### Drydock Stability

• When the ship enters a dry dock, it must have a positive metacentric height; and is usually trimmed by stern. The floor of the dry dock is lined with keel blocks, which are so arranged such that they can bear the weight of the ship. When the ship enters the dry dock, her centerline is first brought in line with the centerline of the keel blocks by using a combination of plum lines and Leica theodolite.
• The dock gates are then closed and the water is pumped out of the dock in stages. Since the ship has a trim by stern, the stern of the ship will first sit on the keel blocks. The rate of pumping out water is reduced as the stern is almost about to touch the keel blocks. The reason is, it is from this stage of the docking procedure when the stability of the ship starts getting critical. The interval of time from when the stern takes the blocks to the moment when the entire ship’s weight is borne by the blocks is called Critical Period. We will understand the details a little later.
• When the stern of the ship takes the blocks, it is fixed to the shores (sides of the dock). This is carried out from aft to forward so that by the time the entire ship takes the blocks, it is fixed to the shores. When the ship is completely borne by the blocks, water is pumped out quickly from the dock.
• When the ship’s stern just touches the keel blocks, part of the ship’s weight is being borne by the keel blocks. The contact between the stern and the keel block creates a normal reaction or upthrust. The magnitude of this upward normal reaction increases as the water level in the dry dock reduces. It is this upthrust that creates a virtual reduction in the metacentric height of the ship. Hence it is very crucial to maintain sufficient positive metacentric height before docking, lacking which, the ship may heel over to either side, or even slip off the keel blocks and capsize.
Three vertical parallel forces acting on the ship:
• Weight (W) acting downward.
• Keel block upthrust (P) acting upward.
• Buoyancy (W-P) acting upward.
The upthrust force (P) can be considered to have an effect similar to that of removal of a weight from the ship. This has the virtual effect of rising the center of gravity of the ship from the point ‘G’ to ‘G1’. The metacentric height therefore reduces from GM to G1
H            Virtual rise in CG during dry docking.
The virtual reduction in metacentric height at any stage of the docking process can be calculated by the following expression:

• This calculation must be carried out for the condition when the ship has just touched the keel blocks throughout its length.
• It is at this point that the keel block upthrust is maximum, and the risk of tipping over or slipping from keel blocks is most likely if the metacentric height is too low or negative

### Differences between MC/MC-C and ME/ME-C engines

The electrohydraulic control mechanisms of the ME engine replace the following components of the conventional MC engine: Chain drive for camshaft Camshaft with fuel cams, exhaust cams and indicator cams Fuel pump actuating gear, including roller guides and reversing mechanism Conventional fuel pressure booster and VIT system Exhaust valve actuating gear and roller guides Engine driven starting air distributor Electronic governor with actuator Regulating shaft Engine side control console Mechanical cylinder lubricators. The Engine Control System of the ME engine comprises: Control units Hydraulic power supply unit Hydraulic cylinder units, including: Electronically controlled fuel injection, and Electronically controlled exhaust valve activation Electronically controlled starting air valves Electronically controlled auxiliary blowers Integrated electronic governor functions Tacho system Electronically controlled Alpha lubricators

### Main engine interlocks

Interlocks are provided so that the engine can be started or reversed only when certain conditions have been fulfilled. When there is a remote control of engines, it is essential to have interlocks. This reduces the possibility of engine damage and any hazards to the operating personnel. Turning gear Interlock . This device prevents the engine from being started if the Turning gear is engaged. Running Direction Interlock . This prevents the fuel from being supplied if the running direction of the engine does not match the Telegraph. Starting Air Distributor in end position . This prevents starting from taking place if the shifting of the Distributor has not been completed. Main Lube. oil pressure, Piston cooling pressure, Jacket water pressure, and important parameters must be above the required minimum. Auxiliary Blower Interlock . The Auxiliary Blower is provided in case of Constant pressure turbo charging. Air Spring pressure Interlock . In case of the present generation

### Why is a man hole door elliptical in shape?

Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole  is lesser in size than the corresponding circular hole. More over it is prime concern to have a  smoothed generous radius at the corners to eliminate stress concentration. Hence other  geometrical shapes like rectangle and square are ruled out.  To compensate for the loss of material in the shell due to opening, a doubler ring has to be  provided around the opening. The thickness of the ring depends on the axis length along the  dirrection in which the stresses are maximum and the thickness of the shell. It is important to  align the minor axis along the length of the vessel, as the stress in this direction is  maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t=  thickness of the shell plating  Circumferential stress: Pd/4t  More over a considerable material and weight saving is achieved as minor is along the  direction of maximum stress.