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3.0 LITERATURE SURVEY

3.1 WATERJET PROPULSION BACKGROUND

Vessels throughout history have used various methods to provide forward motion. Sail along with

oars and paddles were the main sources of propulsion before the invention of steam ships in the

1800’s. Other methods have been developed in the last two centuries that include the screw propeller,

the azimuth thrusters and the waterjet. Each of the methods has their advantages over the others and

therefore more favoured to certain applications. The advantages of waterjet propulsion are explained

by Hamilton Waterjets (2007) to be:

Shallow draft - Waterjet intakes are flush with the hull of the vessel therefore

allowing the vessel to operate in shallow areas with reduced risk of

damage to the jet.

Safety – As the impeller of the jet is totally enclosed the risks when operating

around people are very low.

Low cavitation – At high speeds a waterjet is less likely to cavitate than the

conventional screw propeller making them more efficient.

Waterjet propulsion is widely regarded for use in high speed vessels. As they do not have the

tendency to cavitate like a screw propeller at higher speeds, nominally 30+ knots. Commercial vessels

that predominately use waterjet propulsion as their main source of propulsion are high speed ferries.

3.2 WATERJET PROPULSION – TRADITIONAL USES .

In personal water craft (PWC) waterjets are the only source of propulsion and the majority of the

general public would identify a waterjet with a PWC and not particularly with any other type of

vessel. The fact is that in commercial vessels waterjets are widely used where their advantages

provide good reason for their installations. In the high speed ferry industry they are used not only

because they can provide good power to weight but also because they give the advantage of high

speed efficiency. Other commercial vessels to use waterjets as propulsion are offshore support vessels

that service floating and fixed structures within the offshore Oil and Gas industry. These types of

vessels often prefer waterjets as propulsion as it gives the vessel flexibility and safety when working

around shallow pipe lines and cables as well as providing the capability of high speed transfer of

personnel and supplies to and from the structures (Carlton, 2007).

Waterjets are usually only used at high speeds; this is because they generally possess low efficiencies

at low speeds compared to other propulsion options. Vessels will only use jet propulsion at low speeds

if one of their advantages, such as providing shallow draft or high manoeuvrability, is more important

than the efficiency. An example of this is the use of waterjet propulsion in the Swedish Navy Visby

class corvettes (Technology, 2009) where the decision was made to use waterjet as its main source of

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propulsion for two reasons. The first being that waterjets provide greater stealth for the vessel as the

noise signature of a waterjet is a lot harder to detect than that of a conventional screw propeller and

the second being that the waterjet provided a reduction of draft of approximately 1 meter over a screw

propeller design. Throughout the research for this project a vessel in which it was designed for speeds

below 20 knots was not located where it had selected waterjet propulsion as its main source of

propulsion based purely for its efficiency.

3.3 DIAMETER MATCHING NOZZLE TO JET EFFICIENCY.

• Has jet speed to vessel speed matching been investigated

The literature that has been reviewed for this project is not decisive in the approaches into how to

match a jets exit velocity to that of the vessel. The research certainly did not uncover a step by step

process which should be followed to optimise a jet to match a vessel of certain particulars. The

research that includes most of the necessary theory and formulas in regards to optimising a waterjet is

that of Bulten (2006). The thesis is based around a waterjet of similar shape to that of the DOEN DJ

60. This means that some of the assumptions and values used throughout the thesis are relevant to this

project but some of the information needed to complete the calculations, i.e. the pump curve, is not

available for the DJ 60 waterjet. Bulten (2006) uses the overall propulsive efficiency graphed against

the JVR to determine the optimum operating point for the jet system. In calculating the propulsion’s

overall efficiency in his thesis, Bulten(2006) used several parameters that were not are not available in

this project, which were the thrust deduction factor (t), wake fraction (w) of the vessel, the pump

efficiency which uses pump head. To be able to use Bulten’s (2006) method the missing values will

need to be estimated for the Greenliner and the DJ60 jet. Bulten states that generally for waterjets the

optimum JVR lies within the 0.5 -0.7 range.

To optimise the DJ60 waterjet, the flow rate of the initial setup was needed. It was decided the

simplest way of measuring this flow rate was to traverse a pitot tube across the jet stream to find the

differences in static and dynamic pressure. This then gave a velocity of the stream which was used to

obtain the flow rate of the jet. Brander and Walker (2007) investigated jet intake velocities and

pressure distributions using a pitot tube. This research investigated several different Inlet Velocity

Ratios (IVR) to try and obtain the different phenomenon which are present in the jet at various IVR’s.

This paper does not state which IVR’s are commonly the most efficient but it does state that typically

large high speed craft cruise at IVR values between 1.5 and 2.3 implying that this could be the most

efficient range of IVR values. The Inlet velocity ratio defined in their paper is the free stream velocity

divided by the mean jet exit velocity.

A third method of obtaining the jet velocity and therefore the mass flow rate was considered. This

method was to use to Brandner’s (2007) pump face axial velocity distributions along with the

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measured velocities to establish the mass flow rate. Brandner’s research showed that the IVR of a jet

varied the velocity distributions greatly. An example of the pressure mapping is shown in Figure 2.

The mapping showed that the velocity in the bottom half of the intake was greater than that of the

upper half of the intake. The five pressure maps at various IVR’s were compared to the measured

velocities shown in Figure 17. A relationship between the pressure mappings and the measured

velocities did not exist as the velocities measured through the majority of the jet were symmetrical

between the upper and lower sections. This factored into the decision not to use the mappings as a

way to estimate the mass flow rate of the jet. The major reason in not using the mappings as an

estimation method was because the pressure mappings were all modelled in the Cavitation tunnel. In

the tunnel an inlet velocity ratio could be simulated by forcing water into the jet. Whereas the IVR for

the measured velocities in this project were infinite as the vessel was moored stationary for the

testing. This would cause the pressures within the jet intake to differ largely as the water is being

speed up from stationary. It was decided that it was unknown how Brandner’s(2007) pressure

mappings would correlate to the DJ60 jets outlet pressures. So therefore Brandner’s (2007) pressure

mappings were not used to estimate the jet outlet pressures. To be able to use the pressure maps as a

guide to obtain the mass flow rate of the jet the velocity readings could be undertaken with the vessel

travelling under its own power.

Figure 2: Pressure mapping of jet intake (Brandner, 2007)

3.4 ABOVE SURFACE VERSUS SUBMERGED JET DISCHARGE

• Why is surface widely used?

The reason that has been uncovered as to why surface discharge is used is the fact that Sir William

Hamilton, the developer of the modern waterjet, was using submerged discharge in his initial designs

and was able to achieve a modest speed of 11 miles per hour (mph). He then moved the discharge

above the surface and was able to achieve 17mph of vessels speed. Sir William explained this by

concluding that the drag of the jet and the pipes were eliminated by placing them above the waterline

when the vessel was planing (Hamilton, 2007). In can be assumed that this hasn’t been changed

because predominately waterjets are used as propulsion in planning vessels where it is relatively

simple to maintain the waterjet above the water at the stern of the vessel. Additionally surface

discharge is preferred because of the drag created by the low pressure region explained in section 3.5.

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This means by keeping the discharge location above the surface it eliminates the issue of a low

pressure region causing drag upon the hull.

3.5 OPTIMUM JET DISCHARGE LOCATION TO REDUCE VESSEL DRAG.

No research into using the low pressure region formed by the waterjet when exiting the nozzle to

assist in propelling the boat forward has been found. The conclusion that has been drawn into why

there hasn’t been any research undertaken is that when a planing hull vessel is travelling at high

speeds it would cause separation of air off the rear of the hull. This separation would create a low

pressure zone as shown in Figure 3. The low pressure region created by the waterjet when discharging

would be insignificant with respect to the low pressure caused by the air flow around the hull.

Therefore by minimising the effect of the low pressure region created by the jet would have minimal

impact on the overall drag upon the vessel. The effects would be much greater when discharging into

water but as waterjets have seldom been used in displacement vessels that operate at one constant

speed, this research has not evolved.

Figure 3: Low air pressure region around planing hull