Docks, Locks & Navigational Aids

Docks

A dock is an enclosed water area for ships to be loaded and unloaded (a wet dock) or repaired (a dry dock). Docks are needed where the tidal range is large, so that the water level in the dock can be maintained at a nearly constant height, and ships can work at all states of the tide.

Wet docks (impounded docks)

• A basin with quay walls, enclosed and connected to the sea by a lock with gates.
• The water level is kept at about high water level, so ships remain afloat at low tide, and the quay height is the same for loading.
• Entrance lock: allows ships to pass in and out; the lock chamber is filled or emptied to equalise with the sea or the dock.
• Used at ports with a large tidal range (London, Liverpool, Kolkata — the Kidderpore docks, and Mumbai's Indira and Victoria docks).
• Disadvantages: the delay in locking, the high cost, and the difficulty in large ships' movements; today most new ports use open (tidal) berths because ships are big and the tidal range is small.

Dry docks (graving docks)

A dry dock is a large basin from which the water can be pumped out so that a ship's underwater part can be inspected, cleaned, painted and repaired.

• Structure: floor and walls (usually of concrete or masonry) with altars (steps), a gate or caisson at the entrance, keel blocks and bilge blocks on the floor to support the ship, and pumping arrangements (a pump house and culverts for filling and emptying).
• Procedure: the ship is floated in, positioned over the blocks, the gate closed, the water pumped out (the ship settles on the blocks), and after the work the dock is flooded and the ship floated out.
• Design: floor and walls resist the hydrostatic uplift and the pressure when the dock is empty (the floor is designed as a slab under uplift, the walls under the earth and water pressure), the concentrated load from the ship on the keel blocks, and seepage. Drainage below the floor is often provided to reduce uplift.
• Example: Cochin Shipyard's dry docks; the Mumbai Naval Dockyard's dry docks.

Floating docks

A floating dock is a large U-shaped pontoon structure that can be submerged by flooding ballast tanks so that a ship can enter, then raised by pumping out the ballast, lifting the ship clear of the water. It can be towed and used where a permanent dry dock is not justified.

Other repair facilities

• Slipways (marine railways): an inclined track on which a cradle carries the ship out of the water — for small vessels and fishing boats; patent slips and side-slip.
• Syncrolift (ship lift): a platform lowered into the water and raised with the ship and moved on tracks.
• Shipbuilding berths: inclined or horizontal building berths, with launching ways.

Locks

A lock is a chamber with gates at each end used to move ships between two water levels — from the sea to a dock at a different level, or between reaches of a canal or a river.

Working of a lock

1. A ship enters the chamber when the water is at the same level as outside (gates open).
2. The gate closes behind the ship.
3. Water is let in or out through culverts and valves (or through sluices in the gates) until the chamber level equals the level on the other side.
4. The gate on that side opens and the ship leaves.

The time of a lock cycle is 20–60 minutes for large locks; the capacity is the number of ships per hour, considering the chamber size.

Lock components

• Lock chamber with walls, floor and fenders;
• Lock gates at both ends (upper gate = head gate at the higher level; lower gate at the lower level);
• Filling and emptying system — culverts in the walls or floor and valves, so that the water flows smoothly without causing excessive forces on the ship (the hawser forces);
• Approach walls (guide walls) and waiting areas;
• Operating machinery — hydraulic or electric drives; capstans; and control systems.

Types of lock gate

Mitre gate geometry: the two leaves meet at the centre of the lock, and the mitre point is set back (the "rise") by about one-fifth of the lock width in the direction of the higher water. The water pressure then pushes the leaves against each other, so they act like a three-hinged arch: the load goes into the quoin posts (hinge side) and the mitre posts (meeting side). If the lock width is $B$ and the rise is $B/5$, each leaf makes an angle $\alpha$ with the transverse line, where $\tan \alpha =\frac{B/5}{B/2}=0.4$, so $\alpha \approx 21.8^{\circ }$, and the length of each leaf is $l=\frac{B/2}{\cos \alpha }$.