Category Archives: Naval Architecture And Ship Construction

MEO Orals on Naval Architecture Function 3 Part 4

MEO Orals on Naval Architecture Function 3 Part 4

Load line

  • The load line is a term given to a mark located amidships on both sides of a ship to show the limiting draught to which the vessel may be loaded.
  • This limiting draught is obtained by measuring from the uppermost continuous weather tight deck (normally the freeboard deck) down to the load line mark amidships. This distance is called the freeboard of the ship.

load line marking on ships

Pilgrim nut

  • The pilgrim nut provides a predetermined grip between the propeller and its shaft.
  • The propeller boss is fitted with a S.G cast iron internally tapered sleeve, which is secured (fixed firmly in position) into the boss. This sleeve is bedded to the shaft cone before mounting in the boss so that better fit is achieved which, combined with the pilgrim nut push up, ensure a good friction grip. No key is required.
  • The pilgrim nut is a threaded hydraulic jack, which screwed on to the tailshaft. A steel ring receives thrust from a hydraulically pressurised nitrile rubber tyre. This thrust is applied to the propeller to force it onto the taper sleeve.
  • Propeller removal is achieved by reversing the pilgrim nut and using a withdrawal plate, which is fastened to the propeller boss by studs. When the tyre is pressurised the propeller is drawn off the taper.

Advantages

  1. The sleeve has a similar thermal expansion to the steel shaft, thus reducing the risk of slackness in hot water.
  2. An extension to 5 years survey period is allowed by classification societies.
  3. The boss stress is reduced.
  4. The cast iron/ steel contact has a higher friction coefficient than bronze/ steel.

Pressure Test of Cargo Tanks

  1. A structural test by testing with water to a height of 2.45 meters above the tank crown, or
  2. A leak test consisting of a soapy solution test while the tank is subjected to an air pressure of 0.14 bar. It is recommended that the air pressure is initially raised to 0.21 bar and then lowered to the above test pressure before inspection is carried out.

Water tight doors

  • Watertight bulkheads are constructed to ensure their water tightness.
  • Where openings are necessary in these bulkheads special watertight doors must be fitted.

Difference between Stiff ship and Tender ship

Stiff Ship

  1. Greater GM due to high density cargo on bottom
  2. Ship rolls very fast
  3. Very  uncomfortable

Tender Ship

  1. Small GM (but not negative ) due to loading on top
  2. Ship rolls very slowly
  3. Uncomfortable but better than stiff ship

What are tunnel water tight door requirement ?

  1. The door may be either vertical or horizontal sliding
  2. The means of closing the door must be positive ie. They must not rely on gravity or a dropping weight 
  3. They capable for operating with a list of 15 degree and to be capable of being quickly closed form an accessible position above the bulkhead deck
  4. It must be operated from the vicinity of the door in addition to a point above the bulk head deck 
  5. If no power is available in hydraulically operated system, the door may be closed and opened by manual operated pump
  6. Must have an index at the operating position showing whether the door is opened or closed.

Reference:

Oral Guide by – MIN ZAR TAR

MEO Orals on Naval Architecture Function 3 Part 4