In an echo sounder the stylus is rotating with certain constant speed and transmission takes place when the stylus passes the zero mark.
When higher range scale is selected, the transmission will still take place when the stylus comes to zero. But the stylus speed is reduced because the stylus has to remain on the paper for longer period of time since the echos are returning from greater depths.
This system is known as “ RANGING “.
The range scales are generally provided as :
0 – 100 mtrs
0 – 200 mtrs
0 – 300 mtrs
0 – 400 mtrs
and so on.
Since the same length of paper now covers a larger depth the graduations become closer and it becomes difficult to read the depth accurately.
PHASING arrangement is used to avoid this.
In Phasing arrangement, the speed the stylus motor is kept constant, but the transmission point is advanced.
As shown in Fig, four sensors are positioned around the stylus belt and stylus is rotating at a constant speed.
A magnet mounted on the belt generates the pulse when it passes the sensor, which in turn activates the transmitter.
When the minimum range ( 0 – 100 mtrs ) is selected, the sensor 1 is used and the delay circuit arrangement is such that the transmission occurs exactly when the stylus passes the zero mark.
The bottom scale of the paper will depend on the speed of the stylus motor and it is set to 100 mtr.
On selecting the higher range ( 100 – 200 meters ), the sensor 2 is used for activating the transmitter and the zero of the scale so shifted that the top of the paper corresponds to 100 meters.
Since the speed of the stylus motor remains same, the bottom graduation corresponds to 200 mtrs.
Similarly, for measuring higher ranges different sensors are selected and the transmission time will be advanced accordingly.
It is always advisable to start the echo sounder at minimum range scale when phasing facility is provided, so that shallow depths are not missed.
Thus the danger of missing shallow waters can be avoided.
The Autopilot is basically used when a ship has to steer a set course for a long time without alteration because any deviation from the set course is controlled electronically and automatically.
3 Types of controls:
a) PROPORTIONAL CONTROL The effect on steering when only proportional control is applied causes the rudder to move by an amount proportional to the off-course error from the course to steer and the ship will oscillate on either side of the required course-line.
b) DERIVATIVE CONTROL
The rudder is shifted by an amount proportional to the rate of change of ship’s deviation from the course. The ship will make good a course which is parallel to the required course and will continue to do so until the autopilot is again caused to operate by external force acting on the ship.
There are certain errors due to design parameters of the vessel which have to be corrected. Data signals are produced by continuously sensing heading error over a period of time and applying an appropriate degree of permanent helm is used for this purpose. The permanent helm acts as mid-ship.
The output of these three controls is combined and the net resultant drives the rudder. This type of autopilot is also called as PID Auto Pilot.
The output from a gyro or magnetic compass is coupled to the comparator, in the control unit , along with the input signal from manual course setting control. Any difference between the two signals causes an output error signal whose magnitude is proportional to the difference between the two signals and hence the comparator is also referred to as proportional control. In addition to the proportional control, the control unit also consists of derivative and integral controls which analyse the signals from the gyro or magnetic compass and the course selector
A summing amplifier is used to obtain a resultant error signal from these controls. This error signal is fed to the error amplifier which also gets feedback signals from the rudder, consisting of the rudder position and its movement. The output of this error amplifier is fed via telemotors to the steering gear unit and in turn operates the rudder. The telemotor has two units, i.e. Transmitter and Receiver situated on the bridge and steering gear compartment respectively. There will be no output from the control unit when the difference between the two signals is zero and hence no movement of the rudder results.
Bulkheads in ships are similar to internal walls dividing a building into separate rooms. Vertical partitions arranged either transversely or longitudinally in ships are known as transverse bulkheads and longitudinal bulkheads respectively-
Functions of bulkheads
1. Bulkheads divide the main hull into different compartments and in the event of a damage to the shell plating bulkheads limit the extend of flooding and hence of loss of buoyancy.
2. Bulkheads also prevent spread of fire fromone compartment to another.
3. Transverse bulkhead prevents racking and torsional distortion on a ship.
4. Longitudinal bulkheads contribute to the longitudinal strength of the ship.
5. Bulkheads divide the main hull of a ship into different compartments such as the aft peak tank, engine room, cargo holds,deep tanks, cofferdam space, and the fore peak tank.
Bulkheads are either watertight or non-watertight although such terms as oil tight and gas tight bulkheads have been used.
Transverse watertight bulkheads divide the main hull into many different watertight compartments. Watertight bulkheads are attached to the shell, the deck, and the bottom or tank top by welding.
Non watertight bulkheads are any other types of bulkhead which are non water tight such as centreline wash bulkhead in the peak tanks, partial bulkheads in the accommodation spaces, stores and cargo holds.
The number of transverse bulkheads in a ship isdependent on the length of the ship. However all ships must
a collision or fore peak tank bulkhead
an aft peak tank bulkhead
a bulkhead at each end of the engine room
The number of transverse bulkheads in passenger ship is determined by flooding calculation. There are generally more transverse bulkheads fitted in passenger ships than in cargo ships due to the more stringent damage stability requirements.
The collision bulkhead is positioned such that it is not too forward to be damaged by collision. The distance of this bulkhead from the fore end of the waterline is stipulated by classification society to be 5 to 7.5% of the ship’s length.
The aft peak (tank) bulkhead is intended to enclose the stern tubes in a watertight compartment preventing any emergency from leakage where the propeller shaft pierce the hull. It is located well aft so that the peak when flooded would not cause excessive trim by the stern.
Engine room bulkheads provide a self-contained compartment for engine room spaces preventing damage by flooding from an adjacent compartment/hold.
There are two types of bulkhead construction:
i) Plain bulkhead
ii) Corrugated bulkhead
Plain bulkheads consist of plates stiffened by rolled sections such as bulb plates and angles spaced approximately 760mm apart.
The thickness of the plates are generally thickest at the bottom because of the maximum hydrostatic pressure experienced there, and thinnest at the top and generally not less than 6.5 mm thick. The plates of the bulkhead are laid in a horizontal direction.
Where the depth of the bulkhead is great, horizontal stringers or girders are fitted as well as vertical girders with face plate and tripping brackets.
A corrugated plate is stronger than a flat plate if subject to a bending moment or pillar load along the corrugations.
Corrugations (or swedges) are formed on a corrugated bulkhead to eliminate the need to fit the vertical stiffener, as in those of the plain bulkhead.
A corrugated plate is stronger than a flat plate without stiffening if subject to bending moment or a pillar load along the corrugations.
The elimination of vertical stiffeners also results in saving in steel weight and cost of stiffeners.
The angle of corrugation is normally about 45 degrees.
The troughs are vertical on transverse bulkheads but must be horizontal on continuous longitudinal bulkheads, which form part of the longitudinal strength of the ship.
Diaphragm plates or horizontal stringers are fitted on the bulkhead to keep the corrugation in place.
In cargo ships where various liquid cargoes are carried, cofferdams or simply void spaces between two bulkheads are fitted between tanks to arrest contamination of liquid of different density. Cofferdams are also fitted between tanks carrying fresh water and oil. Pump rooms and ballast tanks can be designed to take the place of a cofferdam. The spacing of adjourning bulkheads of a cofferdam can be 760 mm, a generally accepted space through which a person can pass through.
BULKHEAD DECK: IS THE UPPERMOST DECK UPTO WHICH THE TRANSVERSE WATERTIGHT BULKHEADS ARE CARRIED.
MARGIN LINE: IS AN IMAGINARY LINE DRAWN 76MM BELOW THE UPPERMOST SURFACE OF THE BULKHEAD DECK.
PERMEABILITY: IS THE PERCENTAGE OF SPACE WHICH CAN BE OCCUPIED BY WATER TILL THE MARGING LINE, IF THE SPACE EXTENDS ABOVE THE MARGIN LINE.
WEATHERTIGHT: MEANS THAT IN ANY SEA CONDITION WATER WILL NOT PENETRATE INTO THE SHIP.
TYPES OF BULKHEADS:
WATERTIGHT: SUBDIVIDE THE SHIPS INTO VARIOUS COMPARTMENTS AND THEIR NUMBER IS DICTATED BY CLASSFICATION SOCIETY REGULATIONS.
NON WATER TIGHT: FORM THE BOUNDRIES OF TANKS USED FOR CARRIAGE OF LIQUID CARGO.
OIL TIGHT: ARE ANY OTHER BULKHEADS SUCH AS, ENGINE ROOM CASINGS,ACCOMODATION PARTITIONS, STORES COMPARTMENTS ETC.
FUNCTIONS OF BULKHEADS:
AS STRENGTH MEMBERS THEY GIVE STRENGTH TO THE HULL, THEY RESIST THE HULL FROM DEFORMING,
HELP IN SPREADING HULL STRESSES OVER A LARGE AREA,
DIVIDE THE VESSEL IN SMALL COMPARTMENTS, SO AS TO PREVENT PROGRESSIVE FLOODING AND ULTIMATE FOUNDERING OF THE VESSEL IN CASE OF FLOODING/COLLISION/GROUNDING.
IN LIQUID CARRIERS THEY DIVIDE THE VESSEL INTO TANKS AND THUS REDUCE THE FREE SURFACE EFFECT,
REDUCE / PREVENT DAMAGE CAUSED BY SLOSHING OF THE LIQUIDS IN THE TANKS,
THEY ALSO PREVENT SPREADING OF FIRE, THEY ARE SPECIALLY DESIGNED TO WITHSTAND HIGH TEMPERATURES.
CONSTRUCTION OF WATERTIGHT BULKHEADS:
FORMED OF SEVERAL STRAKES OF PLATING,
WELDED TO THE SHELL DECK AND TANK TOP,
STRAKES ARE WELDED HORIZONTALLY AND STIFFENED VERTICALLY,
STRENGTH INCREASES TOWARDS THE BASE BY INCREASING THE THICKNESS OF THE STRAKES AT THE BASE,
THE COLLISION BULKHEAD IS 12% THICKER THAN OTHER WATERTIGHT BULKHEADS,
THE AFTER PEAK BULKHEAD IS ALSO THICKER TO REDUCE VIBRATIONS,
THE STIFFENERS ARE GENERALLY VERTICAL BULB PLATES SPACED 760MM APART, IN CASE OF COLLISION BULKHEADS AND OIL TIGHT BULKHEADS IT IS SPACED 610MM APART,
IT SHOULD BE OF SUFFICIENT STRENGTH TO BE CAPABLE OF REMAINING WATERTIGHT WITH A HEAD OF WATER UPTO THE TOP OF THE BULKHEAD,
IT MUST EXTEND UPTO THE BULKHEAD DECK IN ONE PLANE WITHOUT ANY RECESS AS FAR AS POSSIBLE AND PRACTICABLE. ANY RECESS MUST COMPLY WITH THE APPLICABLE SUBDIVISION REQUIREMENT AS PER SOLAS,
THE NUMBER OF PENETRATIONS IN A WATER TIGHT BULKHEAD MUST BE MINIMISED, ANY PENETRATIONS MUST BE AS INBOARD AND AS HIGH AS PRACTICABLE AND BE WATERTIGHT,
SLUICE VALVES ARE NOT PERMITTED IN WATER TIGHT BULKHEADS.
LOCATION OF WATERTIGHT BULKHEADS:
COLLISION BULKHEAD IN THE FRONT END,
A AFTER PEAK BULKHEAD,
WATER TIGHT BULKHEAD ON EITHER SIDE OF THE MACHINERY SPACE,,
IF THE VESSEL IS LARGE, ADDITIONAL BULKHEADS ARE REQUIRED TO PROVIDE GREATER STRENGTH AND TO INCREASE THE AMOUNT OF SUB-DIVISIONS,
PASSENGER SHIPS HAVE TO COMPLY WITH SOLAS FOR THE NUMBER AND SPACING OF WATERTIGHT BULKHEADS WHICH IS A STATUTORY REQUIREMENT.
TESTING OF WATERTIGHT BULKHEADS:
THOROUGH INSPECTION OF WATER BULKHEAD SHALL BE CARRIED OUT,
HOSE TEST SHALL BE CARRIED OUT AT THE MOST ADVANCED STAGE OF FITTING UP OF THE SHIP,
TESTING MAIN COMPARTMENTS WITH WATER IS NOT COMPULSORY,
IF TESTING NOT CARRIED OUT BY FILLING WATER, HOSE TEST SHOULD BE CARRIED OUT WHERE PRACTICABLE,
IF HOSE TEST NOT PRACTICABLE DUE TO DAMAGE TO MACHINERY OR ELECTRICAL EQUIPMENT IT IS REPLACED BY CAREFUL VISUAL EXAMINATION SUPPORTED BY MEANS SUCH AS DYE PENETRATION TEST OR ULTRASONIC LEAK TEST,
FORE PEAK, DB TANK INCLUDING DUCT KEEL AND INNER SKINS SHALL BE TESTED BY FILLING UP WITH WATER,
TANKS INTENDED TO HOLD LIQUIDS SHALL BE TESTED FOR TIGHTNESS WITH WATER TO A HEAD UP TO DEEPEST SUB-DIVISION LOAD LINE,
EVERY SHIP SHALL BE FITTED WITH A COLLISION BULKHEAD IN THE FORWARD UPTO THE FREEBOARD DECK, AT A DISTANCE OF 5 % OF THE LENGTH OR 10 MTRS (WHICHEVER IS LESS) FROM THE FORWARD PERPENDICULAR,
WHERE THERE IS A BULBOUS BOW OR ANY OTHER STRUCTURE EXTENDING FORWARD OF THE FORWARD PERPENDICULAR, THE ABOVE DISTANCE SHALL BE MEASURED FROM A POSITION:
AT THE MIDLENGTH OF SUCH EXTENSION, OR
AT A DISTANCE OF 1.5 % OF THE SHIP’S LENGTH FORWARD OF THE F.P., OR
AT A DISTANCE OF 3 MTRS FORWARD OF THE FORWARD PENDICULAR,
WHICHEVER FROM THE ABOVE IS AFT.
THE COLLISION BULKHEAD MAY HAVE RECESS OR STEPS, PROVIDED THEY TO COMPLY WITH RULE (1) AND (2) ABOVE. PIPES PIERCING SUCH BULKHEADS SHALL BE FITTED WITH VALVES WHICH ARE OPERABLE FROM ABOVE THE FREEBOARD DECK AND THE VALVE CHEST SHALL BE SECURED TO THE BULKHEAD IN THE FOREPEAK. THE VALVE MAY BE FITTED AFT OF THE COLLISION BULKHEAD WHERE SUCH VALVE IS EASILY ACCESSIBLE UNDER ALL SERVICE CONDITIONS, BUT THE SPACE SHALL NOT BE A CARGO SPACE. NO DOORS, MANHOLE, OR ANY OTHER OPENING SHALL BE FITTED IN THE COLLISION BULKHEAD.
IN EVERY CARGO SHIP PROVIDED WITH A LONG SUPERSTRUCTURE IN THE FORWARD, THE COLLISION BULKHEAD SHALL BE EXTENDED WEATHERTIGHT UPTO THE DECK IMMEDIATELY ABOVE THE FREEBOARD DECK. THE EXTENSION SHALL BE WITHIN THE LIMITS SPECIFIED IN (1) AND (2) ABOVE.
IN EVERY CARGO SHIP PROVIDED WITH A BOW DOOR OR SLOPING LOADING RAMP FORMING PART OF THE EXTENSION OF THE COLLISION BULKHEAD ABOVE THE FREEBOARD DECK, SHALL BE WATERTIGHT AND THE PART OF THE RAMP WHICH EXTENDS 2.3 MTRS ABOVE THE FREEBOARD DECK MAY EXTENDS FORWARD OF THE LIMITS SPECIFIED IN (1) AND (2) ABOVE.
THE NUMBER OF OPENING IN THE EXTENSION TO THE COLLISION BULKHEAD ABOVE THE FREEBOARD DECK SHALL BE CAPABLE OF BEING CLOSED WATERTIGHT AND SHALL BE RESTRICTED TO MINIMUM, COMPATIBLE WITH THE DESIGN AND NORMAL OPERATION OF THE SHIP.
OPENING ARE NOT ALLOWED IN WATERTIGHT BULKHEADS BUT WHEN IT IS NECESSARY TO HAVE AN OPENING IT IS IN THE FORM OF A WATERTIGHT DOOR. WHERE AN OPENING IS CUT IN THE FORM OF A WATERTIGHT DOOR, CARE MUST BE TAKEN TO MAINTAIN THE STRENGTH OF THE BULKHEAD. THE OPENING IS TO BE FRAMED AND REINFORCED. IF THE VERTICAL SPACING BETWEEN STIFFENERS IS INCREASED TO ACCOMMODATE THE DOOR OPENING, THE SCANTLINGS OF THE STIFFENERS ON THE EITHER SIDE IS INCREASED TO GIVE AN EQUIVALENT STRENGTH TO THAT OF AN UNPIERCED BULKHEAD.
THESE DOORS ARE MADE OF MILD STEEL OR CASTE AND ARE EITHER VERTICAL SLIDING OR HORIZONTAL SLIDING TYPE AND CAPABLE OF BEING OPERABLE UPTO A LIST OF 30 ON EITHER SIDE. THESE SHOULD BE CAPABLE OF OPERATION FROM THE VICINITY OF THE DOOR AS WELL AS FROM A REMOTE LOCATION. POSITION INDICATORS SHOULD BE PROVIDED NEAR THE DOOR AS WELL AS AT ALL REMOTE OPERATING POSITIONS. THERE SHOULD BE AN AUDIBLE ALARM, DISTINCT FROM ALL OTHER ALARMS IN THE VICINITY, WHICH WILL SOUND WHEN THE DOOR IS BEING REMOTELY OPERATED. REMOTE OPERATION CAN BE IN THE FORM OF A VERTICAL THREADED SHAFT GOING UPTO THE DECK OR CAN BE HYDRAULICALLY OPERATED BY MOTOR AS WELL AS A HANDPUMP AT REMOTE LOCATION AND ALSO NEAR THE DOOR. DOORS WHICH ARE NORMALLY CLOSED AT SEA BUT ARE NOT PROVIDED WITH REMOTE OPERATION SHALL BE MARKED “TO BE KEPT CLOSED AT SEA” ON BOTH SIDES OF THE DOOR. DOORS WHICH ARE TO BE PERMANENTLY KEPT CLOSED AT SEA SHOULD BE MARKED “NOT TO BE OPENED AT SEA”.
THE OUTER RAMP SECTION AND THE OUTER BOW DOOR ASSEMBLY ARE MOUNTED ON INDEPENDENT HYDRAULICALLY OPERATED FRAMES,
FIRST THE BOW DOOR IS OPENED, THEN THE OUTER RAMP FRAME IS UNFOLDED, THEN FINALLY THE COLLISION BULKHEAD DOOR OPENS AND SITS ON THE RAMP FRAME AND COMPLETED THE RAMP ASSEMBLY,
THE RAMP FRAME IS LOCATED FORWARD OF THE COLLISION BULKHEAD DOOR,
INTERLOCKING OF VARIOUS PARTS PREVENTS ACCIDENTAL / OUT OF SEQUENCE OPENING, i.e. FIRST THE BOW DOOR WILL OPEN, THEN THE RAMP FRAME AND FINALLY THE COLLISION BULKHEAD DOOR.
WHEN CLOSED THE DOOR IS CLEATED BY HYDRAULICALLY OPERATED WEDGES AROUND THE ENTIRE OPENING, IN ADDITION STRONG LOCKING DEVICES ARE POSITIONED IN THE LOWER PART OF THE DOOR SECTION AND THE HULL,
FULL WATER TIGHTNESS IS ENSURED BY A SINGLE LIP RUBBER SEAL MOUNTED AROUND THE DOOR OPENING.
CONSTRUCTION AND INITIAL TESTS OF WATERTIGHT DOORS AS PER SOLAS
IN PASSENGER SHIPS THE FRAME OF VERTICAL WATERTIGHT DOORS SHALL HAVE NO GROOVE AT THE BOTTOM IN WHICH DIRT MIGHT LODGE AND PREVENT THE DOOR CLOSING PROPERLY.
IN PASSENGER SHIPS THE WATERTIGHT DOOR SHALL BE TESTED BY WATER PRESSURE TO A HEAD UP TO THE BULKHEAD DECK OR FREEBOARD DECK.
AIS system does not communicate any globally available data, whereas LRIT is a globally available, satellite-supported system which meets the requirements of the authorities of having access to the data of individual ships globally and at any time. Time and frequency must be freely configurable by the authority requesting the data at all times. Manipulation by ships (e.g., entering incorrect data) must be eliminated.
One of the most important differences between LRIT and AIS, however, is that AIS is a so-called broadcast system, i.e., is public, whereas LRIT data are only available to institutions which have a (governmental) entitlement to the data and guarantee the confidentiality of these data.
Another main difference is that AIS is a Collision avoidance system mandated by the IMO whereas LRIT is a Reporting system mandated by the IMO.