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CAPSTAN GEAR

Posted by: Engineers Zone Posted date: Friday, December 10, 2010 / comment : 10 ,


Capstan gear is normally fitted forward and aft in ships. The forward gear may comprise a separate cable/warping head which can be used for warping or working the anchors and cables and two cable holders which are designed to work the cables only, or two cable/warping heads which can be used for warping or working the cables. Anchor gear is not now carried aft but an after capstan head is fitted for warping, replenishment-at-sea and storing duties. Capstan gears can be powered nowadays by two methods, electric or electric-hydraulic.

·         ELECTRIC OPERATED CAPSTAN GEAR
This type is in common use in ships, where it consists, generally, of Duplex capstan gear in which the functions of a cable holder and capstan are combined in each of a pair of heads as mentioned previously. It is powered by a three-speed electric motor through a slipping clutch, a primary worm reduction gear, and a secondary worm reduction gear driving the capstan heads through vertical shafts. The driving gear is mounted under the forecastle deck, underslung on a main baseplate bolted to the deckhead. Each capstan head comprises a warping barrel mounted on the same spindle as a cable holder. The warping barrel always revolves with the spindle and so is always ready to use for warping. Vertical movement is provided so that the warping barrel can engage the cable holder which can then be used to haul or veer the anchor. When veering cable the cable holder is disengaged and controlled by handbrake. Neither head can be worked by hand. In larger ships, where space is less of a problem, the motors and gearing are seated on the deck below the forecastle, the vertical shafts being carried up through the latter.
The capstan gear, in this instance, consists of port and starboard wing cable holders and a middle line warping and anchor capstan, driven by two reversible electric motors through a slipping clutch, a geared driving unit and dog clutches. The wing cable holders, a handed pair, can be driven through an arrangement of dog clutches in either direction together, or independently, or be disconnected from the vertical shafts to run freely under the control of hand operated handbrakes. The body of each holder, like the lower barrel of the duplex head, is made of cast steel Grade I and formed with snugs into which the links of the cable fit neatly. The cable holders are not keyed to their spindles and each can revolve freely on a gunmetal bush and can be connected or disconnected from its spindle by a dog clutch. The skirt, or bottom flange, of the barrel forms braking surface for a band type of control brake which controls the speed at which the cable can be run out when the cable holder is disconnected.
The middle line capstan, normally operated independently of the wing cable holders, will revolve only in one direction relative to the motor direction and may be connected to, or isolated from, the motor drive through a dog clutch or by insertion or removal of two driving pins which couple the vertical shaft to a worm wheel. In the isolated condition the capstan can be worked by hand with bars fitted into sockets in the barrel head. These bars are made of ash wood bound with steel shoes which fit into the barrel slots snugly. They are secured by pins passing through the crown plate and shoes. Pawls dropped into a ratchet track located at the base of the barrel prevent the head from running back. Portable whelps can be fitted to facilitate working the ML head as a warping barrel but the wing cable holders are designed only for working the anchor cable. The electric motors are controlled by a portable T-handle fitted in a deck socket.

·         Electric-Hydraulic Capstan Gear
There are two forms of this type of gear. In the first the hydraulic motor and gearing which drive the capstan are incorporated in the actual capstan head as shown in the sketch. By virtue of its compactness this type is usually fitted aft in more modern ships designed with a comprehensive hydraulic system serving several equipments including the after capstan. In the second type the hydraulic motor drive replaces the electric motor drive described for the electric type of capstan gear and is otherwise similar to the latter as regards the design of capstan/warping heads, gearing, etc. Generally speaking this type is replacing the electric drive type in more modern ships forward. In both cases the electric motor and pump which supplies pressure oil to the hydraulic motor, whether the latter is incorporated in the capstan head or not, are situated on the deck below the capstan gear together with the control gear. In each a hydraulic control box is sited adjacent to the capstan and it is possible to work the capstan by means of a hand pump if electric power fails or is not available.

Brake Gear
In most types of capstan gear the brake is unidirectional and its Ferodo lined brake strap (of forged steel) is controlled by a hand wheel which operates through bevel gears to rotate a screwed spindle working in a trunnion nut. The brake strap is in two parts, connected by a joint pin with its head fitted in a recess in the underside of the forked part of the strap. The strap assembly is pivot anchored by a brake bolt in the deck plate. The other end of the band carries the trunnion nut in which the screwed end of the brake spindle works to contract or expand the band.

Controls
In all cases the capstan heads are controlled from the weather deck by means of a portable handle which fits into a socket let into the deck. The handle incorporates a pointer to indicate the handle position relative to the control markings marked on the flange of the socket. In addition the electrically driven capstan includes an emergency stop button set in the deck near to the control handle to stop the capstan in emergency.

TYPES OF FANS

Posted by: Engineers Zone Posted date: Friday, December 10, 2010 / comment : 2 ,


Two types of fan are in general use in ships are, centrifugal fans and axial flow fans.

Centrifugal fans consist essentially of a 'wheel' or 'runner' made up of radial blades rotating in a casing of the scroll form. The wheel is rotated at high speed by an electric motor on the same shaft.' Air is drawn in at the 'eye' of the fan, i.e. into the centre of the wheel and thence between the blades which give it rotation. The centrifugal force due to its rotation expels the air outwards towards the circumference and into the delivery trunk. A fan is said to be 'right handed' or 'left handed' according to whether the rotation is clockwise or anti-clockwise when viewed from the motor to the wheel.

Axial flow fans work on the same principle as the propellers of aircraft. They consist primarily of an impeller (or propeller) with blades mounted on a 'nacelle' enclosing an electric motor inside a circular trunk having a small clearance from the blades. Provided a straight length of trunk can be arranged at the inlet and outlet ends they can be designed to be as efficient as the centrifugal type For a given volume output and pressure, and can be used to distinct advantage where economy of space is important. The modern practice is to use this type for 1 group systems and machinery spaces.

Fans are not placed in bathrooms, wash places, drying rooms or other compartments containing humid air, nor in compartments where sparking could cause an explosion. They are usually suspended from the deck head or seated on the deck in a position affording an efficient arrangement of trunking. Non-air conditioned spaces of ships serving in tropical climates are fitted with fans of the open propeller type to provide additional means of rapidly circulating the air. Three types of open propeller fan are used. Slow-running overhead fans with wooden blades about 3 ft 6 inch diameter are fitted in wardrooms, officers' messes, dining halls, recreation and similar spaces. Small table fans with rubber blades 10 inch diameter are fitted in cabins and with rubber or steel blades 12 inch diameter in offices and mess spaces. Larger 16-in diameter fans with aluminium alloy blades (Hurricane type) are fitted in dining halls and large mess spaces. 

TYPES OF DAVITS

Posted by: Engineers Zone Posted date: Thursday, December 09, 2010 / comment : 4



TYPES OF DAVITS

·         HINGED SCREW TYPE
The majority of the older frigates and destroyers retain the hinged screw type of davits for general use. These davits can be turned in or out to the desired out reach by rotating a handle connected to a worm and wormwheel, the latter operating a screw thread on the extending arm. The davit arms are of I-bar section and so shaped that the boat, in the inboard stowed position, is upright when bearing against the griping pads. The weight of the boat is taken on keel chocks fitted to each davit.
The disadvantage of this type of davit lies in their hand operation and the fact that independent control of each of a pair can result in undue strains on the davits and operating gear due to unsynchronized movements.

·         GRAVITY TYPE
This consists of two portions, the davit arm and the deck frame which forms the runway for the arm. When the boat is being lowered the davit arm travels down the runway until it reaches a fixed stop by which time the boat is clear of the ship's side and disengaged from the davit head hook or 'tusk' which takes the weight of the boat when turned in. Continued veering on the winch allows the boat to travel vertically downwards. During the hoisting operation the ball-weight on the hoist wire engages a stop at the davit head, whereupon the boat and davit arm move as one until the fully housed position is reached. During this latter stage the weight of the boat is automatically transferred to the hook or tusk, thus relieving the tension in the hoist wire.

·         PIVOT TORQUE TYPE
This has a deck frame and davit arm which together with the boat hinges about the deck pivot. To overcome the initial resistance to hinging outboard, should the ship have an adverse heel, a coiled spring is fitted between deck frame and davit arm. This spring is designed with sufficient effort to bring the C.G. of the davit outboard of the pivot pin under all normal angles of ship heel. As in the gravity type davit the weight of the boat is taken by a hook or tusk at the davit head, engagement or disengagement with which occurs during the hinging process.

·         TRAVERSING GANTRIES
Traversing gantries are employed in aircraft carriers where the boats are stowed in boat bays at gallery deck level. These enable the boats to be lifted clear of the crutches (usually hinged), traversed outboard and lowered well clear of the ship's side. To allow for the rise and fall of the boat whilst still attached to the falls, a compensating mechanism is fitted in the lead of wire from the winch. This gear is designed to automatically 'shorten' or 'lengthen' the falls in phase with the boat's vertical movement. A man at the forward and after ends of the boat is able to retain the disengaging hook and the fall block together without difficulty.

The latest ships are equipped with either gravity or pivot torque types. Both are fully power operated and employ a single wire for boat hoisting and turning the boat to the stowed position, thus economizing considerably in manpower. Both davits of a pair are operated by a common drive and the problem of synchronized operation does not arise.

Power is not required for lowering a boat, control being exercised simply by a brake on the winch. The winches are fitted with cranked handles for operation should power fail when raising a boat. Neither of these types of davit have arrangements for combating wave motion so that the use of the nylon grommet or strop (foul weather pendant) becomes an essential safety measure.

All davits are tested with a static load of twice the working load and a running load of one and a half times the working load. In the latter case the boat is raised and lowered (or traversed if applicable) so as to test all parts of the system throughout its designed range.
               

ASSESSMENT OF REQUIREMENTS FOR NON-AIR CONDITIONED SPACES

Posted by: Engineers Zone Posted date: Thursday, December 09, 2010 / comment : 0 ,


For some compartments air conditioning is not practicable or desirable. These include compartments from which large quantities of wild heat and/or moisture must be removed; compartments in which unpleasant, toxic or explosive gases are liable to collect; and compartments like storerooms the contents of which are not sensitive to temperature or humidity. Such compartments must be supplied with sufficient air to maintain habitable conditions and for each type of compartment a maximum acceptable operating temperature is stated in °C or °F above the entering air dry bulb temperature.

ASSESSMENT OF REQUIREMENTS FOR AIR CONDITIONED SPACES

Posted by: Engineers Zone Posted date: Thursday, December 09, 2010 / comment : 0 ,


The basis of design of the normal air conditioning system for any compartment or group of compartments is the removal of sufficient heat and moisture to maintain a reasonable standard of bodily comfort for the personnel to carry out their duties. The heat generated or 'heat load' as it is called, arises from four sources:

a.        The men in the compartment.
b.       Machinery, electrical apparatus, lighting, etc. in the compartment.
c.        Conduction through the boundaries of the compartments from adjacent compartments,from the sun and from the sea.
d.       Fresh air which has to be cooled from outside air temperature to the compartment temperature.

The outside air temperature is assumed to be 31°C DB/27°C WB for tropical conditions and 34°C DB/30°C WB for extreme tropical. The maximum acceptable temperature within the compartment is taken as 30°C DB/22°C WB unless otherwise stated in the design requirements. The total heat load is calculated using standard formulae to obtain the quantity of air required. The capacity of the plant is then chosen to suit this heat load and after grouping of compartments the task of deciding the sizes of fans, coolers and layout of trunking can proceed.

RULE FOR WATERTIGHT AND GASTIGHT CONTROL MARKINGS

Posted by: Engineers Zone Posted date: Wednesday, December 08, 2010 / comment : 1


Observance of the rules for the control of watertight and gastight openings is the responsibility of ALL PERSONNEL and they must be continually aware of which watertight and gastight condition is in force and also of the requirements of each condition. Watch on the maintenance of the condition set is kept by NBCD patrols or by frequent rounds by the Watch on deck or Duty Watch according to the state of readiness, and the type and size of ship. The rules for control markings are:-

·         ‘X’ SHUT IN ALL WATERTIGHT CONDITIONS. To be opened only by permission. If required to be kept open, then a sentry must be posted or other arrangements made for the opening to be shut instantly on order, or a ‘MBLO’ disc must be used.
·         ‘Y ’SHUT IN CONDITION Y AND Z. Open in condition X-ray. When shut, may normally be opened for passage or use but must be immediately shut again. If required to be kept open in conditions Yankee or Zulu, the rules as for X-ray opening apply.
·         ‘Z’ SHUT IN CONDITION  Z. Open in conditions X-ray and Yankee. When shut, the rules as for Yankee openings apply.
·         ‘A’ SHUT IN CONDITION A. When condition Alfa is in force, openings so marked are not to be opened without specific permission from HQ1.
·         ‘M’ in condition Alfa only, openings so marked are under the control of the user department. User departments must ensure that the orders regarding their ’M’ openings are clear and fully understood by their personnel.
·         ‘R’ Fittings and equipment so marked must continue to run or remain open for recirculation.
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