High Speed Propulsion Systems
High Speed Propulsion Systems
High Speed Propulsion Systems
ABSTRACT
The paper presents an overview of current marine transmission designs suitable for high speed applications at
sea. The paper concentrates on diesel-mechanical installations with single in single out configurations. It was
originally presented at the FAST 2005 Conference in St Petersburg, Russia in June 2005.
INTRODUCTION
Recent developments towards increasingly higher speeds for ships and boats have lead to the adoption of
advanced propulsion and hull designs. These new systems and hull forms challenge existing marine transmission
designs.
1. Propulsion systems
35
30
25
Time [%]
20
15
10
0
10 20 30 40 50 60 70 80 90 100
Load [%]
This arrangement results in an engine room location close to the vessel centre. In an attempt to move the engine
further to the aft of the vessel U-Drive transmissions were introduced. These transmissions again feature parallel
in- and output shafts but both in- and output are located on the same side of the gearbox, facing the aft of the
vessel.
However U-Drive installations still require rather large engine room layouts due to the fact that the cardan shaft
can not be tilted above a certain limit. So in order to shorten the distance between engine and gearbox V-drive
transmissions are nowadays widely used in smaller high speed vessels. Having an angle of up to 14 between the
horizontal input shaft and the output shaft axis these transmissions considerably reduce the required length of the
shaft connecting engine and gearbox input. This will also reduce the angle in the cardan shaft which prolongs its
lifetime and reduces vibrations. Another advantage of such installation is the fact that the engine can be installed
horizontally.
An angular arrangement can also enhance the installation situation using a flanged gearbox on the engine. In this
case too the engine can be leveled using the gearbox down angle to meet the propeller shaft inclination.
All these installations can feature an integrated thrust bearing in the gearbox or alternatively separately in the
shaft line.
Especially for high speed vessels the ship speed in engine idle speed, even with only one engine engaged, often
exceeds a speed of five or six knots. This is considered to fast for maneuvering in tight marinas, long channel
systems or when boarding or recovering other vessels. So instead of continuously engaging and disengaging the
engines a gearbox equipped with a trolling system can meet this operational need.
ZF Autotroll for example is an electronically controlled trolling system that provides a monitored slipping of
the input shaft clutch. This system allows a controlled operation between approx. 20% and 80% of the fully
engaged propeller speed. For reversing the full engagement pressure will be applied to assure a fast and reliable
reversing even in trolling operation. Due to the increased heat build up in the clutch trolling operation is limited
to approx. 40-50% of the full engine speed.
Operation at slower or medium speed is typical for navy, police and coast guard applications. Most engines are
optimized for operation at approx. 85% of their nominal power. The efficiency as well as the lifetime of the
engine will be reduced if operated for long periods of time in at only 40% load. One way of dealing with this is
to use only one engine in a two shaft installation. Another way, affecting the gearbox design, is the use of a
power take in (PTI) on the gearbox. In this way the slow speed patrol can be performed using a small electric or
hydraulic motor powered by a generator set and leaving the main engines disengaged. The PTI will provide an
additional reduction ratio to meet the input speed requirement for the smaller engine.
hub of the Propeller. There are generally two ways of getting this accomplished. The oil distribution box for the
hydraulic system can be located in the shaft line behind the gearbox, this does not effect the transmission. The
other, more common option is an OD-Box directly connected to the fore end of the gearbox output shaft. This
requires a connection face on the shaft and on the housing. This usually prohibits a flanged connection of
gearbox and engine. (Fig. 06)
To provide the hydraulic system for the CPP with oil the oil pumps are usually connected to a power take off
(PTO) on the gearbox.
Fig. 06: Reduction transmission with Oil distribution box and CPP
Since CPP do not need a reversion of the shaft rotation to change from ahead to astern direction, the gearboxes in
such installation do not need to be reversing gearboxes. However depending on the type of vessel and engine, the
gearbox still may need to be a three shaft transmission. For a single screw vessel a two shaft transmission is
sufficient, in that case the output rotation is counter engine wise. If installed in a two shaft vessel, the propellers
need to be counter rotating. If the main engines can be supplied with opposing rotation directions, then two shaft
transmissions are still sufficient. If the engines only have one rotation direction, the gearboxes need to be two
shaft (counter engine wise) and three shaft transmissions (engine wise).
In larger commercial vessels CPP installations do not even have a clutch for engagement, in those vessels the
propeller will be set to zero pitch for engine startup.
Due to the fact that the hub of a CPP is larger than that of a FPP, the propeller outer diameter is usually larger
and it therefore runs at slower speeds. For the gearboxes this results in higher ratios.
But also the vertical position is important to keep the engine as low in the vessel as possible. This is important
for the stability of the vessel but will also assure that the engine room height can be reduced. To achieve a low
engine room profile, the traditional vertical offset with input above output is changed. In many cases gearboxes
with horizontal offsets are used. In some cases however the input shaft is placed below the output shaft as seen in
the center shaft line in (Fig 09). Due to the limited design space in catamarans, flexibility to follow the engine
room design is required.
To steer the water jets a hydraulic system is used. The oil supply is usually assured via a hydraulic pump
connected to a PTO on the gearbox.
In most cases the thrust bearing is included in the Water jet system, however in some cases the thrust bearing is
not included and must be provided in the transmission housing
Modern high speed propulsion systems and their impact on marine transmission design
Fig. 10: Italian customs interceptor with ZF Trimax Surface drive system
Since most vessels using surface piercing propellers are very fast, the speed at engines idle speed is usually much
to high for maneuvering in tight areas. Therefore these boats are quite often equipped with a trolling system.
Trimmable surface drives require some experience to operate. The tendency of a vessel during acceleration to lift
the front of the hull will automatically force the propellers deeper into the water. In this situation the operator
needs to lift the propellers out of the water with the hydraulic trim system to run the propellers in their optimum
position. If the propellers are submerged below the waterline, they will considerably increase the load on engine
and shaft line. The torque in that situation can easily exceed 130% of the nominal torque. This peak torque can
of course cause problems for the gearbox too. This is especially true if the gearbox is operated in trolling mode.
For this reason the ZF Autotroll system can be calibrated in a way to protect the gearbox from any damage
caused by submerged surface propellers.
For the transmission the requirements are similar to those for FPP. Reversing reduction gearboxes are required as
well as often a trolling system. Due to the size and speed of the vessels a light weight and compact design,
usually using aluminum housings is demanded. For extra performance during the acceleration a gearbox with
two ratios can be provided. This gearbox will start with a higher ratio, reducing the load on the engine. Once
planing conditions have been achieved, the gearbox will change to the second gear ratio allowing for full top
speed. ZF marine two speed gearboxes are designed for automatic power shifting from first to second gear
without disconnecting propeller and engine.
Modern high speed propulsion systems and their impact on marine transmission design
2. Hull forms
However in mono hull vessels all of the variations described in paragraph 1.1.1. are frequently encountered.
Additionally the requirement for a low helicopter or working deck may lead to a coaxial or horizontal offset
transmission for some government applications.
Modern high speed propulsion systems and their impact on marine transmission design
2.2. Catamaran
Many fast ferries are commonly built as Catamarans. These vessels operate in speeds between 25 and 45 knots.
The size varies from approx. 25 meters up to well above 100 meters. For two engine installations, water jets as
well as propellers are used. For multi engine installations however, water jets are commonly used. While in
mono hulls the length of the engine room is often crucial, for catamaran hulls it is the width of the machinery
that needs to be observed closely.
In both cases there may still be a requirement for a gearbox. Since the propeller shaft is located in the center of
the hull, the gearbox needs to connect the output shaft of the engine with the propeller shaft. This results in a
coaxial or step up (input below output shaft) installation.
Fig. 17: Step up gearbox ZF 53000NR2B with input (front) below output shaft (rear)
The step up version is used for diesel-mechanical propulsion with the diesel engines crank shaft usually below
the propeller shaft centre line.
2.5. Hydrofoil
There are two typical types of hydrofoil vessels, mono hull vessels and catamarans with t-foils.
Both types have similar requirements as other mono hull or catamaran vessels. However due to the elevation of
the hull, V-drive and Down-angle gearboxes are often preferred for these vessels.
Modern high speed propulsion systems and their impact on marine transmission design
CONCLUSION
Modern propulsion systems and the requirements for improved speeds of todays vessels lead to a variety of
installation requirements. Facing these is a continued task for every designer of marine transmissions. At the
same time navy and other government applications are being conducted under higher commercial pressure.
Together with the increased demands from classification societies the transmissions have to fulfill a number of
tasks.
To meet these market demands, a gearbox manufacturer has to follow closely all new developments and has to
continuously implement new features and strategies into its products.
The good price / benefit ratio of todays fast craft transmissions assure the effectiveness and reliability of modern
high speed vessel designs.
In the future we will see new propulsion systems and hull designs that are currently under development. Low
wash designs, new tactical requirements for patrol vessels, reduction of urban automotive traffic, high speed
transport of goods are just some examples for the future. These will impose new demands for everyone within
the industry. Facing these challenges will be the task for all of us.
Modern high speed propulsion systems and their impact on marine transmission design
ACKNOWLEDGMENTS