Naval Shiphandling_Cap2.pdf

- ,. ,,::.. . - : ( ... a.ti-o, ã .., ã ,I L B I t3 l I \.: ·: E. \ 1\ l 2 forces affecting the ship In order to predict the movement of our ship accurately, we must thoroughly understand the nature and magnitude of the forces which affect her. There are six general sources of force which can be brought to bear on our ship independent of any other vessel. They are the propellers, the rudders, the mooring lines, the the WiQ.
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  -  . ,,: :.. . -: a. ti-o, ã .., ã I L B I t l I \ : ·: E \ 1\ l   forces ffecting the ship In order to predict the movement of our ship accurately, we must thoroughly understand the nature and magnitude of the forces which affect her. There are six general sources of force which can be brought to bear on our ship independent of any other vessel. They are the propellers, the rudders, the mooring lines, the th e WiQ.<;I. and finally the The -first four are controllable from the ship itself. The wind and the current (and this includes tidal currents), though not controllable, can be utilized to serve our aims if properly handled. Each of these forces can produce important effects, as indicated in Figure 2 1 , so it is worth while to take the time to study and understand each of them. Let s re- member from the beginning, however, that these only, and that motion results only after i JillJJ.g has played its part · · · ----- · .  A mociernsiiTp may  fiave a distributed mass of many thousand tons and may be several hundred feet long. Such a body not only has tremendous inertia to resist linear acceleration, but it also has a tremendous moment o inerti to resist rotational accelerations. The ship is resting in a fluid (water) coveredby another fluid a1r), both of which will offer a resistance to relative motion. Thus. when we apply a single force to the ship, we can expect an acceleration until the fluid resistance produced by the motion balances out the srcinal force. This will apply to angular motion produced by an off-center force as well as by force appli ed through the center of gravity. Thus, when we apply any force to the ship, we can expect motion to gradually build up until a state of equilibrium is reached, at which time the velocity of the motion will become constant. Basic principles - --·----·-· 13  WIND FORCE PRODUCED BY 3 KNOT WIND FROM e ã .ON THE BOW . 23 LBS. RUDDER 30ã RUI)DER PRODUCES A SIDE FORCE at 15 KNOTS OF ... CURRENT FORCE REQUIRED TO HOLD SHIP STATION-ARY N A 3 KNOT CURRENT 45ã .ON THE BOW.. . 55.000 LBS. MOORING LINES ND GROUND TACKLE 6-INCH MANILA 30.000 LBS . %-INCH WIRE 25,500 LBS . 1-INCH WIRE 64.400 LBS . Pt a INCH SPRING . LAID WIRE ROPE 93.000 LBS. 1-Va INCH DIE LOCK CHAIN 161.000 LBS PROPELLER THRUST AT 15 KNOTS . 55.000 LBS. FIGURE 2-1. Forces which can bear on a ship (figures for a 2,200-ton DO .  water are being set in motion by the movement of the oar. Without the force we apply to the oar, there would be no motion, no pressure difference, and no resis-tance. Thus we see that force, resistance, and motion are irrevocably interlocked when dealing in a fluid medium. The above discussion illustrates the fact that all forces in water manifest themselves as pressure differences. If we are going to apply force on a waterborne object, such as our ship, we can do it only by creating a pressure difference across a part of the ship s structure. And if at any time our ship moves in any way we know that some force is acting somewhere on our ship s structure and we can locate that force by looking for the pressure difference that causes it. In any large body of water, there are always two components of pressure present at any point: one is the static pressure due to depth, or sheer weight of the water above the point; the other is the dynamic pressure caused by motion in the surrounding water. In the sea the static pressure does not cause motion, because it is the same everywhere at any given depth level, and hence balances out as far as we are concerned. Bernoulli s Theorem tells us that at any given depth in an open body of water like the sea, the sum of the static pressure and the dynamic pressure is always constant. Normally there is little motion of the water, so the static pressure is all that needs to be considered. When a ship passes through the water or a propeller blade slices into it, however, the water is set into motion and the static pressure is reduced by the amount of the dynamic pressure. Though it is usually the decrease in effective static pressure brought about by setting the water particles in motion that produces our hydrodynamic effects, knowing the magnitude of the dynamic pressure will tell us the pressure difference to be expected from the motion. The magnitude of the dynamic pressure is given by the expression: p rtf _ 2g where: p Dynamic pressure in lbs/ft2 p = Density of the moving fluid in lbs/fP v Velocity of flow in ft/sec g Acceleration due to gravity, 32.2 ft/sec   The resulting pressure difference caused by the motion of water is therefore proportional to the density of the fluid and the squ re of the velocity of motion. Bernoulli s Theorem and the above expression apply to air movement also, but since salt water at 64.4 lbs/ft3 is 855 times more dense than standard moist air at .0752 lbs/ft 3 the dynamic forces on a ship resulting from the flow of water past its hull and appendages is vastly greater than those caused by the flow of air. On the other hand, the velocity of air relative to the ship may be much higher than normal water velocities, and since the dynamic pressure component increases as the squ re of the velocity, the dynamic effects of strong winds on a ship s struc-ture can be quite large. It may be useful to remember that the velocity of air must e approximately 30 times the velocity of water for the resulting dynamic pressure 15 FORCES AFFECTING THE SHIP  to be the same. Stated another way 30 knots of wind is the equivalent of 1 knot of current. A last general characteristic of water tnat is important in our study is its continuity; it tends to exist as a continuous body, without gaps or holes except as caused by extraordinary forces. If a volume of water is moved away so quickly, oy a pro;::>eller biade, for instance. that the pressure differences there are insufficient to accelerate water in as fast as it is being moved away then a gap would occur on the back side of the propeller blade. This gap is known as separation . A companion phenomenon occurs when, in a high velocity stream, the ve-locity gets so high and the pressure so low that the pressure in the stream drops to the vaporization point of water. In this case drops of water become vaporized in the area described, in a manner similar to boiling. This phenomenon is known as cavitation. Separation and cavitation are of interest to the shiphandler, because, when they occur, they upset the pattern of streamlines and change the resulting forces. These phenomena are likely to occur around abrupt changes m the underwater body of a ship moving at high speed. or about the blade of a propeller that is being rotated rapid y. Hydrofoils Before going into the various hydrodynamic effects associated with a ship and its motion through the water, some definitions are in order: Hydrofoil. Any relatively thin, plate-like member, such as a propelle-r blade or rudder:-d esigned to obtain a lift force when inclined to the flow of the water. Angle on.ttack. I he angle afwhid1a nydrofoi ITs inclined h the relative free stream flow -   -   - --   --- ·- _ -· Lift. That component of the reaction force on a hydrofoil which lies in a directiOi1perpenaiCU1att6._th   reiatiVe_ :ee of the . Drag. That com_ponent of the reaction force on a t}ydrofoil which lies in a directionparallei ' tothe reiative free streain fiow ofthe -A flat plate placed at an angle in a stream of water, as indicated m Figure 2-2(a), acts as a hydrofoil and causes the water to move out of the way on the leading side and to accelerate to move in behind the trailing side. This creates a high pressure on the leading side and a low pressure on the trailing side This difference of pressure exerts a lift force on the plate as indicated in the figure. With smooth flow, this force is proportional to the angle of inclination, the dynamic pressure, and the area of the plate. Since an abrupt change of flow is required at Point A separation could exist at such a point, and could alter the pressure distribution over the surface of the plate It the plate is shaped as indicated in Figure 2-2 b} , however, the acceleration of the water will be more gradual and separation will be avoided. The rudder is obviously a hydrofoil, designed to produce the lateral forces used in the control of the ship's head mg. The rudder force acts through the rudder 16 NAVAL SHIPHANDLING

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