Main propulsion by thruster concept

Main propulsion by thruster

_concept

The azirnuthing thruster unit has for some

time now been used to propel small, low powered vessels such as barges. ferries, etc, while it has made a significant impact

in the tug propulsion market. However. according to the authors of a recent paper, extracted from here, this type of propulsion system offers many advantages for ships n

the future, where the cargo carrying capacity

of a given hull size is to be maximised But. to introduce thrusters as the main propul-sion of vessels of bulk or container ship

type is a technical novelty, but is feasible.

For any normal self propelled ship the followirq functions are essential;

Propulsion system which provides the necessary thrust to give the vessel in ques-tion a specified speed and in some cases.

the static thrust is of importance. The normal answer to this function is a conven-tional marine propeller with shaftline, stern tube with seals, line shaft bearings and reduction gearbox (in the case of medium or high speed diesels>. Remote control of the machinery from the bridge s normally

provided as well.

A steering system which provides control of the ship's steering and course keeping at ship speeds at which the rudder is efficient. The standard solution is. of course, a

con-ventional rudder with rudder post and bearings and steering gear with auxiliaries. In the wheelhouse is located a means of rudder control (wheel, lever or similar>, with a control link to the steering gear system.

An interface

of an

autopilot system is

normally arranged.

A manoeuvring system for safe and rapid ship's control in confined waters (harbour, canal, etc) where the combination of the propulsion and steering system is

in-adequate. To achieve better control in this

respect, transverse (tunnel) thrusters can be

Thruster and Combined Propeller-Thruster Propulsion, Two Concepts for the Future' by O. Bjorheden and L. Larberg. KaMeWa AB. Presented at the Tenth International Marine Propulsion Conference. London.

March 1988. 30 Diagram showing the thruster propulsion unit location.

nstalled at the bow and in some cases at

the stern. Such a thruster system includes a tunnel thruster with a drive system (normally

an electric motor), auxiliaries and remote

controls.

During the lifetime

of the ship these

various systems must be serviced and repaired. lt must be borne in mind that the systems above are supplied by up to three or four different makers, with a correspond-ing number of service contacts to be made in case of problems. In comparison with the conventional systems described above. the rotatable thruster concept offers a unique

package.

A new propulsion system should/must

have approximately the same efficiency as a

conventional one. In the case of thruster propulsion the position of the unit in the stern is normally further aft compared with a conventional propeller. In general this allows for the adoption of a relatively larger propeller diameter. Furthermore, the intro-duction of highly skewed propeller blade design allows for smaller tip clearance and

large propeller diameter. The above increase

in diameter will, in combination with an optimised shaft speed, increase the effi-ciency.

Two alternatives with regard to the installa-tion of the concept are available:

Cpp type thruster in combination with

skeg. 1I'1'fiI Machinery arrangement with a thruster propulsion system.

Traction thruster (cr' or fpp type>. The features of these two alternatives can be described as follows:

The reason the skeg is fitted s to create a

more efficient environment for the propeller (higher friction wake and higher hull effi-ciency). In addition. the skeg "shadows" the strut and gear housing of the thruster thus reducing the drag from these parts. The skeg can also be an important member for supporting the stern at drydocking. In this case the turning angle of the thruster is limited to 180° (aft sector) because of the skeg. As the thrust force cannot be directed into the forward sector, thrust reversal is

achieved by pitch reversal (cp propeller) or

by engine reversal.

The alternative traction thruster has no turning angle limitations and can for this

reason be either of the cpp or f pp type. The

main feature lies in the arrangement with the main stay and "bottom vane" down-stream of the propeller. These two com-ponents are given any asymmetric profile, with angle of attack and camber optimised to act as guide vanes which recover energy from the rotating slipstream. Substantial

efficiency improvements are achieved with

this arrangement.

To clarify the open water efficiency of the traction thruster described above compared with that of a conventional propeller-rudder arrangement. free-running tests were

con-ducted

n one of the KaMeWa marine

laboratory's cavitation tunnels with propeller models 395-B in three different

configura-tions.

Thruster propulsion offers great flexibility in the choice of propulsion machinery. The propeller unit includes one built-in bevel gear. An upper bevel gear may be added. The gear ratio of these gears can be selected to give a total speed reduction from engine to propeller ranging from 3.1 to 18.1 (depending on the torque situation). Because of this it _{is possible to select an}

optimum propeller shaft speed in

combina-tion with any practical rev/mm of the power

plant (diesel, electric motor, etc).

By choosing suitable

lengths of the

couplings/intermediate shafts between

engine, upper gearbox. and the propeller unit it is possible to locate the power plant at the most suitable position. both vertically and longitudinally. The upper gear can be arranged for a single. twin, or triple input.

When a cpp thruster is installed one can make full use of constant speed driven

auxiliaries connected to the engine or gearbox power take-off shaft.

Diesel engines are natural for most ships.

If, however, the vessel has high demand on electric power capacity (dredgers. crane vessels, maintenance/service vessels, f

lo-tels and similar), diesel-electric drive may be an attractive solution. The simplest and least expensive thruster drive is arranged through

simple ac non-synchronous motors with relatively high shaft speeds in combination with a cp type thruster. If space allows, the

motors can be directly coupled to the

thruster input shaft.

The thruster propulsion system is most suitable for remote control and monitoring (ie. unmanned engine room operation), as the number of control areas are minimised when compared with a conventional ship's

propulsion/manoeuvring system.

Advantages

The most striking advantages with the propulsion thruster installation regarding

hull design, production and hull disposition

are:

A simple stern, for instance the so-called

barge stern, can easily be adopted. This not

only reduces production costs but also allows the possibility of adopting a broader stern, gaining displacement and improving

stability.

The number of individual seatings and bed plates is diminished with a reduced number of main systems. The installation work for the whole propulsion/manoeuvr-ing system is reduced for the same reason.

as is the time for tuning and testirtg.

The engine room length can be remark-ably shortened. For a given overall ship's length this means thai the length of the payload section can be increased which means increased bale capacity, normally in the order of S to 10 per cent. In most cases the deadweight capacity increase is also of

Right.

Counter-rotating propel/er

thruster system.

Be/ow. Open water test resu/ts.

the same magnitude when compared with a

ship of conventional design.

The thruster unit with upper gearbox can be mounted in a "container" (circular or rectangular) which is flanged to a corres-ponding well in the stern. By this arrange-ment the thruster.can be installed at a late stage in the building program, even after

the hull has been launched.

In summary, a thruster-equipped vessel needs less engineering and a less qualified shipbuilding technique than a conventional ship. Production of such vessels can, there-fore. take place at yards with simpler

facili-ties. If builders of main engines and thrusters arrange for package delivery, including

engineering (possibly also containing some major direct-driven auxiliaries), this would mean additional simplification for the

build-ing yard.

Each potential ship project for which

pro-pulsion thrusters are being considered needs

to be studied for evaluation of the produc-tion costs and operaproduc-tional economy. lt is

important to take full advantage of Thruster propulsion and not to consider the

mach-inery arrangement alone. The investigations

performed by KaMeWa so far show that based on initial costs, the price of a thruster system is about the same as for a conven-tional system (a stern tunnel thruster has not been assumed n these studies). The

MANOEU VR/NG DEVICES

effect on hull production costs need, further

detailed studies. The present studies of operational economy show positive figures. The potential for more payload (cubic and

deadweight) is the most important plus

factor for the thruster concept.

Special tests

In co-operation with the Transatlantic Ship-ping Line, self-propulsion tests have been performed with the counter-rotating pro-peller-thruster system fitted on a model of the latest class of ACL ro/ro vessels (the "Atlantic Companion" class). The tests were

carried out in

the towing tank

of the Swedish Maritime Research Centre (SSPA).

The cr system was laid out with roughly the

same total power as that of the conventional

system, distributed with about 75 per cent on the forward and 25 per cent on the aft propeller. Moreover, the diameter of the forward propeller was chosen to be the same as that of existing single screw, and shaft speeds were chosen as optimal for all three propellers. For the 20 knot design condition, the SSPA trial prediction for the conventional as well as the CR system appears in Fig 1. As can be seen from the diagram. the gain in propulsive effciency with the CR system is about 10 per cent somewhat dependent on the distribution of power between forward and aft propellers.

I P P P P n (1 Propeller alone P p. p.

t

p.. I. n t n a,

Propeller plus rudder

(T _{- Rrudà} ) V er a net 2ir n O Traction thruster MAY/JUNE 1988 31

In addition to resistance and self-propulsion

tests. comparative measurements of steering

forces were also carried out with both

systems operating at different rudder angles and Ship speeds. The cr system requires

rudder angles which are 2.5 to 3 times

larger than those of the conventional rudder at a certain desired steering force. This is a quite logical result considering the fact that

the thruster rudder area in this case amounts

to only one third of that of the conventional rudder. Depending on the application and the need for high speed steering forces. larger rudder areas may. of course, be

considered for the cr system.

The cr thruster offers a maximum

flex-ibility _{n adapting to a wide range of prime}

movers. On large cargo vessels, where optimum fuel economy might be the primary concern, a medium speed diesel engine driving the thruster over an upper bevel gear is probably the most natural arrange-ment. This solution also implies a greatly improved operating safety and redundancy. with two independently driven propulsors

replacing an ordinary single screw

installa-tion.

On ships where regular operation with one or two propulsors is foreseen. ie not only in case of emergency. cpp's of the

featherable type seem rather attractive since

they allow proper loading and

_efficient utilisation of the engines installed in any possible operating condition. In addition. the drag of a disengaged propeller is

mini-m sed.

On smaller vessels such as. for example. cruise liners, electrically driven cr thrusters may imply advantages with regard to the

machinery arrangement. Dependingon how the electricity is produced (shaft alternator,

or separate diesel generators. or a mix), this

type of transmission may offer the same amoun of flexibility and redundancy as the pure diesel alternative. On many passenger vessels cruising at low speed seems to bea

Right. Comparison of steering forces

with conventional

and CR system on ACL ships.

Be/ow left. Model

arrangement for propulsion test with

ACL ship. Be/ow right. Fig. 1. Triai

prediction for conventional and

cr system on ACL ship.

frequent operating mode and for such

vessels running on one or two electrically

driven cr thrusters and feathered fore

propel-lers should come close to the ideal for

engine utilisation (reduced main engine wear) as well as manoeuvrability. noise and

vibration generation.

Up to now, the rotatable thruster has mainly found applications for propulsion and positioning of special vessels.. such as

offshore semi submersibles and service

ships. Based on the accumulated experience

with thrusters of moderate rating. it seems

logical to also make use of thrusters for propulsion of more conventional ships such as coasters and bulk/container vessels of small and medium size. Small ferries and passenger vessels are to a limited extent

Total paver

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already using thrusters, and thrusters with power suitable for medium size vessels are

now available as well.

The main advantages are improved manoeuvrability and flexibility, together with

a shortened engine room and hence

in-creased cargo space. Simplified control and a reduced number of systems are additional features.

The cr propeller-thruster system, je the

rotable thruster applied as a

counter-rotating propeller working behind a larger conventional propeller, is a practical way to recover energy from the rotating slipstream. The gain in total efficiency will be about 5to

10 per cent which for a vessel with medium to long range operation gives a substantial

saving in fuel costs.

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Vessels equipped with normal side thrusters

require only a manual control system but for some ships fitted with side and azimuth-ing thrusters, and indeed electrically driven main propellers as well, a dynamic position-ing system with computerised control is

essential. A very good example of lust how complicated some vessels can be is seen

with the pipelaying vessel LORELAY. owned

by Allseas Engineering BV. Holland. An important feature of this vessel is that it is

equipped with the Sami Megastar thruster control system developed by ASEA Ström-berg Drives. Finland. using variable speed

electric a.c.

rotors

driving fixed pitch.

propellers. A superior performance and considerable fuel savings is claimed for this system compared with a constant speed

c.p. propellers arrangement.

For normal pipelaying operations such a

vessel is held in position by several anchors

which are moved according to the progress of pipelaying. However, modern pipelaying equipment and trench diggers can move faster than anchoring systems allow, and to employ powerful anchor handling tugs is expensive and using anchors close to

existing pipelines is also potentially danger-ous. A solution to forget the anchors and rely on dynamic positioning is considered

ideal, but this requires several high powered

thrusters with short response time even though the bulk of work s still performed in fair weather when the average actual thrust is only about 20 per cent of the maximum

installed. With conventional controllable pitch thrusters this results in a heavy fuel

consumption.

A new solution was developed by StrOm-berg to use variable speed thruster drives to

reduce the fuel consumption considerably.

Stromberg's newly introduced Sami Mega-star frequency converter was found to meet all the requirements and uses conventional

a.c. cage induction motors to drive the thrusters. In practice this solution has also turned out to be superior to two-speed a.c.

motors and variable speed dc. motors.

Thrusters arrangement

The result is the eight thrustèrs arrangement aboard the LORE LAY. Two 3000 kW motors

are geared together to drive the single main propeller. Two 2550 kW and one 1000 kW unit comprise the fixed side thrusters. n

additin. one 2550 kW retractable azimuth-ing thruster and two 3000 kW azimuthazimuth-ing thrusters complete the propulsion system. Four 5730 kVA diesel generators provide

the primary power supply.

Sami Megastar frequency converters are provided for the eight propulsion motors with each converter located near to the

induction motor it controls. The total installed

power of these motors is 20 650 kW and it

is not necessary to start diesel generators to start up thrusters as ll can be connected to Just one or two generators without rotating and any of them can give full thrust within

seconds. The system ensures that the

optimum number of diesel generator sets is

running in all operation situations.

The Sami Megastar drives are controlled by a Selma marine thruster assignment

system which allows the ship to be controlled either manually, by auto pilot, or by automatic Dynamic Positioning System using a

Kongs-berg NMD class Ill type. The thruster assignment system is integrated in the total Selma system which provides control at exceptional response for the following:

Power management for three 3300 Volt

MANOEU VR/NG DEVICES

A.C. thruster drives for pipelayer

busbars with four main generators and one

harbour generator.

Ballast and bilge control including control

of valves and pumps. Standby pump control.

Temperature/viscosity controller

func-tions.

Alarms and monitoring of machinery and

piping.

Process stations in the Selma system carry Out dedicated automatic monitoring and control functions, of parameters such

as pumps and

thruster motors, engine

rev/mm, clutches, valves, etc.

Economic benefits

Such a system based on variable frequency control of a c. induction motors is said to greatly reduce running costs because of the higher overall electrical system efficiency. Other benefits claimed are there is better propeller efficiency using fixed pitch types

rather than c.p. propellers and there is more efficient use of the diesel engines as there is

no reactive power generation, soft starts of

motors, and immediate availability of

thrusters from zero speed.

According to the owners of LORELAY. a vessel of this type with constant speed c.p. propellers would normally burn 40 tonnes of fuel per day in fair weather, but actual consumption aboard this vessel has been below 24 tonnes/day or 40 per cent less.

Thus. it is claimed that the higher initial investment cost of the frequency converters

is _{more than compensated for}

_{by the}

cheaper propellers, smaller diesel engines and generators and of course the

consider-able fuel savings. For an 11 000 kW

installa-tion the annual fuel cost savings are

esti-mated as US $1 .14 million.

¿

Retracting thrusters for small craft

Twin engines are now the norm on most powerboats but, there is one area where doubling up for safety is not often consi-dered, and that is with the steering. lt could be argued that with a twin screw boat the engines can be used for steering, certainly enough to get the boat to safety. However. having two engines and two drive systems on a boat is an expensive installation and

now there

is a way to get many of the

benefits of a twin engine nstaflation on a single crew boat and still have an auxiliary

means of steering available.

Bow thrusters are used on some larger

and slower motor boats

to help with manoeuvring in harbour. These are usually of the tunnel type where a tube is fitted through the forefoot of the boat. Such bow thrusters are fine, but they can add quite a lot of resistance particularly to a planing boat, which is why they are not fitted very often to this type of craft. However, there is

a new type of thruster which not only

overcomes the resistance problem of the tunnel thruster, but can also be used as an

auxiliary means of propulsion.

Small retractable thruster

The type of thruster, which has been used very successfully for many years on ships and drilling rigs, can be retracted when not in use. A fairing piece is fitted below the thruster propeller which matches the shape

of the hull when the unit is fully retracted, so that there is little or no added resistance. The thruster unit is extended by means of

the hydraulic cylinder on which it is mounted

MAY/JUNE 1988

and the simplest type has the propeller in a fixed direction so that the thrust can be

directed

to one side

or the other to

manoeuvre the boat n the same way as the

tunnel thruster.

This type of thruster can add the expert touch to close quarters manoeuvring and it

also provides an emergency form of steering,

but by adding a bit more in the way of

hydraulics, the bottom

thruster can be

made to turn through 90 degrees around the vertical axis. This allows the thrust to be vectored in any desired direction, so that in addition to steering, there is a very viable. alternative means of propulsion. Even more

sophisticated is the type of retracting

thruster, where the thrust can be directed in any desired direction, so that steering and

propulsion are combined.

The attraction of a unit of this type would be on single engined boats where they not only provide emergency steering and pro-pulsion. but also a means of improving the handling of the boat in tight situations in harbour. The thruster is powered hydraulic-ally and so it needs a pump which has to be driven by an engirte. The simplest way is to link this pump to the main engine, but, of course, there will be no emergency

propul-sion. if the main engine fails.

The best solution is to drive the hydraulic pump from an auxiliary engine, perhaps an

auxiliary generator. The power requirements

for the auxiliary thruster could create the incentive to fit an auxiliary engine because. not only can it drive the auxiliary prbpulsion thruster, but can also be used to charge

Left. Artist's

impression of the

thruster both retracted and extended.

Right. The thruster unit is extended by means of a hydraulic cylinder.

batteries if the starting batteries are flat. lt

can provide electrical power in the harbour and it can also be used to drive a bilge

pump.

In a single engine installation having an auxiliary means of propulsion and steering

can be a comforting thought. A single

engine may be more efficient than two

engines, or t may create more space on board and it will certainly be less costly. By the time the generator and thruster have been added, there probably won't be much difference in the cost, but the result will be a much more flexible engine installation. As well as the improved manoeuvrability in tight corners, and with the hydraulic system available, it can also be used to power a windlass or perhaps the trim tabs. The main maker of retractable thrusters of this type is Richfield Marine Thrusters Ltd. which offers units up to 75 hp. whilst an Italian company. LCS. make units which are not retractable,

upto60hp.

¿ii

MANOEU VR/NG DEVICES

High lift rudders for British ferry

A recently developed device to enhance the manoeuvrability of vessels has now been

extensively tested aboard ship. The High Lift

Rudder, produced by Ulstein Trading Ltd

A.S., Norway. is the result of several years of

research leading to a rudder said to have a

considerably higher stall angle than conven-tional flap rudders.

The rudder is provided with a bulbous

leading edge, an active

flap and vane

elements. Tests at the Norwegian Marine

Research Institute have proven that the shape has reduced resistance compared to other high lift rudders. Because of the considerably higher stalling angle (where flow separation occurs) vessels fitted with the Ulstein rudder are claimed to be opera-ting more effectively and with improved economy. The rudders are individually

designed for each order, are delivered fully

assembled and can be supplied with a

detachable solepiece as an optional extra. Components are easily accessible and

materials with high resistance to wear are

used in areas where necessary.

The full range of High Lift Rudders is now designed and manufactured in Britain by

Ulstein (UK) Ltd. Dunfermline. The most

recently supplied rudders from this company

were a pair for a ferry being built by Richard

Dunstan. for Shetland Islands Council. Each

Two tugs on order from the J. L. Mosor

Shipyard. Yugoslavia. for Brodospas Salvage

and Towage Enterprise, are to be equipped with twin Compass thrusters manufactured by Liaaen Helix. Norway. Each of the type TCN 92/62-250 thrusters is powered by a diesel engine of 1450 kW at 900 rev/mm with the 2500 mm diameter propellers in a nozzle turning at 247 1ev/min and providing a bollard pull of 50 tonnes. Also supplied

for these vessels is a Helitron rerriote

control system which prevents overload of

Diagram showing how the bulbous

leading edge of the U/stein High Lift Rudder exposes flow separation.

rudder has a chord length of 1 200 mm. height of leading edge 1 700 mm. an area of 2 sq metres and designed to operate up

Compass thrusters for Yugoslav

_tugs

the main engines.

Another interesting Liaaen contract was for the Kloster Cruise liner being built by Wärtsila Turku Shipyard. The two main propellers are 4600 mm diameter Liaaen highly skewed c.p. units driven at 150 rev/mm by 5280 kW diesel engines. Also provided for this liner are two resiliently

mounted type T'PiN 73/56-200 tunnel

thrusters. Each is

driven by a

vertically

mounted electric motor of 1000 kW and

the propellers operate at 314 rev/mm in an

to 45 degrees each way, The steering gear is a Tenfjord unit supplied by Ulstein (UK)

Ltd.

inner tunnel of 1700 mm diameter. The inner tunnel which carries the thruster is mounted elastically into an outer tunnel which is welded to the ship's structure. Between the two tunnels

is an air filled

space which insulates the propeller induced

noise from the vessel. For this vessel all the

propellers, engines and electric motors are controlled by Liaaen Helitron electronic

re-mote control systems to protect the

engines/motors and optimise fuel con-sumption.

General arrangement of the Yugoslavian built tug which is to be fitted with twin Compass thrusters.

38