(a) Describe with sketch for the terms of stress exerted on the ship structure.
(a) Hogging (b) Sagging (c) Racking (d) Surging
(b) Discuss, with the aid of sketches, the methods used to minimize the effect of those in ships exceptional length.
(c) What portions of the structure resist the stress?
- When buoyancy amidships exceeds the weight due to loading.
- When the wave crest is amidships, the ship will ‘hog’
- This is called hogging of ship.
- It can be known that the ship will curve upwards
- When weight amidships exceeds the buoyancy due to loading.
- When the wave trough is amidships, the ship will ‘sag’
- This is called sagging of ship.
- It can be known that the ship will curve downwards
- When a ship is rolling, tendency for the ship’s structure to distort transversely, call racking.
- This effect will be greatest when the ship is light or ballast condition.
- Racking stresses are highest at the corners of the box section.
- To resist distortion, transverse bulkheads, brackets, beam knees joining horizontal and vertical structures used
- In heavy weather, when a ship is heaving and pitching, direction of forward/aft motion of ship, called surging. It is linear motion.
- dynamic forces created longitudinally, cause local stress in ship structure.
- The movement of a ship in a seaway, results in dynamic forces which largely of a local nature.
- These forces cause structure to vibrate, and transmit stresses to other parts of structure.
(b) Methods used to minimize the effect of ‘hogging’ and ‘sagging’
- Longitudinal bending stresses. When ship is in a seaway, or in loading her.
- Hogging cause too much weight in the ends of ship, sagging cause too much weight amidships of ship.
- At bottom, all longitudinal work, gives necessary strength.
- At the top, thickened deck stringer and sheer strake, to make a strong ‘L shaped’ girder.
- Deck girders and longitudinal bulkheads, to resist this stress.
- In large ships, use special steels for the sheer strake and bilge strakes; to strengthen the ship- longitudinal frames and beams in the bottom and under the strength deck.
- The stresses are greatest amidships, strength is greater amidships than at the ends.
- In long ships, shearing stresses occur near neutral axis.
- To strengthen the hull at about the half-depth of the ship, in the neighborhood of one-quarter of the length from each end.
(c) Portions of the structure which resist hogging and sagging stresses
- Dynamic forces cause damage to any faulty part structure.
- This distribution of weight and buoyancy result in a variation of load, shear forces and bending moments along length of ship.
- Ship’s structure subjected to constantly fluctuating stresses resulting from shear forces and bending moments as waves move along ship’s length.
- Depending upon direction- bending moment acts, ship bend in a longitudinal vertical plane.
(a) How do you understand “Fin Stabilizers”?
(b) Draw a sketch of the mechanism controlling it.
(c) Explain how the angle and direction through which the fins are turned controlled.
(a) Fin Stabilizers
- Two fins-Extend from ship side at about bilge level.
- Turn in opposite directions as ship rolls
- Create force on each fin by forward motion of ship
- Produces moment opposing roll
- When fin is turned down, water exerts an upward force.
- When fin is turned up, water exerts a downward force.
- Fins are Usually rectangular, having aerofoil cross-section and turn through about 20˚.
- Fitted with tail fins, turn relative to main fin through a further 10˚.
- Turned by electric motor driving variable delivery pump, delivering pressurized oil to fin tilting gear.
- Oil actuates ram coupled to fin shaft through a lever.
(b) Mechanism to control the fins
(c) How the fins are controlled and operated to stabilize the ship
- Tilt of fins angle and resulting ship moment determined by sensing control system.
- Fins are rectangular in shape and streamlined in section
- Use of movable flap or fixed and movable portion is to provide greater moment to ship.
- Forward speed of ship fins to generate thrust results in righting moment
- Effect of fins stabilizer depends upon their movement speed, from one extreme point to other.
- Fin movement by hydraulic power unit incorporating type of variable displacement pump
- Control unit causes fin movement, when reaches desired value, to rest
- Control system have two gyroscopes, one senses movements from vertical, other rolling velocity
- Result of control system, function of fin movement is roll angle, roll velocity, roll acceleration, natural list.
- Fin stabilizer provide accurate and effective roll stabilization, complex installation, merchant vessel is usually limited to passenger ships.
- Low ship speed stabilizing power falls off, when stationary no stabilization is possible
Sketch and describe an oil lubricated stern bearing and associated propeller shaft stern seal arrangement.
Oil Lubricated Stern Tubes
1. Cost – High
2. Friction – Less coefficient friction
3. Lubrication – Hydrodynamic lubrication
4. Measuring – Use poker gauge at aft end seal
5. Clearance – 0.0015-0.002 mm of shaft diameter
6. Wear limit – 2-3 mm of shaft
7. Bearing material – Grey cast iron with centrifugally lined white metal
8. Bearing length – Fore end=0.6 x shaft dia Aftt end=1.5 x shaft dia
9. Sealing Arrangement – fit inboard & outboard seal, Inboard seal prevent LO leaking, Outboard seal prevent SW leak in
10.Shaft – Without sleeve
11.Construction – Cast iron construct stern tube pressed into stern frame. Two white metal bearing bush press into stern tube
Type of Stern Tube Seal Arrangements
There are basically three types of sealing arrangements.
1. Simple stuffing box type.
2. Lip seal type.
3. Radial face seal type.
- Sketch shows a lip seal, known as Simplex Compact seal.
- Consist a number of sealing rings held in contact, by means of garter springs, with a renewable corrosion resistant liner fitted to shaft.
- Rings are made of elastomer able to withstand both tearing and high temperature.
- Forward seal retains bearing lubricating oil has two rings and aft seal prevents ingress of sea water has three rings.
- Check plugs fitted on top and bottom of aft seal to check for stern tube wear down and oil leakage.
- They are scarfed-rings renewed without drawing out propeller shaft.
(a) Sketch the three methods for connections of Sheer Strake and Deck Stringer.
(b) Explain about the Shell Plating of the ship.
(c) Explain about the Deck Plating of the ship.
(b) Shell Plating
- Bottom and side shell plating, form a major part of longitudinal strength members of vessel.
- Bottom shell plating, slightly thicker than side shell plating.
- Keel plate is about 30% thicker than the remainder of the bottom shell plating,
- Strake adjacent to keel on each side of ship is known as garboard strake, same thickness of bottom shell plating.
- Uppermost line of side shell plating and attached to main deck is known as Sheer strake.
- Thickness of shell plating depends mainly on length of ship,
- Maximum bending moment of a ship occurs at or near amidships.
- Shell plating at amidships are thicker than at ends.
(c) Deck plating
- Deck forms a cover over cargo, accommodation and machinery space must watertight.
- Deck plating is made up of longitudinal strakes of plating across its width.
- Plates or strakes nearest to deck edges are known as stringer plates.
- Thicker than rest of deck plating, form the join between sheer strake of side shell and deck plating.
- Towards ends of ship, deck plating is reduced in thickness.
- At large openings in deck for hatchways, engine casing, pump room entrance, etc. require to increase in thickness of plating.
- Plating between hatches of cargo ship is thinner than rest of deck plating.
- Plating of weather decks is cambered towards ship’s side to assist drainage of any water falling on deck.
(a) Sketch and describe a watertight bulkhead.
(b) Explain how water tightness is maintained for pipes piercing watertight bulkheads.
(a) Watertight Bulkhead
- divide longitudinal subdivisions, protect against fire and foundering.
- Transverse strength, against racking stresses.
- The number of bulkheads, depends position of machinery space.
- Collision bulkheads extend to upper deck.
- After peak bulkhead only extend to first deck.
- All others extend to the uppermost continuous deck.
- Platings are usually fitted horizontally, increasing from the top downwards.
- Plating in after peak bulkheads, doubled or thickened around the stern tube to resist vibration.
- Stiffeners may be spaced 750 mm apart, collision bulkheads and deep tank bulkheads, spacing 600 mm.
- Corrugated bulkheads are now common and normally vertically.
- Pipe piercing collision bulkheads operable from freeboard deck.
- Valve fitted on after side of bulkhead.
(b) How water tightness is maintained for the pipes piercing watertight bulkhead
- Numbers of openings kept minimum.
- Any necessary penetration of watertight bulkheads maintains watertight integrity.
- Pipes piercing suitable valves (steel, bronze or other approved ductile material, other than cast iron).
- Operable from above freeboard deck.
- In passenger ships, ship below margin line only one piercing allow
- No manholes, doors etc. allowed
Sketch and describe sea-inlets with refer to the fitting through of a ship.
- Sea inlet valve fitted to sea inlet box to draw sea water for machinery cooling system, fire, ballast
- This opening has rounded corners, kept clear of the bilge strake
- The sea inlet box is to have the same thickness as the adjacent shell,
- Fit grill over opening and sacrificial anode to prevent a galvanic action
- All discharges from above or below freeboard deck from enclosed spaces fitted an efficient non-return arrangement
- Arrangements and their control, according to the discharge distance from the summer load waterline.
- Manned machinery space inlets and outlets are easy controls and valve position indicators.
- Scuppers from open spaces led directly overboard.