When navigating in narrow channels the systems of water flow and water pressure will be considerably increased. The ship will thus push water ahead of her and the surface of the water will rise noticeably several ship's lengths ahead of the ship.Along the ship's side, water will flow back with higher velocity than in open water of the same depth and abaft of the ship, water will be pulled along in the direction the ship is sailing.If the ship sails in the centre line of the channel and if the channel is symmetrical about her she will not be exposed to a turning effect from the channel, … [Read more...]
Anchoring – An Effective means of Controlling Ship’s Movement
Anchors The specific requirements for anchoring equipment in ships are laid down by the Classification Societies and is determined as per each ship's displacement, her wind area down to the summer load line. These requirements are the weight of anchor, number of anchors, length of chain cable and breaking strength.Requirements also specify that the cable must be secured in the chain locker by an arrangement whose breaking strength is greater than 15% and less than 30% of the tensile proof stress of the chain (Norske Veritas). Also, the breaker of the windlass must be able to withstand a … [Read more...]
Slow Speed Control
The estimation of speed and knowing when to reduce speed when approaching a berth is not always easy and confidence can only come with experience. On very large ships, such as VLCCs, some guidance may be available from reliable doppler logs, but on many ships, a doppler log is not available. In any case, total reliance upon instrumentation is not wise and is no substitute for experience. A pilot jumping from one ship to another, sometimes several during one duty period has to develop a "feel" for the type of ship he boards and con "by the seat of his pants." Speed Many casualties are proven … [Read more...]
Basic Propeller Types
Introduction A propeller is a type of fan which generates an adequate thrust to propel a vessel at some design speed with some care taken in ensuring some “reasonable” propulsive efficiency. Considerations are made to match the engine’s power and shaft speed, as well as the size of the vessel and the ship’s operating speed, with an appropriately designed propeller. Generally, propellers are of two types - Fixed Pitch Propeller ( FPP) and Controllable or Variable Pitch Propeller ( CPP). Controllable Pitch Propeller or CPPThese propellers now have a relatively good track record for … [Read more...]
Principles of Ship Handling
Ship Handling and Maneuvering is defined as the art of proper control of a ship while underway, especially in harbours, around docks and piers. It is one of the skills that any ship handler finds very satisfying when well accomplished. The most basic thing to be understood in ship handling is to know and anticipate how a ship behaves under all circumstances and what orders should be given in order to make the ship behave and move exactly the way you want her to. The difference between the ships' heading and the actual direction of movement of the ship should also be constantly attended to as … [Read more...]
The Pivot Point
A Pivot Point is a central point on a vessel which remains fixed as the bow and stern swing around it. Fixed is not really correct, because it moves forward or aft as the vessel moves through the water as stated below: 1. Ship stopped Unless stated otherwise. Each example assumes a ship on an even keel in calm conditions and still water. In this situation, no forces are involved and the ship has a pivot point coinciding with its centre of gravity approximately amidships.2. Making Headway Two forces now come into play. Firstly the forward momentum of the ship and secondly, longitudinal … [Read more...]
What is Transverse Thrust?
Transverse Thrust The force with which the water acts upon the propeller blades is not just in the longitudinal direction but also has an athwartships component. This is known as transverse thrust. Going Ahead If we look at a ship with one right-handed propeller (i.e. the propeller rotating n a clockwise direction when going ahead and as viewed from astern) and the rudder on the centre line the following situation is the norm: In the initial stages of the ship getting underway, the vessel's speed is low but the engine (and, therefore, the propeller) is at high Rpm ahead. During this … [Read more...]
Shallow Water Effect on Ship
Shallow water effects on a ship w.r.t "Draft & Maneuverability." • As the hull moves through shallow water, the area which it displaces is not so easily replaced by surrounding water, therefore, leading to a state of partial vacuum as the propeller and rudder are still working. The vessel take-ups longer to answer her helm, and response to engine movements becomes sluggish.• In these circumstances, vibration will be set up, and it will be extremely difficult to correct a yaw or sheer with any degree of rapidity.• At normal speed it is found that steering becomes erratic when the … [Read more...]