S P I H S F O G N I L 1 1 L O P I C R O T
DESCRIBE THE EFFECT ON GM OF ROLLING The first point to be made clear is that GM is by no means the only factor involved in the manner in which a ship rolls, although it is an important one. We know that with increased metacentric height a vessel will roll more quickly, i.e., her period of roll in seconds will be short. The effect of GM on amplitude is less well known. It should be clearly recognied ship!s officers that a stiff ship in heat! weather not only has a short period of roll but also a large amplitude. "onversely, a tender ship is apt to have a long period of roll with a small amplitude. "onsider the effect on angular acceleration of a short period and a large amplitude roll#
DESCRIBE THE EFFECT ON GM OF ROLLING The first point to be made clear is that GM is by no means the only factor involved in the manner in which a ship rolls, although it is an important one. We know that with increased metacentric height a vessel will roll more quickly, i.e., her period of roll in seconds will be short. The effect of GM on amplitude is less well known. It should be clearly recognied ship!s officers that a stiff ship in heat! weather not only has a short period of roll but also a large amplitude. "onversely, a tender ship is apt to have a long period of roll with a small amplitude. "onsider the effect on angular acceleration of a short period and a large amplitude roll#
CONTINUATION To understand the reason why a large GM produces large amplitudes, compare a stiff ship with a raft. The raft as a type of vessel represents the highest point of stiffness. $nd how does a raft behave in waves% &oes it not assume e'actly the slope of the seas, thus inclining to large amplitudes% The stiff ship attempts to do the same. (he is quick and alive, responding immediately as a wave rolls up her side and under her bottom.
CONTINUATION.. It is apparent then that a moderate GM should be the ob)ective of the well*informed ship operator. The racking stresses associated with a stiff ship are to be shunned, and equally the danger of waves breaking on deck. +nly e'perience with your ship can inform you, the ship!s officer of the best possible GM. In general, however, and in the absence of knowledge to the contrary, the ship will be better off with a moderately small rather than with a large GM.
CONTINUATION.. The stiff ship move up and down about like a cork. The tender ship, on the contrary, is slow. (he lags behind the motion of waves and thus tends to roll to lesser amplitudes. utand this is important-the wave mounting the side of the tender ship finds it easier to sweep on up and over the bulwarks, damaging topside equipment and structure as well as endangering the lives of personnel.
EXPLAIN HOW INCREASE OF DRAUGHT AND OF DISPLACEMENT INFLUENCE ROLLING The e'tent to which a ship is immersed has an important bearing on the way in which she will roll. The primary reason for this is related to the structure of ocean waves. +ceanographers have long known that the surface slope of a wave is much steeper than its subsurface slope, the slope becoming progressively flatter with increase of water depth. ven a small increase in water depth has a pronounced effect.
COTINUATION… The student should remember that the true criterion of stability is the righting moment /displacement ' righting arm0. Therefore, an increase of displacement, all other things being equal, increases the true stability of the ship. $ ship at the lighter drafts, requires a larger GM to offer a proper amount of stability, while a more heavily laden ship can afford to have a smaller GM.
CONTINUATION.. 1owever, an increase in stability due to an increase in the displacement affects rolling differently from an increase in GM. The heavier ship tends to have an easier motion. ut once again, only e'perience with a given ship can offer quantitative answers to the effect of draft and displacement on rolling.
DESCRIBE HOW THE DISTRIBUTION OF MASS WITHIN THE SHIP AFFECTS THE ROLLING PERIOD With a given displacement and GM, the weight of cargo or ballast aboard a ship can be distributed in many ways. 2or e'ample, some weight can be shifted up from the lower to upper levels and some can be shifted down to the lower hold without changing either GM or displacement. +r, weight can be shifted out into the wings of a compartment rather than concentrated on the centerline. ither of these changes would distribute the mass of the ship!s displacement away from the ship!s center of rotation and increase what is known technically as the 3mass moment of inertia.3
CONTINUATION… The effect on rolling is not unlike that of the tightrope walker who, when he attempts his routine without a long pole, must )itter back and forth rapidly /but over a small arc0 in order to maintain his precarious equilibrium. When equipped with a pole, his movements are much slower, but he must lean to the side to a greater angle.The modern ship tends to have a large built*in mass moment of inertia compared with ships of thirty or more years ago, since the superstructures are heavier and the double bottom and deep tank capacities have been increased. To the e'tent that this is true, the modern ship can afford to sail with larger GMs han earlier ships since their motion is dampened by mass moment of inertia.
EXPLAIN WHAT SYNCHRONIZATION IS AND THE CIRCUMSTANCES IN WHICH IT IS MOST LIKELY TO OCCUR The practical implications of this very important phenomenon will be discussed in more detail here. In the ma)ority of times when a ship rolls violently, it is because the ship!s natural rolling period is synchronied with the apparent wave period. $nd, it is this connection that the case for the moderate or small GM is enhanced. 2or it is a fact that the wave periods apt to be encountered on the oceans of the world are much more likely to coincide /or nearly coincide0 with the rolling period of a stiff rather than with a tender ship.
CONTINUATION.. To put it another way, the 4* or 56*second roll associated with a large GM of the usual merchant ship is similar to a great many of the apparent wave periods which the ship will encounter, while the 57* or 58*second roll associated with a moderately small GM will hardly ever find a matching 57* second wave period. 1owever, one warning should be given. It is possible that a tender ship may find herself synchroniing, and in this case the resulting heels can be severe. /(uch as the case of a tender ship at sea with e'tremely large waves on the quarter.0 ut even in this case, a smaller change of course, speed, or GM is necessary to eliminate the synchroniation than would be the case with a stiff ship.
DESCRIBE THE ACTIONS TO TAKE IF SYNCHRONIZATION IS EXPERIENCED
DESCRIBE HOW BILGE KEELS, ANTIROLLING TANKS AND STABILIZER FINS REDUCE THE AMPLITUDE OF ROLLING Many devices have been designed to reduce the amplitude of ship9s rolls, and in some cases to increase the period of roll. The principal factor leading to dangerous and uncomfortable rolling is the angular acceleration, so that reducing the amplitude of roll does not in itself lead to a more comfortable and seaworthy ship. "onversely, if the period of roll can be increased, this will improve rolling characteristics even though the amplitude is not decreased.
CONTINUATION.. :ets us consider some of the antirolling devices which have been developed and analye their advantages and disadvantages. In this connection, it should be noted it is not beneficial to eliminate rolling entirely since the yielding of a vessel to the tremendous pounding of the seas is a necessary characteristic of a seaworthy vessel. Too much success in dampening rolling may result in serious shocks and structural damage.
ilge ;eels The installation of fins or 3keels3 at or near the turn of the bilge has been known to be beneficial for many years. 2roude was the first, however, to show their effectiveness e'perimentally, around 5<=6. (ince then almost all large vessels have been fitted with bilge keels. :ongitudinally, bilge keels e'tend from >7 to =7 percent of the length and vary in depth from less than a foot to about ? feet.
$lthough the effectiveness of the bilge keel increases with depth, practical considerations limit keel depths. These considerations include the necessity of keeping the keels within the e'treme depth and breadth of the vessel@ difficulties in drydocking@ necessity of limiting the stress on the plating of the keel and thus reducing the probability of leakage where the keel is attached to the hull@ and increase in hull resistance and the consequent loss of speed or increase in horsepower.
CONTINUATION.. ilge keels derive their roll*quenching ability by setting in motion a mass of water which is carried along by the vessel, thus increasing virtually the mass moment of inertia of the vessel. The eddying of water behind the keel results in a loss of energy which otherwise would go into an increase in the amplitude of the roll. $lso, not only do the normal pressures increase on the leading side of the keels, but the reduction of velocity of water on the following side leads to an increase in pressure with components acting around the a'is of rotation of the ship in a direction opposite to the ship!s rotation.
ilge keels increase in effectiveness with amplitude of roll producing greater periods of roll than would otherwise e'ist at these angles of roll. 1owever, the principal purpose of bilge keels is to reduce the amplitude of roll. ilge keels increase the period of roll only slightly, normally. ilge keels also increase in effectiveness with speed of the vessel, $nother factor influencing the effectiveness of bilge keels is the mass moment of inertia of the vessel. /The less the mass moment of inertia, the greater the effectiveness.0
'periments with different forms of bilge keels have shown that discontinuous keels are more effective than continuous keels. Modern practice dictates the installation of bilge keels along the streamlines in the vicinity of the bilge. This prevents cross*flow across the keels and a consequent increase in hull resistance. With this practice, bilge keel resist* ance is almost entirely frictional and is thus held within acceptable limits.
CONTINUATION.. ANTIROLLING TANKS "onsiderable attention has been given in the past to the use of antirolling tanks, and various types of installations have been made with varying degrees of success. 'perimental work in this field is continuing. $round 5<=A, antirolling tanks cook the form of simply creating free surface in tanks located in the upper decks of the ship. These so*called 3water chambers3 operated, obviously by reducing the stability of the ship but were dangerous in some situations, especially if the period of the water in
CONTINUATION the tank and the period of roll of the ship were synchronied. 2or this reason, this type of antirolling tank was abandoned progress in the creation of antirolling tanks since then took two directionsB Conactivated and activated tanks. The nonactivated tanks are usually an application of the D*tube principle with honontal and vertical ducts.
CONTINUATION.. In these nonactivated tanks, the water can only move 3downhill,3 the theory being that as the ship rolls the water will move to the low side, achieving its ma'imum heeling moment when the ship starts to roll back to the other side, creating a moment which acts in opposition to the direction of roll. In these tanks care must be taken to provide proper dimensions to the ducts as well as proper venting at the top of the vertical ducts.
CONTINUATION.. $nother form of nonactivated tank has a pair of narrow tanks about 5<6 feet in length located around amidships with appro'imately half of the tank above the load waterline and half below the waterline. The tanks are open to the sea at the bottom and vented at the top. Thus, as the ship rolls, the tank on the low side fills up and as the ship rolls back, the full or almost full tank creates a heeling moment in opposition to the direction of Eoll In the 5486s a nonactivated installation called 3flume stabiliation3 was remarkably successful and is aboard many merchant vessels today.
CONTINUATION… $ctivated antirolling tanks have used various methods to obtain a more precise control over the movement of water in D*tube arrangements. $pplications using antirolling tanks in the activated mode are generally limited to military applications.
CONTINUATION.. ANTIROLLING FINS $ntirolling fins have been considered for use in dampening ship rolling since before the turn of the century. $ntirolling fins are rudderlike in appearance and pro)ect out from the side amidships )ust above the bilge keel. In the &enny*rown installation they are retractable so that they can be withdrawn into a pocket in the ship when they are not in use. The fins operate by creating a couple opposing the roll of the ship.
CONTINUATION… 2or e'ample, if the ship rolls to starboard the fins are angled so that the forward side of the starboard fin is pointing diagonally upwards and the port fin is pointing diagonally downwards. Then, the forward motion of the ship causes the water to e'ert an upward force on the starboard fin and a downward force on the port fin. This couple tends to roll the ship to port and thus offsets the starboard roll. The movement of the fins are controlled by sensitive gyroscopes. +n military craft anurolling fins can be used to counter angles of heel created by high speed turns aboard aircraft carriers.
CONTINUATION.. $ntirolling fins perform as well as antirolling tanks in eliminating roll amplitude. Their disadvantage is that the vessel must be moving before it benefits from the antirolling fins, whereas antirolling tanks work well even with the dead in the water, i.e., a merchant ship that has lost its plant. $nother principle disadvantage is the increase in hull resistance.
CONTINUATION.. $lthough the antirolling fins do not contribute very much to the deadweight of the ship, their use does increase hull resistance, and therefore fuel consumption. In these days of soaring fuel costs antirolling fins would most likely be found aboard military vessels and specialied ships where their unique abilities are more optimum than an antirolling tank installation.
GYROSCOPIC STABILIZERS Gyroscopic stabiliers have been installed on many vessels, but due to the deadweight they consume and the space they require they have been used mainly aboard passenger type vessels. Gyroscopic stabiliers operate on the principle of gyroscopic inertia, the characteristic of a gyroscope that resists motion. "onsider the followingB The largest gyroscopic stabilier was installed on the (( "onte di (avoia.
STATE THAT A SHIP GENERALLY HEELS WHEN TURNING The installation, consisting of three 5? foot diameter rotors weighing ?AA tons was successful in reducing rolling, but it is doubtful whether the use of such stabiliers will be practical because of the high cost of purchasing and operating as well as the loss of deadweight and space. "onte di (avoia was virtually restrained from rolling because of the brute force of gyroscopic inertia.
STATE THAT A SHIP GENERALLY HEELS WHEN TURNING
STATE THAT, WHILE TURNING, THE SHIP IS SUBJECTED TO AN ACCELERATION TOWARDS THE CENTRE OF THE TURN
ACCELERATION ACTS AT THE UNDERWATER CENTER OF LATERAL RESISTANCE, WHICH IS SITUATED AT ABOUT HALF-DRAUGHT ABOVE THE KEEL
STATE THAT THE FORCE IN THE ABOVE OBJECTIVE IS CALLED THE CENTRIPETAL FORCE, GIVEN BY F MV!"R WHERE# M MASS OF THE SHIP IN TONNES V SPEED IN METRES PER SECOND R RADIUS OF TURN IN METRES F CENTRIPETAL FORCE IN KILONEWTONS
THE CENTER OF LATERAL RESISTANCE CAN BE REPLACED BY AN E$UAL FORCE ACTING THROUGH THE CENTRE OF GRAVITY AND A HEELING COUPLE E$UAL TO THE FORCE X VERTICAL SEPERATION BETWEEN THE CENTRE OF LATERAL RESISTANCE AND THE CENTRE OF GRAVITY
