ship’s pivot point
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Ship’s Pivot Point
Is the point where the ship turns around.
Imaginary point on the ship’s centerline about which the ship pivots.
Forces which affect location of the Pivot Point
Headway or Sternway
When the ship is dead in the water, the pivot point is generally in the center if the ship.
When initially ordering engines ahead, the pivot point shifts forward as the speed increases. Once the ship is steady steaming, the pivot point settles back at about 1/3 from the bow.
When ordering engines astern, the opposite takes place; the pivot point shifts aft and settles at about 1/3 from the stern.
Acts on the freeboard or sail area of the ship
Ships tend to back into the wind
30kts of wind = 1kts of current
Acts on the underwater part of the ship.
Creates set and drift.
Wind and Current Effect on Vessels
The following figures illustrate the position of the Pivot point as a vessel moves from a position of stop to one moving ahead and astern,
The pressure of the water that acts on the bow or at the stern brings about a shift in the position of the Pivot point.
In this situation no forces are involved and the ship has a pivot point coinciding with its centre of gravity approximately amidships.
Two forces now come into play. Firstly, the forward momentum of the ship and secondly longitudinal resistance to the forward momentum created by the water ahead of the ship. These two forces must ultimately strike a balance and the pivot point moves forward. As a rough guide It can be assumed that at a steady speed the pivot point will be approximately 25% or a 1/4 of the ship's length from forward.
The situation is now totally reversed. The momentum of sternway must balance longitudinal resistance this time created by the water astern of the ship. The pivot point now moves aft and establishes itself approximately 25% or a 1/4 of the ship's length from the stern.
Although not intended some publications may give the impression that the pivot point moves right aft with sternway. This Is clearly not correct and can sometimes be Misleading. It should also be stressed that other factors such as acceleration shape of hull and speed may all affect the position of the pivot point. The arbitrary figures quoted here however, are perfectly adequate for a simple and practical working knowledge of the subject.
his is an example of a ship of 160 metres. It is stopped in the water and two tugs are secured fore and aft on long lines through centre leads. If the tugs apply the same bollard pull of say 15 tonnes (t) each. It is to a position 80m fore and aft of the pivot point. Thus two equal turning levers and moments of 80m x 15t (1200tm) are created resulting in even lateral motion and no rate of turn.
With the ship making steady headway however, the pivot point has shifted to a position 40m from the bow. The forward tug is now working on a very poor turning lever of 40m x 15t (600tm), whilst the after tug is working on an extremely good turning lever of 120m x 15t (1800t-m). This results in a swing of the stern to port.
The efficiency of the tugs will change totally when by contrast the ship makes sternway. Now the pivot point has moved aft to a position 40m from the stern. The forward tug Is working on an excellent turning lever of 120m x 15t (1800tm) whilst the after tug has lost its efficiency to a reduced turning lever of 40m x 15t (600tm). This now results in a swing of the bow to port.
In meteorology, winds are often referred to according to their strength, and the direction from which the wind is blowing. Short bursts of high speed wind are termed gusts. Strong winds of intermediate duration (around one minute) are termed squalls. Long-duration winds have various names associated with their average strength, such as breeze, gale, storm, hurricane, and typhoon. Wind occurs on a range of scales, from thunderstorm flows lasting tens of minutes, to local breezes generated by heating of land surfaces and lasting a few hours, to global winds resulting from the difference in absorption of solar energy between the climate zones on Earth. The two main causes of large-scale atmospheric circulation are the differential heating between the equator and the poles, and the rotation of the planet (Coriolis effect). Within the tropics, thermal low circulations over terrain and high plateaus can drive monsoon circulations. In coastal areas the sea breeze/land breeze cycle can define local winds; in areas that have variable terrain, mountain and valley breezes can dominate local winds.
In most ships the pivoting point is well forward when moving ahead, so that the pressure on the greater exposed area abaft this point tends to turn the ship into the wind. When going astern, the pivoting point moves aft and the stern tends to fly into the wind. The degree to which these effects are felt depends largely on the shape and disposition of the ship’s superstructure. For example, a ship with a very high forecastle is not affected a great deal when going ahead, but her stern seeks the eye of the wind rapidly as soon as she gathers sternway.
The effect on the ship’s turning circle usually is to expand the curve in the two quadrants in which her bows are turning away from the wind, and to contract it elsewhere. When turning away from the wind the ship is sluggish in answering her rudder. She may be carrying lee rudder already to keep her on her course, so that in order to start the turn more wheel than usual must be applied. When avoiding a danger ahead remember that the advance will be greater’ when turning away from the wind.
Wind effects are felt more strongly when speed is slow, and when she is lightly laden. As ahead speed is reduced the bow usually falls off the wind more and more rapidly until, when the ship has lost all way, she lies approximately beam-on to the wind
Effect when turning at rest. When turning at rest in calm weather a ship pivots about a point somewhere between her centre of gravity and the centre of area of her underwater profile. This point is normally somewhat forward of amidships, but it moves forward or aft with trim by the bow or stem respectively. Under the influence of wind the attitude of a ship when stopped depends on the relation between the area exposed to the wind before and abaft the at-rest pivoting point. Usually a warship lies with the wind within 20 degrees of the beam, and when settled there she requires a greater turning moment than normal to start her turning at rest.
Effect when turning at rest
Drift. Any ship drifts to leeward under the influence of wind, the rate increasing progressively with loss of headway or sternway and with an increase in the angle of wind from the fore-and-aft line. When stopped and beam-on to the wind, the ship, as she drifts to leeward, begins to transmit her motion to the water surrounding her. The rate of drift increases up to a point at which both the ship and a body of surrounding water are moving bodily to leeward. Immediately the ship moves ahead or astern she will then enter water that is not drifting and so will reduce her own rate of drift to leeward.
Once a ship has been obliged to reduce to slow speed in a storm the pressure of the wind on her hull will have an increased effect on her handling qualities. The effect is greater if the ship is lightly laden, or is of shallow draught, or has large superstructures. When going very slowly or when stopped, most ships tend to lie broadside on to the wind, and in exceptionally strong winds it may be difficult to turn them up into the wind, though it may be possible to turn them away down-wind. In a typhoon or hurricane it may be impossible to turn certain ships into the wind, which is one good reason why any seaman avoids such conditions with land or dangers to leeward.
EFFECT OF WIND ON A SHIP
EFFECT OF WIND ON A SHIP
The amount of leeway a ship makes in a gale depends on her speed, draught and freeboard, and on her course in relation to the direction of the wind and sea. In winds of gale or hurricane force the leeway with the wind abeam can be very considerable, and may amount to as much as two knots or more, particularly if the ship is steaming at slow speed.
It is a common mistake among inexperienced seamen to make insufficient allowance for leeway, particularly in a prolonged gale when, in addition to the wind, there will be a surface current caused by it. The amount of leeway made by a ship in various circumstances can only be judged by experience, but it is wise to allow a liberal margin of safety when passing dangers to leeward, because cases abound of ships having gone aground through failure to make sufficient allowance for leeway in the course steered.
Leeway caused by the wind
Leeway is the amount of drift motion to leeward of an object floating in the water caused by the component of the wind vector that is perpendicular to the object’s forward motion. The National Search and Rescue Supplement to the International Aeronautical and Maritime Search and Rescue Manual defines leeway as "the movement of a search object through water caused by winds blowing against exposed surfaces". However, the resultant total motion of an object is made up of the leeway drift and the movement of the upper layer of the ocean caused by the surface currents, tidal currents and ocean currents. Objects with a greater exposure to each element will experience more leeway drift and overall movement through the water than ones with less exposure.
Clearly the ship’s handling qualities are not affected in any way if the whole body of water covering the area in which she is manoeuvring is moving at a constant speed. In narrow waters, allowance must be made for the distance the ship will be moved by the stream during a manoeuvre. But it frequently occurs in confined waters that the stream differs considerably within a small area, so that the bows and stern may be exposed to quite different currents.
Current and Tidal Stream
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. In extreme cases, and particularly in ships of low freeboard aft, the deck aft may be flooded by the stern wave.
The effects of shallow water on the speed of the ship and on the flow of water past the hull when moving ahead have already been described. 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.
A slipstream is a region behind a moving object in which a wake of fluid (typically air or water) is moving at velocities comparable to the moving object (in comparison to the ambient fluid through which the object is moving). The term slipstream also applies to the similar region adjacent to an object with a fluid moving around it. "Slipstreaming" or "drafting" works because of the relative motion of the fluid in the slipstream.
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. If the attempt to turn at rest in shallow water with ahead revolutions on one shaft and astern on the other fails, or the turn is very sluggish, the situation will almost certainly become worse if the revolutions are increased. Stopping the engines to allow the eddies to subside, and then starting again with reduced revolutions, is more likely to be successful.
The effects of shallow water on the speed and steering of a ship, are intensified in a canal or similar narrow shallow passage, because the movement of water around the ship is confined. A ship moving along a canal pushes ahead of her a volume of water proportionate to her size and speed. A lateral wave is formed just ahead of the ship, constituting a zone of increased pressure, just astern a similar but smaller wave travels along with the ship. Between these two waves there is a trough along the length of the ship constituting a suction zone. Anything floating is repelled by the wave at the bows, and similarly the bows of the ship itself are repelled from anything solid such as the canal bank. The suction zone tends to attract any floating thing towards the sides and quarters of the ship, and also to cause the after part of the ship to be attracted towards the bank. The water level in the canal ahead of the ship is raised; while astern of her it is lowered. If speed is increased and the depth and width of the canal are little more than the draught and beam of the ship, the effects are noticeable a long way ahead and stern of the ship.
Passing through canals and narrow channels
To maintain the level of water in the canal an opposing current is set up that flows rapidly past the sides of the ship. This current is strongest close to the ship and near the surface, and weakest at the bottom of the canal and near its sides. Combined with the shallow-water effect, this opposing stream retards the ship’s progress. For example, a heavy ship passing through the narrow sections of the Suez Canal may make good only 5 knots at revolutions for 7 knots, while passage through the Gaillard Pass of the Panama Canal may reduce the ship’s speed by as much as 40 per cent.
Effect of canal on ship’s speed
To prevent damage to the banks and to craft moored, a speed limit is imposed in canals and in many rivers, and this must be rigidly obeyed. If the draught is such that there is only a little water under the keel, the ship’s speed should be kept well down, and a careful watch kept on the state of the wave formation caused by the ship’s passage. An increase in the bow and stern waves indicates that the ship is going too fast. She tends to settle deeper in the trough, and her speed may drop suddenly, causing the stem wave to overtake the ship and render the steering uncontrollable. The same effect may occur when the revolutions are reduced rapidly, so it is all the more important not to go too fast, and if obliged to reduce speed, to do so gradually if possible.
In a canal the use of the wheel alone may be quite insufficient to correct a sheer, hence the ship handler should be ready to use the engines on the instant, or to let go an anchor immediately, if the need arises.
Experiments have further shown that it may be less effective to reverse the engine or propeller pitch on the side away from the sheer than merely to stop it. There is also the danger of damaging the propellers by swinging the stem too close two the bank. Meanwhile the rudder may be entirely ineffective in checking the sheer, and, if so, the anchor opposite the direction of sheer should be let go and dragged at short stay.
Correction of a sheer in a canal
In a large ship, if prompt action with the engines and rudder as described has failed to have any effect on the sheer, it is probably best to apply full astern power in order to take the way off the ship, and if necessary also to let go both anchors. If this is not done by the time the sheer has carried the bows past the centre of the channel it is unlikely that the ship can be prevented from striking the opposite bank.
In smaller single-screw ships a sheer is best checked by full ahead revolutions (or full pitch) and full rudder, but on occasions the sideways force of the propeller when going astern may be used to prevent the stern swinging on to the starboard bank.
In any ship quick judgment is necessary when correcting a sheer, to ensure that the correcting action is removed and possibly countered as soon as it begins to take effect; otherwise it is quite easy to produce a sheer in the opposite direction and ground the ship on the bank from which she was originally swinging away.
To sum up, a ship when in a canal has a critical speed above which her steering becomes increasingly erratic because of the shallow-water effects. This is known as the canal speed, which cannot be exceeded with safety.
The effect of water pressure against the bows from the presence of shelving water on one side, causing the bows to swing away into deeper water, is the phenomenon known as smelling the ground. In a narrow passage or canal it can produce a dangerous sheer towards the opposite shore or bank, but it can be beneficial if the water opposite the shoal is deep add safe. The effect is most marked if the bottom shelves steeply.
Smelling the ground
As the ship approaches a bend in a canal or river there will be a tendency for the bows to smell the ground on the outer bank and so to be swung round the bend. In negotiating a bend it may be found that it is unnecessary to use any wheel towards the direction of the bend, because the water pressure on the outer bow will be just sufficient to carry the ship round. In fact, if the ship approaches the bend on the outer side of the channel it may be necessary to use opposite wheel to keep her safely in the channel as she rounds the bend. If she approaches the bend too close to the inner bank there is a danger that she may take an uncontrollable sheer towards the outer bank. Nice judgement is therefore required in selecting the best course to follow and if there is little current it is generally advisable to keep to the centre of the channel, but inclining slightly to the outside of the bend, when it will often be found that very little rudder is required to negotiate the bend.
Rounding a bend where there is little current