ROLLING OF SHIPSTOPIC 11DESCRIBE THE EFFECT ON GM OF ROLLINGThe 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# CONTINUATIONTo understand the reason why a large GM produces large amplitudes, compare a stiff ship with a raft. Theraft 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. ,ut-and 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 ROLLINGThe 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. COTINUATIONThe 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 displacementaffects 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 PERIODWith 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 CONTINUATIONThe 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 OCCURThe 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 EXPERIENCEDDESCRIBE HOW BILGE KEELS, ANTI-ROLLING TANKS AND STABILIZER FINS REDUCE THE AMPLITUDE OF ROLLINGMany 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