Turning Circles and Stopping Distances
The advance of a ship for a given alteration of course is the distance that her compass platform moves in the direction of her original line of advance, measured from the point where the rudder is put over.
The transfer of a ship for a given alteration of course is the distance that her compass platform moves at right-angles to her original line of advance, measured from the point where the rudder is put over
Consider the paths described by various parts of a ship turning under rudder when steaming ahead, see figure above. Each point in the ship must follow a path approximately concentric with that described by the centre of gravity. The angle made by the tangent to the curved path of any point with the fore-and-aft line is known as the drift angle at that point at any given instant.
The drift angle has its highest value at the stern and it diminishes gradually along the Fore-and-aft line in the forward direction until a point is reached, usually nearer the bow than the stern, where it is zero. Forward from this point the drift angle gradually increases in the opposite direction. When drift angle is quoted the value given is normally that measured at the centre of gravity.
The tactical diameter is the amount that the compass platform has moved at right-angles to the ship’s original line of advance when she has turned through 180 degrees. In other words, it is the transfer for an alteration of course of 180 degrees.
The following factors determine the acceleration powers of a ship.
The momentum of the ship depends upon the mass of the ship and the speed of the ship. Thus a lighter ship will gain or lose speed faster than a deeply loaded ship. If a large tanker is taken as an example then at the same speed it will travel longer after the engine is stopped – when the tanker is in full load condition. The reverse will happen when the tanker is on ballast – that is it will travel a lesser distance. For starting up also after the first movement is given a loaded tanker will come to the designed speed slower than the same tanker when it ballast.
The shape of the underwater part of the hull also plays an important part. Two tankers of the same displacement would have entirely different accelerating and decelerating speeds. The tanker which has finer lines than the other would be able to travel further after the engines are stopped as well as start and reach the designed speed faster.
Another factor is the condition of the ships bottom and the underwater part of the hull. If the undersides are fouled with marine growth then there would be a drag and the effect on the start up would not be that affected but the travel distance after the engines are stopped would be shorter.
If the under keel clearance is low then the effect is both ways that is the ship will take longer to reach her designed speed from stop as well as she travel longer when the engines are stopped.
Rates of gaining and losing speed
Knowledge of the rate at which a ship gains or loses speed in different circumstances is invaluable when manoeuvring in congested waters. These rates depend chiefly on the displacement of the ship, her condition of loading, her draught, the power of her engines, the size of her propellers and the depth of water. The corresponding rates for one ship will differ largely from those of another, and the rates for a particular ship may change considerably with her condition of loading.
When increasing or decreasing speed by changing the ahead revolutions, the rate of acceleration or deceleration is affected by so many factors and varies so much in different parts of the total speed range that it is difficult to recommend any practical method of allowing for accurately when manoeuvring. It is common practice to use a standard figure for the ship under all conditions (e.g. 100 metres per knot for a heavy ship). It must be realized that this method may prove extremely inaccurate in certain circumstances, and the ship handier should be prepared to make bold and rapid adjustments of speed during a manoeuvre if it appears that the estimate is wrong.
FACTORS AFFECTING SPEED
If a ship lies for long in harbour, particularly in a tropical harbour, her bottom becomes fouled by weeds, barnacles and other marine parasites or growths, and the speed attainable with a given number of revolutions is reduced.
The growth accumulated during 6 months would cause a reduction of about 10 per cent. Thus normal revolutions for 15 knots would give only 13 knots through the water.
When a ship is moving in shallow water the gap between the ship’s hull and the bottom is restricted, the streamline flow of water past the hull is altered and the result is seen as a greatly increased transverse wave formation at the bows and again at the stern. In fact, the increased size of the stern wave is a sure indication of the presence of shallow water. The energy expended in the waves formed by the ship is a loss from the power available to drive her, and therefore in shallow water her speed is reduced.
Furthermore, the restricted flow of water past the stern reduces propeller efficiency, which also tends to reduce her speed. Usually, the higher the speed the more pronounced is the reduction of speed.
FACTORS AFFECTING A SHIP’S HANDLING QUALITIES
Draught, trim and loading
On a general cargo ship or tanker the difference between the turning qualities when lightly laden and when fully laden is very marked. When deeply laden a cargo ship has a much larger turning circle than when lightly laden, and she is more sluggish in answering her rudder.
Trim by the stern usually increases the tactical diameter, but helps a ship to keep her course more easily when on a steady course. When trimmed by the bows her turning circle is likely to be decreased; she does not answer her wheel as readily as usual, and once she has started to swing it is more difficult to check her. The effect of trimming is to move the ship’s pivoting point towards the deeper end.
The effect of a list is to hinder a turn in the direction of the list and assist a turn away from it. A list to port decreases the tactical diameter of a ship turning to starboard, and vice versa.
The effect of speed on tactical diameter will vary from one type of ship to another. Often higher speed may lead to a greater tactical diameter because the rudder may stall. Modern rudders, on smaller ships, however, are able to operate satisfactorily at higher water speeds and greater angles, and hence the tactical diameter may not vary much with speed. Indeed, on some ships there is a best speed giving the minimum tactical diameter and at higher or lower speeds the tactical diameter is greater. Watchkeeping officers should be fully aware of the,effect of speed on the turning qualities of their ship.
These effects may become excessive if the depth of water is less than one-and-a-half times the draught, particularly if the ship enters such water at high speed. She may become directionally unstable and fail to answer her rudder at all, and the draught aft may increase so greatly as to cause the propellers to touch bottom.
The effects are likely to be particularly pronounced in ships where the propeller slipstream does not play directly on to the rudder. The effects of shallow water on steering in restricted waters such as canals or rivers are usually worse than in the open sea, and are more likely to have dangerous results. The only way to regain control is to reduce speed drastically at once.
When manoeuvring at slow speed or turning at rest in a confined space in shallow water, the expected effects from the rudder and the propellers may not appear. Water cannot flow easily from one side of the ship to the other, so that the sideways force from the propellers may in fact be opposite to what usually occurs. Eddies may build up that counteract the propeller forces and the expected action of the rudder. Stopping the engines to allow the eddies to subside, and then starting again with reduced revolutions, is more likely to be successful.
Effect of hull form on turning circle
A ship of fine underwater form (container ship) will turn in a larger circle than a ship of similar length and draught but of fuller form (tanker). Modern container ships are generally of great length in proportion to beam and thus tend to have large turning circles. The shape of the underwater part of the hull aft, particularly the cut-up area, as shown in Figure, has a most important effect on the size of the turning circle.
Effect of cut-up area on turning qualities
The ship with the larger cut-up area ABC will have a smaller turning circle than the one with the smaller cut-up area ADX
Effect of single screw on turning circle
In a ship fitted with a single right-handed fixed-pitch screw (most of the ships) the sideways force exerted by the propeller creates a tendency for the ship to turn to port when going ahead. With a left-handed controllable-pitch propeller the effect is reversed, the ship turning more easily to starboard, hence the turning circle with this type of propeller is usually of smaller diameter when turning to starboard than when turning to port.
Effect of a turn on speed
The effect of the drag of the rudder and the sideways drift of the ship will result in a progressive loss of speed while turning, even though the engine revolutions are maintained at a constant figure. For alterations of course of up to 20 degrees the reduction of speed may not be very great, but for those between 20 degrees and 90 degrees the speed usually falls off rapidly. For alterations exceeding 90 degrees the speed may continue to fall slightly, but it usually remains more or less steady. The rate of deceleration depends upon the initial speed of the ship and the angle of rudder applied, and it varies greatly between different types of ship.
Roughly, most medium sized ships when under full wheel will have lost about one-third of their original speed after turning through 90 degrees, and their speed will then remain steady as the turn continues.
The time taken to turn through a given angle depends on the initial speed and the angle of rudder applied; usually the faster the speed and the greater the rudder angle the sooner will the turn be completed.
Heel when turning
The initial heel when the wheel is put over is inwards, because the rudder force is acting at a point below the centre of gravity of the ship. As the ship begins to turn, the centripetal force on the hull (which is greater than the rudder force), acting through water pressure at a point below the centre of gravity, overcomes the tendency to heel inwards and causes her to heel outwards. This outward heel is very noticeable when turning at good speed. If the wheel is eased quickly the angle of outward heel will increase, because the counteractive rudder force is removed while the centripetal force remains, until the rate of turning decreases. Should an alarming heel develop, speed should be reduced instantly.