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NATURE 



[April 14, 1892 



twenty years ago, that bilge keels of excessive size — 3 

 feet 6 inches deep, and 100 feet in length, on a vessel 172 

 feet long — had only an insignificant effect upon speed 

 throughout great differences of trim. 



It is strange that the mercantile marine should not yet 

 have adopted bilge keels, and obtained the undoubted 

 advantage they give in steadiness. The number of ships 

 that have them is comparatively few. There is an 

 almost universal opinion and prejudice against their use, 

 and the largest and finest passenger steamers have no 

 bilge keels. This is in spite of the fact that, in cases 

 where bilge keels have been fitted to try to check heavy 

 rolling— and they have been of suitable size and properly 

 placed — it has been found that the angles of rolling have 

 been reduced by nearly one-half. There is a prevalent 

 belief — which h;is no foundation in fact — that bilge keels 

 are very detrimental to speed. We have said that Mr. 

 Froude's experiments showed the contrary, even on 

 trials made in still water ; but it appears certain that at 

 sea any trifling loss of speed which still-water trials might 

 show would be more than compensated for by gain in 

 speed when the vessel is prevented from rolling through 

 large angles from side to side, and undergoing gre.it 

 changes of underwater form at every roll. Experience 

 with ships that have had bilge keels added after running 

 for some time without them shows that there has been 

 no appreciable difference of speed or increase of coal 

 consumption on their voyages. 



Another, and a more heroic, method of stopping or 

 reducing rolling would be to counteract the inclining 

 moment of the ship caused by the ever-changing inclina- 

 tion of the waves by an equal and opposite moment, 

 which would vary as the inclining moment varies. This 

 has been attempted at different times and in various 

 ways. It is essential to any degree of success, however, 

 that the opposing moment brought into operation should 

 be completely under control, so as always to act in the 

 manner and to the extent required. The attempts to ob- 

 tain a steady platform by freely suspending it, and making 

 it independent of the rolling of the ship, have failed — apart 

 from the practical difficulties of carrying out such an 

 arrangement on a large scale— because the point of sus- 

 pension oscillates when the ship rolls, and the platform 

 acquires a rolling motion of its own. Weights, made of 

 heavy solid material, which move from one side to the 

 other of a ship subject to the action of gravity and 

 rotation, fail because they cannot be made to act con- 

 tinuously in the manner required. 



A degree of success has been achieved by admitting 

 water into a suitably prepared chamber and leaving it 

 free to move from side to side as the ship rolls. This 

 has been done in several ships of the Navy, the case of 

 the Injiexible being that which was the most carefully 

 experimented upon. The movement of this internal 

 water follows the inclination of the ship, but it lags 

 behind, and thus tends to reduce the inclination. Its 

 effect can be regulated by the quantity of water admitted 

 into the chamber and by its depth. The Inflexible Com- 

 mittee state in their report that comparatively small 

 changes in depth increase or diminish largely the extinc- 

 tive pov/er of the water. For various reasons — one of 

 which is that while such a chamber is very effective in a 

 moderate sea it fails in a rough sea when the rolling of 

 the ship is greatest — and perhaps partly on account of 

 the destructive and disturbing effect of 100 tons or more 

 of water rushing from side to side of a ship over 60 feet 

 wide— these water-chambers appear to have gone out of 

 use in the Navy, and they have been given up in the City 

 of New York and City of Paris, which vessels were said 

 to be fitted with them when first built and placed upon 

 the Atlantic. 



Mr. Thornycroft has devised a means of checking roll- 

 ing by moving a weight, under strict control, from side to 



NO. I 172, VOL. 45] 



side of a vessel so as to continuously balance, or subtract 

 from, the heeling moment of the wave slope. It consists 

 of a large mass of iron in the form of a quadrant of a 

 circle, which is placed horizontal, with the centre on the 

 middle line of the vessel, and there connected with a 

 vertical shaft. The shaft is turned by an hydraulic 

 engine, which is very ingeniously controlled by an 

 automatic arrangement. The heavy iron quadrant is 

 swept round from side to side, revolving about its centre, 

 to the extent that is required to counteract the heeling 

 moment. In a paper read on the 6th instant before 

 the Institution of Naval Architects, Mr. Thornycroft 

 said : — 



" The manner in which the controlling gear works will 

 be better understood if we imagine a vessel remaining 

 upright among waves, while near the centre of gravity of 

 the ship we place a short-period pendulum suspended so 

 as to move with little friction ; this will follow the change 

 in the apparent direction of gravity without appreciable 

 loss of time, so that any change in the wave angle and 

 apparent direction of gravity cannot take place without 

 due warning, which will indicate the time and amount of 

 the disturbance. It is therefore only necessary to make 

 the motion of the ballast bear some particular and con- 

 stant ratio to the motion of this short-period pendulum 

 to keep the balance true. The inertia of a heavy mass 

 will cause some loss of time, as we can only use a limited 

 force for its control ; but it is possible to accelerate the 

 phase of motion and overcome this difficulty so far as to 

 get good results. 



" If, now, we imagine the ship to roll in still water, the 

 effect of the combination just described will be to balance 

 the ship's stability for a limited angle ; but this defect is 

 removed by the introduction of a second pendulum of 

 long period, which tends to move the ballast in the oppo- 

 site direction to the first one, and enables the apparatus 

 to discriminate between the angular motion of the water 

 and that of the vessel. 



•' I find, however, that the long-period pendulum is 

 rather a delicate instrument, and that its function can be 

 served by a cataract arranged so as to .always slowly 

 return the ballast to the centre, and this device has the 

 effect of accelerating the phase of motion, which, in some 

 cases, we also require. 



" We are therefore able, by very simple parts, to con- 

 struct an apparatus which will indicate the direction and 

 amount of motion necessary to be given to the ballast at 

 a particular time so as to resist the wave effort ; this 

 power of indicating may be converted into one of con- 

 trolling by suitable mechanism. The loss of time due to 

 inertia of the necessary ballast is not always unfavourable 

 when the apparatus has to extinguish rolling motion, the 

 greatest effect being obtained when the ballast crosses 

 the centre line of the ship at a time when it is most 

 inclined to the water surface, and this corresponds to a 

 quarter of the phase behind the motion of the short 

 pendulum." 

 ; The apparatus has been working for some time in the 

 steam yacht Cecile with very good results. What the 

 ; objections may be to applying it to the largest passenger 

 : steamers remains to be seen. A moving weight of some- 

 I thing like 100 or 150 tons would probably be required in 

 ! such vessels. The power necessary to control the move- 

 ment of the weight appears to be small, and Mr. Thorny- 

 croft's invention seems at any rate to show the way 

 towards obtaining the long-desired boon of substantially 

 reducing, if not checking altogether, the rolling of ships. 

 If it succeed in doing upon a large scale only a portion of 

 what is claimed for it in the way of anticipating and 

 counteracting the heeling effect of waves, without the 

 possibility of acting in an erratic or undesirable way, 

 we may hope to see it adopted some day in passenger 

 steamers. 



