checklist_no

1/2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs (ASD)

TABLE OF CONTENTS

GENERAL
MAIN DIMENSIONS

Main Particulars........................................................................................................... 1

Main Dimension Ratios .............................................................................................. 2

Main Particulars Estimate.......................................................................................... 2

HULL FORM

Stern Lines ................................................................................................................... 2

Hard Chine................................................................................................................... 3

Round Bilge.................................................................................................................. 3

Transom........................................................................................................................ 3

SKEG

Skeg Size ..................................................................................................................... 4

WEIGHT, HYDROSTATIC, STABILITY

Metacentric Height, Stability...................................................................................... 5

10.

Weight........................................................................................................................... 5

11.

Trim and Draft.............................................................................................................. 5

HULL STRENGTH

12.

Mounting Adapters...................................................................................................... 5

13.

Navigation Mast........................................................................................................... 6

14.

Shaft Bearing Support................................................................................................ 6

15.

Bulwark......................................................................................................................... 6

16.

Skeg.............................................................................................................................. 7

PROPULSOR INSTALLATION

17.

Propulsor Installation Alternatives............................................................................ 7

18.

Distance between the Propulsor Units .................................................................... 9

19.

Propulsor Tilting and Heeling..................................................................................10

INTERMEDIATE SHAFTS

20.

Shaft Arrangement....................................................................................................10

21.

Shaft Angle.................................................................................................................14

22.

Cardan Joint Phasing...............................................................................................14

GENERAL LAYOUT

23.

Forecastle...................................................................................................................15

24.

Aft Deck ......................................................................................................................15

25.

Towing Hook / Aft Towing Winch............................................................................16

26.

Superstructure...........................................................................................................16

27.

Wheelhouse...............................................................................................................16

28.

Control Layout ...........................................................................................................17

PROPULSOR ROOM SPACE

29.

Propulsor Room Space............................................................................................18

MAIN ENGINE

30.

Main Engine Choice ................................................................................................18

ESCORT TUGS

31.

Special Requirements .............................................................................................18

EXTERNAL CONNECTIONS

32.

Electricity....................................................................................................................19

33.

Cabling........................................................................................................................19

34.

External Tanks...........................................................................................................19

35.

Cooling........................................................................................................................19

1/2001

March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs (ASD)

GENERAL

This “checklist” is compiled in order to enable designers with no or little

experience in azimuth propulsion vessels to make significantly better

preliminary designs and proposals for stern drive tugs. The list can also

well serve the experienced designer as a checklist or reminder that

important aspects have been considered in the design.

The list is based on a the experience of supporting naval architects,

shipyards and owners with comments, hints and suggestions on how to

improve their designs in order to optimise the available performance of

vessels equipped with azimuth propulsion. When compiling the list we

have also used direct input from naval architects on what is special on

vessels with azimuth propulsion and what should be kept in mind when

designing these kinds of vessels.

The list is in no way exhaustive and does not include every important

aspect to a good stern drive design. It is more to be seen as a reminder to

details often not known, forgotten or ignored.

The advice and details in the list should not be taken as requirements, nor

can a tug be designed solely relying on the items in this list – the real

work, and end result is still up to the naval architect designing the vessel

as is the full responsibility. Steerprop Ltd. cannot be held responsible for

any possible negative influence on any design based on the proposals in

this list.

MAIN DIMENSIONS
1.

Main Particulars

Stern drive tugs intended for harbour duty and ship handling should be

dimensioned according to the local requirements, assisted vessel size,

environment (wind, current, tide…), type of jetties, manoeuvring space

etc.

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Typical maximum length of a harbour/ship-handling tug is approximately

33 m (LOA). Correspondingly, maximum bollard pull that normally

reasonably can be accommodated is 60 …65 tons. Nevertheless, some

recent designs indicate even much higher power/length ratio.

Main Dimension Ratios

Typical main dimension ratios for stern drive tugs are

L/B = 2.7 …3.0

B/T = 2.3 …2.6

CB = 0.48 …0.52

Main Particular Estimate

Tug designs are normally based on a certain bollard pull requirement, from

which the necessary engine power can be determined. Once this is

assessed the correct azimuth propulsor and propeller diameter can be

chosen. The required minimum draft of the tug can then be set to 1.6

…1.75 x propeller diameter.

For the first estimate of main particulars the lightweight of the tug can be

approximated to 200 …250 kg/m

(LBH). Ice class and other special

requirements are not accounted for and should be considered separately.

Dead weight capacity is depending on vessel purpose and may vary from

less than 60 ton to more than 400 tons for “same size” tugs.

Check space and height requirements for the main components, propulsor

and main engines to determine required hull height.

HULL FORM
4.

Stern Lines

Stern drive tugs should be designed with a “buttock flow” or - also called -

“pram-type” stern, where the water inflow to the propellers is mainly

along the buttocks, not from the sides. The angle between the baseline

and the buttocks in the stern should be kept as small as possible. A good

rule-of-thumb for maximum recommended angle is 13º + 1º for each

meter of immersion (hull draft). Thus, on ASD tugs the angle should

normally be kept less than 17º …17.5º (fig. 1).

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Fig.1.

Profile of a buttock flow stern. Maximum recommended angle a is 13º + 1º for each meter

of draft

Larger angles will cause the water flow to separate as well as water inflow

from the sides, which is prone to decrease propeller performance

drastically.

The stern profile need not to be S-formed, as there is no need to, or

benefit in straightening the buttocks to be almost horizontal in way of the

propulsors.

The stern should have a slight V-angle all the way to the transom. There is

no need for flattening the area in way of the propulsors. A V-angle – even

a slight one – will reduce the risk for stern slamming in waves.

The propulsors may protrude below base line, as on some of the finest tug

designs they do. The skeg dimensions ought to match.

Hard Chine

Hard chine designs are possible, but only double chine type is

recommended. Alignment of the chine needs attention. Flow separation

may occur where flow-lines cross the chine. This increases the resistance

and deteriorates the operating conditions for the propeller.

The double chine should extend all the way to the transom.

Round Bilge

From hydrodynamics point of view the best bilge form is a round bilge with

the radius growing towards the stern. A hull form with a narrowing stern is

also advantageous.

Transom

The transom should be designed with as little immersion as possible, as

the water “trapped” behind the transom causes a large increase in

resistance.

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

An immersed transom should be “cut off” at approximately 45º to the

waterline in way of the waterline in order to improve astern performance,

fig. 2. A cut off will have a substantial influence on both astern speed and

astern manoeuvrability. The propulsor should not be installed too close to

the transom in order to avoid ventilation of the propeller while going

astern or braking.

Fig. 2. A transom “cut-off”, as shown above, will improve the astern performance substantially

SKEG
8.

Skeg Size

The skeg can normally be very small without losing the directional

stability. A short skeg will make the tug more manoeuvrable and will

improve astern running course keeping and performance. In no case

should the skeg run all the way to the propulsors, when turned for astern

sailing, fig. 3. A too long skeg will also make sidestepping difficult as the

water flow from the propulsor is re-directed by skeg.

Fig. 3. Maximum skeg length shown in the left figure. The right figure shows a too long skeg.

The draft of the skeg should preferably be large enough to go below the

azimuth propulsors by 100…300mm. However, dry-docking procedures

should be taken into account, when the propulsion units are protruding

below the bottom of the tug hull.

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

WEIGHT, HYDROSTATICS, STABILITY
9.

Metacentric Height, Stability

Typical metacentric height for a stern drive tug is approximately 1.5 m.

Tugs with less initial stability need to have their stability checked carefully

and may still end up with stability problems.

The stability for the specified bollard pull needs to be checked at an early

stage, especially if the design is rather narrow. The stability requirement is

not only for a static situation, as – in normal towing conditions – both the

tug and the assisted vessel are moving, the assisted vessel dragging the

tug along. Especially important this is if the tug is assisting vessels at

higher speed or used for escort towing.

10.

Weight

The weight of the azimuth propulsors may come as a surprise and often it

will be difficult to get a proper balance, if not accounted for in the first

basic design. The units are way back in an area with little or no hull

volume. Hence, the centre of gravity for the rest of the tug needs to be

kept more forward than on other tug types.

11.

Trim and Draft

At least the trim of a tug should be possible to be altered to improve the

performance and prevent ventilation of the propellers in braking astern

running. For ice going tugs the possibility to trim the vessel fast is

important, as is the possibility to increase the draft for ice navigation.

HULL STRENGTH
12.

Mounting Adapters

Minimum recommended mounting adapter height is 300 …500 mm at the

lowest point, depending on size and form, fig. 4. Minimum height is

determined by strength requirements and installation procedure.

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

min. 300 mm

Mounting adapter

Fig. 4.

A drawing showing the mounting adapter and its minimum height.

The hull in way of the azimuth propulsors may require extra strength to

accomplish the correct sequence of damage, i.e. the propulsor should

break before tearing off the bottom.

13.

Navigation Mast

The mast needs to be stiff enough and possibly supported to reduce

excessive vibrations, especially in fast manoeuvres at high speed.

14.

Shaft Bearing Supports

The shaft bearing supports need to be stiff and strong enough to take the

load of the rotating shaft.

15.

Bulwark

The bulwark should be inclined inwards to prevent the bulwark from

touching assisted vessels. Ideally the bulwark should not be as far to the

sides as possible, but some tens of centimetres inwards to enable easy

stepping onboard without having to jump the bulwark first.

The bulwark around aft deck need to be strong enough to take the whole

weight of the tow wire without shearing, if the tug is equipped with an aft

winch and intended for towing over the stern.

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

16.

Skeg

The skeg need to be supported inside the hull, not just welded to the

bottom plate. A “soft” skeg may cause severe vibration throughout the

tug, not only to the skeg itself. The vessel is also supported by the skeg

during dry-docking and it is the first part to hit the bottom in case of

grounding, thus protecting the propeller nozzles.

PROPULSOR INSTALLATION
17.

Propulsor Installation Alternatives

The Steerprop propulsors may be installed in several different

configurations and variations thereof. Additional, customized installation

configurations can also be arranged. The main installation modes are:

•

Weld-in installation

•

Small bolt-in mounting cone

•

Large mounting adapter for thru-hull mounting

Weld-in Installation

The weld in installation is the strongest installation into the hull and the

propulsor is an integrated part of the hull structure. The propulsor is

usually installed in two parts – the upper part, that is welded into the hull,

from above and the lower part from below the hull.

Fig. 5.

Weld-in installation: 1) the upper part is installed from above; 2) the upper part is welded at

the hull bottom and at the top flange, the lower part is brought into place from below the

hull; 3) the lower part is bolted to the upper part

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Small Bolt-in Mounting Cone

When the propulsor is installed using the small mounting cone there is no

need to compromise hull strength around the propulsors, as the needed

openings in the hull can be kept to a minimum. Mounting cone installation

allows the propulsor to be installed in one piece from below or in two

pieces, upper gearbox from above and lower gearbox from below.

Fig. 6.

Small bolt-in mounting cone installation – in one piece: 1) the propulsor is brought below

the hull; 2) the propulsor is turned in way of the trunk and lifted into position; 3) the

propulsor is bolted into the trunk and the clutch assembly is installed

Fig. 7.

Small bolt-in mounting cone installation – in two parts: 1) the upper part is installed from

above, the lower part is brought into place from below the hull; 2) the upper part is bolted

into the hull; 3) the lower part is lifted into place and bolted to the upper part

Large Mounting Adapter

The use of a large mounting adapter allows the complete propulsor to be

lifted onboard in one piece. Often it is possible to do so even with the

vessel in water. This feature also enables the propulsor to be lifted off the

vessel for repairs and maintenance without dry-docking the vessel. See

fig. 8.

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Fig. 8.

Large bolt-in mounting adapter installation: 1) the propulsor is lifted onboard; 2) the

propulsor is bolted to the trunk flange; 3) the deck hatch is bolted or welded into place

above the propulsor

18.

Distance between the Propulsor Units

In order to improve the overall performance, especially the

manoeuvrability of the tug, the unit should be installed as far from each

other as possible. Minimum recommended distance between the units is

the unit turning diameter + 500 mm.

It is also recommended that the distance from the vessel side to the unit

centre is more than ½ unit turning diameter + 500 mm, in order to

prevent the units from touching jetties or assisted vessels even when

heeling.

min.

500 mm

min.

500 mm

Fig. 9.

Minimum distance between the units as well as between the side and the units are 500 mm

The tug beam should be designed to allow the required units to be

installed accordingly.

1/2001

March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

19.

Propulsor Tilting and Heeling

The propulsor units need not to be installed vertically, but can be both

tilted (longitudinally) and heeled (laterally) in order to achieve some

benefits. Typically the units are tilted, up to 3º …5º in order to decrease

the cardan shaft angles on the intermediate shaft. To achieve same

lifetime expectancy / maintenance interval for all the cardan bearings the

prime mover should be tilted correspondingly. Note that some main engine

manufacturers have strict maximum angles for engine tilt, thus also

restricting the propulsor tilt angle.

A tilt of 2º …3º is recommended to improve the hydrodynamic efficiency of

the installation.

If the main engines need to be installed close to each other, the distance

between the units can be maximized (and performance improved) by a)

using oblique intermediate shaft angles – note maximum cardan shaft

angles! – and/or b) by heeling the units outwards. Maximum heel angle is

to be determined case by case.

tilt 3

heel 5

Fig. 10. The units can be installed either tilted or heeled, or both. A tilt of 2º …3º is recommended to

optimise the water inflow to the propeller.

INTERMEDIATE SHAFTS
20.

Shaft Arrangements

The power train between the prime mover and the propulsor is by an

intermediate shaft. Depending on application and installation there are

several different possibilities for the intermediate shaft layout. In the

shortest case the shaft is only a tooth coupling and a flexible coupling,

between the engine flywheel and the propulsor input flange.

In order to prevent the forces from the propulsor and the shafts to

damage the prime mover a flexible coupling has always to be installed

between the engine flywheel and the intermediate shaft package.

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Tooth Coupling and Flexible Coupling

This is the shortest possible intermediate shaft installation, where the

shaft is a mere tooth coupling connected to the propulsor flange at one

end and to the flexible coupling on the prime mover flywheel.

Fig. 11. Using only a tooth coupling and a flexible coupling results in the shortest possible

intermediate shaft line arrangement

Stub Shaft

The next shortest intermediate shaft is a short, so called stub shaft. The

shaft should be equipped with a pair of bearings to take the gravity load of

the shaft and thus the shortest possible length is one that provides for

space for the bearings. The stub shaft is usually fixed by flanges to the

propulsor input flange and the flexible coupling on the prime mover

flywheel.

Fig. 12. The stub shaft arrangement is another very compact installation method.

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Centalink

If a short shaft without bearings is preferred a possibility is to use a so-

called flexible shaft ,e.g. Centalink, where the shaft (normally of tube

type) is integrated to torsionally stiff flexible elements at each end of the

shaft.

Carbon Fibre Shaft

A novel solution for straight shaft installations is to use a large diameter

hollow carbon fibre shaft, which requires no bearings. The use of a carbon

fibre shaft is feasible in stern drive tugs, where a conventional straight

shaft otherwise would be used. The carbon fibre shaft will save weight and

installation work compared to a conventional shaft with bearings and

bearing foundations. The carbon fibre shaft is using the same flexible

elements as in the Centalink solution above. Maximum length of a single

piece shaft is approx. 8 m.

Fig. 13. The “flexible” shaft line arrangement – with no bearings – enables a fast and easy

installation of the intermediate shaft.

Single Cardan Shaft

If the engine is installed close to the same level as the propulsor and

rather close to each other a single cardan shaft can be used as the

intermediate shaft. In order not to damage the engine a stub shaft is

required, at least for power ratings above, say 600 kW. The stub shaft

requires a pair of bearings capable of taking axial forces and to take the

shaft weight. Note that the cardan shaft cannot be used with very small

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

angles, a minimum of 1º is normally required to ensure that the joint

bearings are moving.

Fig. 14. Sometimes it is feasible to use the single cardan shaft solution. Also this installation will

require a stub shaft and flexible coupling.

Typical ASD Intermediate Shaft

The typical intermediate shaft arrangement on a stern drive tug comprises

cardan shafts and a rather long straight shaft. Also this arrangement will

require a stub shaft at the prime mover end as well as a flexible coupling

to save the prime mover from damage.

Fig. 15. The typical intermediate shaft on stern drive tugs comprise two cardan shafts with a long

straight shaft in between. At the prime mover end a stub shaft and a flexible coupling wil be

required

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

21.

Shaft Angle

The height difference between prime mover flywheel and azimuth

propulsor input flange should be kept as small as possible. Maximum

allowed height is dependent on distance between the prime mover and

cardan shafts chosen.

Maximum usable angle is approximately 7.5º per joint, due to cardan shaft

rpm, bearing lifetime, and vibration. Use of the maximum angle has to be

separately checked. In order to avoid later problems, maximum

recommended angle is 6 ...6.5º per joint.

max. 15

Fig. 16 The angle of the intermediate shaft should be minimized - maximum recommended angle is

usually 15º

22.

Cardan Joint Phasing.

In order to improve the performance and lifetime of the cardans as well as

to reduce the risk for vibration problems on the intermediate shaft, the

cardan shaft joints at each end of the intermediate shaft should be in the

same phase for an odd number of shaft bearings and in different phase for

an even number of bearings.

Fig. 17 Cardan shaft phasing. Above with same phase for intermediate shaft with odd number of

bearings. Below with different phasing for intermediate shafts with even number of

bearings.

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

GENERAL LAYOUT
23.

Forecastle

Stern drive tugs for harbour use should be designed without a forecastle.

The normal direction of work is “over the bow” and thus the main towing

point and winch are installed on the foredeck. A forecastle will bring the

towing point considerably higher deteriorating the stability. Also on coastal

tugs the forecastle should be kept as low as possible, preferably only as

half height forecastle.

Fig. 18 Stern drive tugs for harbour and coastal duty should have the foredeck as low as possible,

as most of the normal operation is “over the bow”. A high forecastle causes a high heeling

moment due to the towing point position

24.

Aft Deck

The stern “corners” should be rounded with a large radius, enabling

turning against a ships hull during ship handling operations. The fendering

should be smoothly continuing without sharp corners between the bow and

side fendering and the stern and side fendering, as the ASD tug is able to

push with either bow, stern or sides against the assisted vessel.

Fig. 19 A good stern deck has large-radius rounded corners

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March 2001

DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

25.

Towing Hook / Aft Towing Winch

If the tug is equipped with an aft towing point / hook it should be

positioned as far forward as possible on the main deck and have to be in

front of the propulsion units, in order to improve/enable manoeuvring

while working over the stern.

26.

Superstructure

The superstructure should be designed as narrow as possible, with a small

wheelhouse far from the sides to enable the operation under the flair ,

bow or stern of large vessels e.g. container vessels, without the risk for

damage on the tug or the assisted vessel. Also smoke stacks and fire

monitors should be as close to the centreline as practically possible.

Fig. 20 This sketch shows the need for narrow and central superstructure and wheelhouse,

especially for the handling of vessels with high flare.

27.

Wheelhouse

The wheelhouse should be designed as small and compact as practicable,

in order to ensure the efficient use of a single control position layout. The

tug master should have as good view as possible from the steering

position – minimum requirement is that the bow fender as well as the aft

corners are visible as well as the bow winch and aft towing hook or winch .

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Fig. 21 The important lines of sight from the wheelhouse are to the bow fender and stern corners,

ideally to the winch and as much of aft deck as possible. Visibility upwards is also important

for tugs assisting high freeboard / high sheer vessels (see fig. 7).

Fig. 22 An example sketch of a single steering position wheelhouse layout, with “walk -through”

control layout. The helmsman’s chair is installed on tracks to enable it to be moved out of

the way.

28.

Control Layout

The azimuth propulsor controls should be positioned in a way to enable the

helmsman to easily concentrate on the tug operations, not on how to

handle his tug. The optimum solution is to place the control cabinets on

each side of the steering position. The distance between the cabinets

should allow the (possible) helmsman’s chair to fit between them, but the

distance between the propulsor controls should be kept between 55 cm

and 65 cm.

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

55 ...65 cm

Fig. 23 A typical single steering position layout with a “walk-through” control layout. The distance

between the azimuth propulsor controls should be 55-65 cm.

PROPULSOR ROOM SPACE
29.

Propulsor Room Space

There should be enough internal height in the propulsor room to allow for

installation as well as maintenance of the units. Recommended is a height

of 200 …300 mm above the units. A separate bolted-on maintenance hatch

can be installed above the unit if enough height is otherwise not available.

The Steerprop Azimuth Propulsors are designed to be as short as possible,

i.e. the internal height requirement is minimised and thus the units are

suitable also for low deck heights.

MAIN ENGINE
30.

Main Engine Choice

The propeller and main engine should be chosen together in order to

ensure that there is enough of torque available even in negative water

inflow to the propellers. If not, there is risk for engine overload and

stalling in a critical phase of ship handling.

ESCORT TUGS
31.

Special Requirements

In order to improve the escort capability of a “standard” stern drive tug

design. The main requirement is stability. A minimum metacentric height

of 3 m is recommended.

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DESIGNERS’ CHECKLIST Nº 1

Azimuth Stern Drive Tugs

Maximum obtained towline in indirect arrest is dependent on the positions

of the azimuth propulsor, the tow point and the centre of effort of the

underwater part of the hull. For more information please contact Steerprop

office in Rauma.

EXTERNAL CONNECTIONS
32.

Electricity

The Steerprop propulsors require an external feed of electricity for the

control, for the display and emergency steering as well as for the alarms.

The system is of 24 V DC type and the electricity is required to the control

cabinets, normally situated in the engine room.

The electricity demand is approx. 300 W for the primary steering, approx

100 W for the emergency steering and display and approx. 50 W for the

alarms.

The electricity feed is required to be continuous with a battery backup.

33.

Cabling

The controls will require three cables for each propulsor to be drawn

between the wheelhouse control stand and the engine room / propulsor

room. There is one cable for the main control, one for the emergency

control and display and on cable for the alarms.

34.

External Tanks

The Steerprop propulsors require a shaft seal tank of approx. 20 litre

capacity for each propulsor to be installed in the propulsor room. For units

with hydraulic steering (normal for stern drive tugs) a hydraulic unit -

comprising required tanks (capacity 60 …100 litres), filters, valves and

coolers – need to be installed close to each propulsor . The unit is

delivered as a part of the Steerprop delivery. The hydraulic unit need to be

connected to the propulsor with 6 pipes between the propulsor and the

hydraulic unit and 5 pipes between the hydraulic unit and the propulsor.

35.

Cooling

The Steerprop units are equipped with coolers that need to be connected

to a cooling water system (fresh or sea water) the water capacity need is

100 …200 litres/min, depending on propulsor size and power. These values

are for an input water temperature of +32º C.