256 UEPOHT — 1875. 



into matlieuiatical formulfe), but also increases tlie pressure on the stern-post and 

 dead wood for a considerable distance forward by cliecking tLe velocity, the effect 

 of which on the turning of tlie ship would form a considerable item in the whole 

 action. This is in accordance with the principle of stream-lines so ably explained 

 by Mr. Froude, in which he shows that a diminished velocity is always accompanied 

 by increased pressure. When the stream has, however, set up a decided cun-ent 

 towards the outer edge, the velocity increases and the pressure diminishes. This 

 e.xplains the experimental fact before mentioned, that the fore part of the rudder is 

 more efficient than the after part. In accordance with this view, it is understood 

 that some ships in the royal service are being fitted with the balanced rudder, so 

 arranged that, when under canvas only, not only is the fore part of the rudder 

 turned into a stern-post, and the blades of the screw made to act as dead wood, 

 but additional dead wood is made to run out under the counter, so as entirely to fill 

 up the cavity before the screw. 



There are two conditions in the putting over of the rudder to be considered. At 

 the first moment the axis of motion, or instantaneous axis of rotation, is not the 

 centre of gravity, but a point determined as follows :— The first impulse in the 

 rudder is to produce a pressure on it perpendicular to its surface, which on ordinary 

 mechanical principles is equivalent to au impulse applied to the ship at its centre 

 of gravity in a parallel direction, and a couple to turn it round the centre of gravity. 

 In fig. 2, Plate XIII., A is the instantaneous axis, thus formed : B G A is drawn per- 

 pendicular to the direction of the water-pressure on the rudder ; G D parallel to 15 C, 

 and equal to the ship's outer radius of gyration round a vertical axis through G ; 

 D A is drawn at right angles to B D, cutting B G produced in A. Then, evidently 



GD^ R2 

 by the construction, G A =pT> =7Tt> gives A tlie instantaneous axis. 



This is only true at the first moment. The ship afterwards shifts laterally until 

 the lateral resistance so created becomes equal to the lateral force on the rudder, 

 and there remains a couple turning the ship round, whose force is the resistance on 

 the rudder, and the arm the distance between the centre of effort on tlie rudder 

 and the centre of lateral resistance on the ship. This latter point is almost always 

 before, and sometimes very considerably before, the middle point of the ship's 

 length. Hence evidently the stern is the most effective place for tlie rudder, as no- 

 Avhere else can this arm be so large ; and hence also is the manifest disadvantage 

 of a rudder raking forward from the top downwards, as is sometimes the case m 

 vaehts, which arrangement not only diminishes the arm of the turning couple, 

 but practically increases the immersion in consequence of the existence of a verti- 

 cally resolved part of the water-pressiue on tlie rudder. 



Sensibility to tlie helm («. e. quickness and readiness in a ship to go about) is 

 a most important quality. At the first moment the angular acceleration, which is 

 a measure of tliis sensibility, varies directly as the moment of the water-pressure 

 on the rudder, and inversely as the product of the weight of the ship and the 

 .square of its radius of gyration about a vertical axis through the centre of gravity. 

 The moment of water-pressure varies catcris paribus as the area of rudder surface. 

 When large ships wore first built the area of rudder surface was not increased in 

 proportion to the dimensions; hence large ships (cf/. the 'Achilles') were not 

 handy. Ilandiness is also diminished by putting heavy weights at the stem and 

 stern, and so increasing the radius of gyration. A short ironclad would thus carry 

 heavy bow and stern guns with less injury to steering qualities than a larger one. 

 The great difficulty under which naval architects lay on the first introduction of 

 large ships was not in making the rudder big enough, but in finding power enough 

 on the wheel to turn tliem through an efficient angle, rudders of even moderate size 

 being found to require the united efforts of 40 or even GO men to bring them over 

 tol8deg. or 20 deg., not to speak of .38 deg. or40deg. This difficulty has since been 

 overcome by applying steam or hydraulic pressure to the wheel. In' the latter case, 

 however, if Mr. Reed's explanation of the cause of the ' Bessemer ' not answering 

 her helm fast enough on steering into Calais harbour be accepted, there seems to 

 be the inconvenience that it requires an appreciable interval of time to move the 

 rudder ; and hence for instantaneous purposes {e. g. getting clear of an obstacle or 



