systems including blue ARROW and MECS electronic controls
- Provision for an internal rake to remove debris from the intake screen (except HJ212 model)
- DNV, A.B.S., BV, etc. certification
Hamilton waterjets have several advantages over other waterjet makes. These include...
Superior Cavitation Resistance
Pump design offers up to 25% more thrust than other waterjets between 0 and 20 knots. Manoeuvrability at low speeds and acceleration to high speeds are superior.
Precise Steering Control
HamiltonJet's unique JT steering nozzle minimises thrust loss when steering. No central deadband provides higher course-keeping efficiency and higher overall boat speeds.
More Accurate Performance Prediction
HamiltonJet's experience is supported by accurate performance prediction software to ensure waterjets are an appropriate propulsion option for your vessel, and to recommend the most appropriate waterjet size and set up for every different application.
Worldwide Service and Sales Support
HamiltonJet’s global network of Distributors can be found in over 50 locations, support by Regional Offices in the US and Europe and the factory in New Zealand