Ship Construction

Question 1

1. Explain the meaning and purpose of EACH of the following ship terms:
   a) Transverse watertight bulkhead. (4)
   b) Bilge keel. (4)
   c) Shear strake. (4)
   d) Cofferdam. (4)

Answer 1

a) It is a bulkhead which stretches the full breadth and from bottom plates to the main upper deck. It prevents spread of water and fire whilst also providing strength to the vessel which stops racking.
b) A stiffened plate that is welded to the outside hull plating. Acts as passive stabilising device which reduces the severity of ships rolling.
c) The top line of ships side plating running adjacent to the weather deck. The plate is required to be of higher strength, either thicker plate or higher tensile material.
d) A tank used to segregate and resist cross contamination of different liquids, or the engine room. A cofferdam would be found between a fuel storage tank and a ballast water tank, if fuel tank were to form a crack internally, it would leak into the cofferdam instead of contaminating the ballast water.

Question 2

2. Describe, with the aid of a sketch, the structural features of the bow which impart structural strength. (16)

Answer 2

A collision bulkhead is fitted to a vessels bow between 10 & 13 meter (or 5% to 7% of the ships overall length) from the fore peak, it adds structural strength to the vessel and its main purpose being limiting the damage of a head-on collision to the part of the bow forward to it. The plate floors help keep the structural strength transversely of the vessel which prevents racking.

Question 3

3. a) State the structural features to bow and stem which impart structural strength. (8) b) State the stability implications when using water to fight a fire on board a ship. (8)

Answer 3

a) Breast hooks.
Wash bulkhead at centre line.
Painting beams.
Painting stringers.
Transverse straighteners.
Longitudinal straighteners.
Plate floors.
Collision bulkhead.
Transom plate.
Deep floors
Floor spacing is closer at bow, moves from 2m to around 0.8m between.
Thicker plating in painting and pounding areas.
Stern frame.
b) When fighting a fire you must ensure flood quantities of water are used, this is too ensure a large mass of water doesn’t overflow the bilges. It is hard to know how much water has been used to counteract with ballasting of the vessel.
Free surface effect increases the capsize moment.
It increases displacement of vessel in the water.
Could cause listing if mass of water remains at one side due to shift of centre of gravity. Shift in vertical position of G which would decrease stability.

Question 4

4. Sketch the following ship construction features:
   a) A plate floor;(6)
   b) A watertight bulkhead. (6)
   c) Explain how watertight bulkhead integrity may be tested. (4)

Answer 4

c) Watertight bulkheads can be tested by two ways, either by filling the compartment with water or using a fire hose against the wall. Filling the compartment would only be possible if there was a safe means of emptying the tank. With both methods, you ensure that no water passes through the bulkhead.

Question 5

5. With reference to forward collision bulkheads:
   a) Describe their construction; (8)
   b) State their function;(4)
   c) State their position, giving a reason for this positioning. (4)

Answer 5

a) As with watertight bulkheads, the forward collision bulkhead is stronger and thicker than other bulkheads to sustain against water pressure.
A number of large stakes are welded to the ships structure, tank tops, side shell and upper deck.
Thicker plates towards the base, with plate-work arranged horizontally.
Stiffening and strength is arranged vertically and uses vertical bulb plate
stiffeners and/or toe angle bar.
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b) To stop the spread of water throughout the vessel in the event of a frontal collision, maintaining the stability and buoyancy of the ship.
c) A collision bulkhead is fitted to a vessels bow between 10 & 13 meter (or 5% to 7% of the ships overall length) from the fore peak.

Question 6

6. a) Describe, with the aid of a sketch, a freeing-port. (8)
   b) Explain how freeing-ports assist in maintaining the stability of a ship. (8)

Answer 6

a)
A freeing port is an open hole cut into a bulk ward that allows rapid draining of green seas and collected rain water from the weather deck. It can sometimes be sealed by a hinge plate that will open when a weight of water acts on the inside surface. These can also be located on hull side plating, such as on Ro-Ro ferries in order to drain the water from internal car decks.
b) The freeing port maintains stability by quickly allowing large volumes of water to drain away which would otherwise be able to gather on the weather deck. The effect of this water would increase the mass of the vessel which would increase displacement and draft and improve gravity a little. Having this water collecting on the upper deck would raise the centre of gravity therefore decreasing GM, consequently the righting lever GZ would decrease. This would adversely affect stability. Free surface effect could further decrease stability due to the large volume of water accumulating on deck, which could cause a permanent list.

Question 7

7. a) Explain the term free-surface effect. (8)
   b) Explain why free-surface effect is important in ro-ro ferries. (8)

Answer 7

a) When a tank is partially filled, the liquid’s centre of gravity position will change as the ship is inclined. Liquid in partially filled tank always decreases the initial metacentric height GM, righting lever GZ, and angle of vanishing stability.
A partially filled tank is know as a “slack tank”. The reduction of stability caused by the liquids in slack tanks is known as free-surface effect. This adverse effect on the stability is referred to as a “loss in GM”.
The free-surface effect can endanger the ship or even lead to a negative metacentric height. Therefore the number of partially filled tanks should be kept to a minimum. When ballasting the vessel, only one transverse pair or a single centreline ballast tank should be filled up. At
sea, as far as possible, ballast tanks shall be 100% full or empty. When ballasted, wide double bottom tanks must be always 100% full.
b) Free surface effect is important on Ro-Ro ferries due to the lack of bulkheads, this means large open spaces such as car-decks can accumulate large bodies of water, and if not cleared quickly increase the risk of a strong free surface effect. By adding 1 internal bulkhead it reduces the free surface effect by 4 times. Adding 2 bulkheads reduce it by 9.

Question 8

8. a) Describe how the thrust of the propeller is transmitted to the hull of the ship.(6) b) Show with the aid of a sketch the principle of operation of a Michell thrust block. (10)

Answer 8

a) The thrust of the propeller is transmitted axially through the shafting of a ship to a heavily reinforced point on the ship’s hull, where either a separate/engine mounted thrust block will transfer from the shaft to the hull. A thrust collar which rotates with the shaft is located between bearing faces.

Question 9

List EIGHT actions to be taken by the Engineer Officer of the Watch to ensure the safe passage of the vessel through an ar
ea of heavy weather.(16)

Answer 9

Ensure that all bilge wells are emptied sufficiently.
Ensure that all watertight doors are closed.
Ensure that all heavy items are securely stowed and lashed.
Ensure that any chemicals, noxious liquids, flammable or otherwise are stored correctly and securely.
Stop all maintenance work, stow all tools and parts securely.
Ensure all tanks are sufficiently full.
Sufficient water within boiler and boiler system to compensate for rolling.
Sufficient lubricating oil within machinery sumps and service tanks.
All tank vents on deck covered with water-proof hoods.
Ensure that all external openings are securely closed.
All non-essential over-side discharges stopped and valves closed.

Question 10

10. With reference to transferring large quantities of fuel from double bottom storage tanks to wing bunker tanks:
    a) Explain the possible effects on the stability of the vessel. (8)
    b) State the important checks to be made before and during the fuel transfer operation. (8)

Answer 10

a) The wing tanks are higher than the double bottom tanks and are off-centre of the ship which results in the centre of gravity being higher due to the weight of the oil in the wing tanks. This has a destabilising effect on the ship as it decreases its GM. The situation would be made worse if the oil from lower double bottom tanks was used fires due to the weight being taken from a lower point of the ship which would increase destabilisation of the ship
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due to the increase in GM. Therefore taking the oil from the wing tanks first would help to make the ship more stable.
b) Open correct valves.
Check tank levels before and during operation.
Check pump on starting suction and discharge pressure.
Test pump emergency stop.
Check running amps of pump.
Check fuel tank temperature before transfer.
Inform bridge where transferring to so appropriate ballasting can be done – monitor stability.
Test emergency stop.

Question 11

11. a) State the measurement that gives an indication of a ship’s stability.
    b) Explain the meaning of the measurement stated in Q a)
    c) Explain how the measurement given in Q a) indicates EACH of the following: i) Stable stability,
    ii) Unstable stability
    iii) Neutral Stability

Answer 11

a) GM
b) GM is the distance between the centre of gravity and the metacentre of a vessel. Depending on the size of GM would indicate ships stability, i.e. stable, neutral or unstable. c)
i) GM = KM – KG, and since in a stable vessel metacentre is above the centre of gravity GM will be positive, therefore the ship will be stable as a righting lever will bring the ship back to centreline.
ii) In a neutral vessel the metacentre and centre of gravity are the same location which means the vessel is neither stable nor unstable, meaning it’ll stay at the same angle. iii) In an unstable ship, the G is above M. This means the moment is acting to capsize the vessel as there isn’t a righting lever.

Question 12

12. With reference to Bilge Injection System
    a) Describe, with the aid of a sketch, a bilge injection system.
    b) State the purpose of fitting a bilge injection system on board a ship.
    c) Identify the location on board a ship of a bilge injection system.

Answer 12

a)
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A bilge injection system uses largest seawater pump in the engine room which pumps overboard. It will have a suction line that is taking suction from the lowest point in the engine room through a strainer and is isolated by a screw down non return valve. The bore of suction line must be a minimum of 2/3 the diameter of the main sea suction.
b) To remove large volumes of water inside the machinery space.
c) Bottom deck of the engine room.

Question 13

13. With reference lubricating oil or fuel oil tanks that have ventilators on the vessels main weather deck, explain EACH of the following:
    a) The purpose of the ventilation;
    b) How ingress of sea water in bad weather is prevented;
    c) How the risk of fire entering the ventilator is reduced;
    d) How the risk of pollution is reduced.

Answer 13

a) The purpose of ventilation is to let the potential explosive hydro-carbon fumes be released from the tank. Also, to prevent over and under-pressure during filling/emptying of a fuel/lub. oil tank.
b) F.O. & L.O. tanks have vents on the freeboard deck which are fitted with closing devices in order to prevent water entering the tank. In case of vent submersion due to green seas, a floating ball inside will float it against a rubber neck, preventing sea water entering. c) Mushroom vents are fitted with flame gauze for protection against fire.
d) An overflow pipe is equipped to guide the contents to another tank, usually an overflow tank.
Sounding tanks are there to allow you to measure the depth of the oil and only full tank as per design to avoid slop over.
Flame proof mesh and a save-all is compulsory on deck vents.

Question 14

14. Explain, with reference to stability, why, when the vessel has full bunkers, fuel stored in higher tanks, such as wing tanks, is generally used before fuel stored in lower tanks, such as double bottom tanks.

Answer 14

The wing tanks are higher than the double bottom tanks and are off-centre of the ship which results in the centre of gravity being higher due to the weight of the oil in the wing tanks. This has a destabilising effect on the ship as it decreases its GM. The situation would be made worse if the oil from lower double bottom tanks was used fires due to the weight being taken from a lower point of the ship which would increase destabilisation of the ship due to the increase in GM. Therefore, taking the oil from the wing tanks first would help to make the ship more stable.

Question 15

15. Describe the inspection required of a weather deck water-tight door to ensure compliance with load line survey.

Answer 15

A weather deck water tight door prevents the passage of water when exposed to a head of water. A typical head of water could range from 3-10m. A watertight door is designed to be used on the ship deck above the water line, where they can be subject to the adverse weather conditions experienced offshore. Water tight doors are also designed to withstand brief submersion experienced from green seas. Water tight doors are tested using a pressure tank where a hydrostatic pressure can be applied to the door. The door is generally pressurised from the inside as this is the worst case scenario. A watertight door is also tested with a high pressure hose, which is directed at the seal. In both cases no leak can be present.