Section 2.5: Special Features Flashcards
Side Thrusters Advantages
a) Reduction in tug costs in most ports.
b) Reduction in berthing and un-berthing times.
c) Reduction in side shell damage.
d) Less wear on mooring ropes.
Side Thrusters
A greater number of ships today are fitted with side thrusters - more so in the smaller vessels
although many large ships do incorporate side thrusters in their design as stern or bow units.
For many of the vessels built for the offshore oil and gas industry side thrusters are a necessity
and they would be unable to perform their function without them. It is because of their common
use on such vessels that the cost of thrusters for the typical merchant vessel is becoming very
reasonable and more owners can see cost advantages in fitting them.
One important point with the installation of side thrusters is that they must be of a suitable size
to do the job for which they are intended in all reasonable weather conditions.
Side Thrusters Disadvantages
a) Greater initial building cost.
b) Additional cost of maintenance and spare parts.
c) Need for additional auxiliary power if driven by electricity
different types of thrusters used in different ship types
3 basic types of side thrusters
- Tunnel thruster
- Rotatable thruster
- Azipods
Describe Tunnel thruster
2 common types:
i) Tunnel thrusters
ii) Angles Tunnel thrusters
A fixed or controllable pitch propeller is mounted inside a transverse tunnel which is open to port and starboard, the direction of the propeller moves water from one side of the vessel to the other thereby exerting a sideways force on the relevant section of the hull.
Describe Rotatable thrusters
Rotatable Thrusters protrude below the hull and act as a multi directional propellers. The Omni Water Jet Thruster utilizes a mixed-flow impeller that draws water in through a bottom intake then directs it through a steering valve into one of two or three side or end nozzles which penetrate the hull in the direction of the required thrust.
The tunnel thruster can develop the maximum power for its size of any of the types but it does take up a fair bit of hull space. This type is most effective when the ship is nearly stationary but effectiveness reduces rapidly if there is movement ahead or astern. This type is usually fitted to larger vessels that need only to be turned short round or given assistance in berthing
The rotatable thruster has the advantage of being multi directional. This type is often fitted to vessels that require dynamic positioning, two or more can hold a hull stationary in the water against a tide or wind. They can also be used as a main propulsion unit. The disadvantage of this type is that, because the unit protrudes below the hull, it is susceptible to damage.
The Omni water jet thruster has an advantage over the tunnel thruster in that it is effective even when the vessel is moving ahead or astern. This type of thruster is similar to a water jet and, if end and side nozzles are fitted, it can be used to move the hull in any direction with some precision. This type is also effective in rough water or at very light drafts - even if the nozzles are out of the water.
Describe Azipod systems
A section concerning transverse thrusters of course would not be complete without reference being made to Azipods. Put simply an Azipod is an electric motor to which a propeller has been attached. This assembly is housed in a unit beneath a vessels hull that is capable of rotating through 360°. The Azipod is similar to a rotatable thruster but it is much more powerful and so it may be effectively used in much bigger ships.
Notice that the propeller is on the forward side of the pod, also that it is Controllable Pitch. Placing the propeller this way allows unimpeded access of water to the propeller. From the propeller, the water stream is directed onto the mounting arm of the unit which acts as a rudder and this increases the effectiveness of the steering arrangement.
This type of system is in common use on modern cruise liners and container ships where manoeuvrability and space within the hull are at a premium. The diesel/electric or gas turbine/electric power plants required to provide power to the azipods may be located low down in the vessel - saving space and also increasing stability – both significant considerations in cruise liners.
Stabilising Fans
The active fin system is composed of at least one pair of moving fins fitted to the ship’s side below the water line at the greatest distance from the centre of roll. The fins, usually gyro stabilised, are tilted to produce an upward or downward thrust against the rolling action of the vessel at it moves through the water:
The damping effect of this system is very effective and can easily achieve a 90% roll reduction. A drag effect is also produced because of increased resistance to the water flow along the hull. Extra drag means slower speed and increased fuel consumption.
Describe Stabilizers
Trying to reduce the degree of rolling has always been a problem at the forefront of any ship design. Rolling is not only a discomfort for passengers or crew, it can cause cargo damage and puts additional strain on securing systems. Therefore the more we can dampen the rolling action of a ship at sea the greater the chance of the cargo arriving at its destination in the same condition as it left.Trying to reduce the degree of rolling has always been a problem at the forefront of any ship design. Rolling is not only a discomfort for passengers or crew, it can cause cargo damage and puts additional strain on securing systems. Therefore the more we can dampen the rolling action of a ship at sea the greater the chance of the cargo arriving at its destination in the same condition as it left.
Auto Heeling Tanks
Similar to active stabilising tanks except that the pumps work at a slower rate and the system is used in port only to control a list that may develop while working cargo. The common name is an anomaly, these tanks should be called anti list tanks, but the system is simple and very common on container and Ro/Ro vessels today.
Usually fitted to the side tanks within the double skin of a container ship’s hull the system incorporates an automatic pump that transfers water ballast to the high side tank once the vessel develops a list.
Some large vessels are fitted with two or three separate heeling tanks which are used to control the list in whichever section of the vessel that cargo is being worked. This is to overcome torsional stresses that may develop if the heeling tank is not in the same compartment as the weight shift that is causing the list.
The controls on an auto heeling tank incorporate a time delay to ensure that the system does not continually switch on and off as the ship rolls on the berth, and an active variable start control to determine at what angle of list the pump will start the ballast transfer.
A typical setting for such a system would be a 90 second delay to commence pumping at a half degree list.
Roll reduction devices can be grouped under two headings depending on whether they require power and feedback control or not:
Active Systems
Stabilising fins
Active Tanks
Gyroscopic Stabiliser
Passive Systems
Bilge Keels
Passive Tanks
Bilge Keels
Bilge keels are fins in planes approximately perpendicular to the hull at or near the turn of the bilge, such that they do not protrude beyond the extreme width of the hull nor increase the depth of the hull:
They are fitted on nearly all ocean-going vessels, both commercial and military. The longitudinal extent varies from about 25% to 50% of the length of the ship. The metal strip may be from about 0.3 - 1.0 m wide depending on the curvature of the bilge and size of the vessel.
The damping effect is increased with the area of the bilge keel and its distance from the rolling axis. The damping is also increased considerably with the speed of the hull through the water.
In order to reduce the resistance of the hull the bilge keel is placed along the water flow lines.
