A new future for azimuthing thrusters

In terms of column inches in the trade press, azimuth thrusters have been somewhat overshadowed by their azimuthing electric podded cousins of late

.

Yet in the past few years the azimuth thruster has made some big strides into a wide section of the marine market

.

The Ulstein Aquamaster Azipull thruster opens the way for higher speed applications, up to about 25 knots

.

Rolls-Royce, its manufacturer, says that tanker owners have also expressed an interest in the product ? for instance two each are installed on Stena Bulk?s C-Max tankers ? and it has 14 units on order for ferries and offshore service vessels

.

Azipull is a mechanical azimuth thruster equipped with a pulling

propeller

which may be either controllable pitch or fixed pitch depending on the choices made in the rest of the propulsion system

.

The lower bevel gear and pitch-setting mechanism is based on experience drawn from designing and manufacturing thousands of thrusters of many types

.

The mechanical elements are enclosed in a hydrodynamically optimised leg with a wide chord to provide rudder effect and improve the vessel?s course stability

.

The leg incorporates a skeg extending below the gear housing

.

Together, the streamlined leg and skeg recover swirl energy from the slipstream of the pulling

propeller

, raising the overall propulsive efficiency because this otherwise wasted energy is converted to a forward force on the thruster leg

.

Within the hull of the vessel is the upper gear housing, steering gear and auxiliaries taken from the standardised Ulstein Aquamaster azimuth thruster range to maximise commonality of spare parts and minimise technical risk

.

The flow of water to a pulling

propeller

is only determined by the hull so it is possible to obtain a more homogeneous inflow to the

propeller

in this type of thruster

.

More uniform inflow is decisive for reducing unsteady cavitation, and thereby cutting down

propeller

induced noise and vibration and reducing oscillations in shaft torque

.

Initially, production is being concentrated on the AZP85 and AZP120 frame sizes, covering the range from about 800kW up to about 3,200kW, but Rolls-Royce says that the power range could extend to about 6MW

.

The main characteristics are:

l low drag, high efficiency pulling azimuth thruster

l high hydrodynamic and total efficiency

l low noise and vibration levels

l course stability

l continuous service speed of 24 knots while maintaining good manoeuvrability

l can be linked to any kind of prime mover, accepting diesel or gas turbine with mechanical or electrical drive

l flexible with respect to vessel application, enhanced by availability in CP and FP versions

A step further

Taking the azimuthing thruster a stage further is Steerprop, which has recently completed model tests at Krylov Shipbuilding Research Institute in St

.

Petersburg, Russia

.

The dual-end contra-rotating propulsor was tested in both the towing tank and in the cavitation tank

.

The efficiency of the forward

propeller

was measured to be close to 0

.

9 and the propulsor was found to be cavitation-free in normal operating conditions

.

Steeprop recognises the benefits of a pulling propulsor and has combined this with a CRP to offer what it claims is increased efficiency through the aft

propeller

recovering the swirl energy left behind by the forward

propeller

and through splitting power between two

propeller

s and two gear-sets

.

This makes it possible to reduce the

propeller

rev/min and have two large

propeller

s with light load and low speed of rotation

.

The low rev/min reduces frictional losses

.

Steerprop, like Rolls-Royce, recognises that a propulsion unit with a pulling

propeller

offers an undisturbed flow to the

propeller

, which means less noise and vibrations due to better cavitation behaviour and lower pressure pulses

.

And tip clearance can be reduced enabling use of a larger, higher efficiency

propeller

.

However, the company claims that the presence of the pod behind the

propeller

offers an additional gain: The pod geometry may be designed to create a pressure wave in front of it, which acts like an additional wake for the forward

propeller

.

The increase in

propeller

thrust due to this unique interaction between pod and pulling

propeller

practically cancels the pod drag

.

Steerprop claims that its propulsor?s efficiency is 3-5% higher than a pushing CRP, 12-15% higher than tandem

propeller

s and 20-25% higher than a single

propeller

The propulsors are available in a power range suited for the most popular demand today; they can be provided with ducted or open

propeller

s or with contra-rotating

propeller

s; they can be provided with or without release clutch, or with an in-built speed modulating clutch; the steering can be either direct-electric or mechanically (or electrically) driven hydraulic, etc

.

The control system is based on modern automation

technology

, allowing flexible adapting to each vessel type and operation profile, to all available prime movers and to any kind of interface for other ship systems

.

Various propulsion options

One of the azimuthing thruster?s attractions is its mechanical design with bevel gear transmission

.

This means that it is not restricted to diesel electric or gas turbine electric systems, but can also use straightforward mechanical drive from the prime mover

.

Realistic choices, depending on the type of vessel, include:

l diesel mechanical drive and azimuthing thruster with CP

propeller

l gas turbine mechanical drive and azimuthing thruster with CP

propeller

l diesel electric or gas turbine electric with either CP or FP azimuthing thruster

.

In the case of the two latter solutions the choice is between using a constant speed propulsion motor with a CP

propeller

to give speed control, or a fixed pitch

propeller

and variable rev/min motor

.

Which is best depends on the operating profile of the vessel

.

An advantage of the mechanical drive azimuthing thruster with CP

propeller

is that the engine can be run at constant speed and can at the same time drive a shaft generator, thus efficiently supplying the vessel?s electrical loads as well as propulsion power and sensitive and rapid manoeuvring capability

.

Double-ended ferries

To date, 12 Ulstein Aquamaster Azipull thrusters have been selected to propel three of a new design of passenger/vehicle ferry, the Fjellstrand FerryCat, an innovative vessel that radically updates the traditional double-ended ferry

.

FerryCat uses a symmetrical catamaran layout under the car deck, with an Azipull unit under each ?corner?

.

Each azimuth thruster is coupled to its own diesel engine by a short drive shaft, giving a high level of propulsion system redundancy and excellent manoeuvrability

.

Extensive calculation and test tank work has been carried out to integrate the hydrodynamics of the thrusters and the slim catamaran hulls

.

One significant result is that all engines can be run at about the same power (1,500kW each) regardless of direction of travel, instead of having to supply the majority of power to the aft

propeller

s, which is the case with traditional designs if a good efficiency is to be achieved

.

The result will be a better fuel consumption since each engine operates in the most efficient part of its power band

.

Service speed will be 22 knots

.

The low structural weight of the FerryCat will give brisk acceleration and deceleration, minimising the top speed required to provide a given berth-to- berth time

.

The first vessel has been ordered by Rogaland Trafikkselskap for the Stavanger-Tau route in Norway, where it is scheduled to cut journey times almost in half

.

Two FerryCats, propelled by Azipulls, are also on order for operation in Turkey in the Istanbul area

.

FerryCat is an unusual concept, and it is likely that for many other types of ferry two Azipull units would be chosen for operation at higher than usual transit speeds

.

Offshore support

Two Azipull thrusters will be installed in a UT745 offshore platform supply vessel, in a diesel electric installation using electric motors driving into the upper gear houses

.

Of all types of offshore service vessel, Rolls-Royce says the Azipull thruster can probably be used to best advantage in platform supply vessels, and the industry is showing strong interest in the propulsor

.

A typical PSV operating profile shows the vessel spending about 25% of the time in harbour loading and unloading, 40% sailing at a service in the 14-16 knot range and 35% loading or discharging at sea in close proximity to rigs and platforms, often in strong winds, high seas and strong currents

.

No propulsor meets all requirements perfectly, but Rolls-Royce says that Azipull can be a very good match

.

The unit offers high efficiency at transit speed, and the CP version provides good manoeuvrability and secure positioning at platforms

.

Other types of offshore vessel can benefit from fitting Azipull thrusters, including ROV/inspection vessels and cable maintenance ships needing a fairly high transit speed

.

Azipull is not, however, suited to lower speed applications where bollard pull is an important consideration

.

Tanker interest

The tanker industry is proving receptive to the Azipull principle

.

Rolls-Royce says that its NVC-Design concept for products tankers is exciting interest

.

Its main application is likely to be in vessels from 3,000-15,000 dwt

.

Propulsion is by two Azipull thrusters with CP pulling

propeller

s

.

A design that can be used to illustrate the concept is the NVC 3100, a products tanker of about 4,000 dwt

.

It has two Azipull units for main propulsion, each rated at 1,250kW and directly driven by a medium speed engine, for example the new C-series unit from Bergen

.

A clutch and short shaft connects the engine to its thruster

.

At the same time a shaft generator is driven through a clutch from the forward end of each main engine

.

At sea the generators supply the ship?s service; when berthing they also power a small tunnel bow thruster

.

At the terminal one engine can be stopped, the other one then being declutched from its thruster and connected to its shaft generator to cover the vessel?s electrical requirements and also power the cargo pumps

.

The system provides a high level of mechanical and electrical redundancy and has the advantage of permitting main engine maintenance to be undertaken when at a terminal, since one main engine and propulsion train will still be available

.

Further advantages of this concept are the simple hull form with a clean run in the afterbody which, combined with the Azipull units, gives a high propulsive efficiency

.

Also the light compact system and short engineroom far aft open for increased cargo volume and deadweight within the vessel?s overall dimensions

.

Similar Azipull solutions can also be applied to other small merchant vessel types

.

Superyachts are a specialised niche market, reflecting an owner?s personality rather than fitting a specific commercial market

.

In this area the attributes of the Azipull thruster can open the way for designers to come up with new solutions

.

Positioning voyages can be made at continuous speeds up to about 22 knots with a low fuel burn because of the high propulsive efficiency in a suitable hull

.

At the owner?s selected cruising ground twin Azipull azimuth thrusters would provide quiet cruising and precise control when manoeuvring

.

They could also be integrated in a dynamic positioning system to hold the yacht in position without anchoring, avoiding damage from anchors in areas with delicate ecosystems

.

More twin

propeller

technology

Schottel has also been busy developing its twin

propeller

technology

and in September succeeded in winning an order from the Finnish shipyard Uudenkaupungin Tyovene Oy in Uusikaupunki for the propulsion systems of two double-ended ferries

.

These 86

.

2m long ferries will be fitted with two Schottel type STP 550 Twin Propellers, and are to go into operation in the Vaxholm area on the east coast of Sweden in the beginning of 2003

.

The STP 550s have a designed power of 602 kW, and are classed to Finnish/Swedish Ice Class 1 B

.

The contract includes an option for equipping two additional ferries

.

The company has also devloped a new version of the Twin Propeller especially for river cruise ships

.

The first of four luxury cabin-class vessels built for the Seetours shipping company, and fitted with two type STP 550 Twin Propeller units, successfully completed its trials in spring 2002

.

Building on this success, Schottel decided to develop a new Twin Propeller type, which it has called the STP 440

.

The unit has been developed for use in shorter cabin-class vessels but it is also a high-performance, attractively-priced propulsion unit for yachts and catamarans, says Schottel

.

The STP 550 is a propulsion unit originally developed for sea-going ships with significantly higher power ratings, but is very well-suited for the Seetours vessels, whose length of 125m, beam of 14m, draught of 1

.

48m and displacement of 2,040m3 greatly exceed the dimensions of previous cabin-class vessels for river navigation

.

However, the unit is too large and heavy for smaller vessels and it is for this reason that the company decided to develop the new STP 440

.

The expertise gained in adapting the STP 550 has now been transferred to the development of the STP 440, which is designed for a continuous output of 783kW

.

A power output of up to 860kW can be accommodated for short periods, however

.

The STP 440 is equipped with gear sets providing particularly quiet operation for an input speed of 1,600 and 1,800 rev/min

.

To satisfy the requirements for operation in shallow waters, the front pull

propeller

and the aft push

propeller

have a diameter of 1,400mm

.

The clearance between the

propeller

s and the hull is 200mm

.

The distance between the power input shaft and the

propeller

shaft can vary between 1,700mm and 2,000mm, depending on the installation situation

.

The pneumatically-operated clutch is particularly advantageous when used in hotel ships

.

The clutch engagement is completely noiseless, gentle and jerk-free, which is of great importance for passenger comfort

.

The connected load of the compressor units installed on each STP 440 is only 1

.

5kW

.

The STP 440 is designed for installation very close to the engine, to which it is connected via an elastic shaft

.

This makes the overall length of the propulsion system very short, meaning that the engine room can be made smaller, and more space is available for cabins

.