258 REPORT— 1875. 



when the angle of one 1)lade was reduced, tliat of the other was increased to an equal 

 extent. The model was towed hy the nose with an arrangement which allowed 

 perfect freedom of motion in a vertical direction, but inexorably resisted any attempt 

 at lateral motion, while the stern was left perfectly free to move sideways in 

 either direction. The set of the rudder was varied until she followed neutrally; 

 and it was found that when she did this, the upper half required just 20 deg. of 

 inclination to balance the lower half with 10 deg., thus proving incontestably the 

 higher efficiency of the lower half. Possibly, however, an advantage inay accrue 

 from raising the centre of effort on the rudder, in some ca.ses by reason of the arm 

 of the turning couple, viz. the distance between this centi-e of effort and centre 

 of ship's lateral resistance becoming less inclined ; while this is inclined, there is 

 a tendency to heel, which may sometimes be troublesome, especially in sailing- 

 ships. 



Stream-lines modify the velocity and the direction with which particles of water 

 impinge on the rudder less perceptibly in screw-propelled than in sailing-ships. 

 Another phenomenon may be noticed, viz. that a vessel or model moving in the 

 direction of its lengtli is, on stream-line principles, in a position of unstable equi- 

 librium. She will always have a tendency to deviate on one side or the other — 

 on which seems a matter of indifference ; but this tendency will, when once ex- 

 hibited, go on increasing until she turns broadside on to her original direction. In 

 a Sixiling-vessel moving obliquely to the direction of the wind the relative position 

 of the centre of efi'ort of sail and of lateral resistance may oblige her to carry lee 

 or weather helm according as she is ardent or slack. In any case the less helm a 

 ship requires, the less is her way retarded, and tlie better she will sail. 



I can only glance <at the improvements which have been made in the forms of 

 rudder ; of these the most important, as far as screw-ships are concerned, is the 

 pivot-balanced rudder, in which, by placing tlie axis of rotation at or near the 

 centre of eilort, very little power is required at the tiller or wheel to bring it over 

 to any required angle. The most usual form is that fitted to the ' Bellerophon,' in 

 whicii the proportion of the fore and aft part (determined by experiment) is as 1 

 to 2 ; when under steam the column of water driven backwards by the screw acts 

 powerfully on each side and produces a stron": turning power. When under sail 

 the fore part is disconnected from the aft, and being fixed in the direction of the 

 keel, becomes a stern-post. The means of still further increasing this stern-po.st 

 or dead wood by a movable plane mider the counters now fitting in screw-ships has 

 already been adverted to. 



Lumley's rudder, in which the aft part is capable of being brought over by a 



by checking the velocity of the im- 

 pinging stream a very considerable additional pressure is brought to bear on the 

 fore part, and thus the steering quality is much improved. It was formerly fitted 

 in several of II.M. ships, but has been latterly discontinued in consequence of the 

 rusting and corrosion of the chains and self-acting machineiy under water. Chap- 

 lin's rudder, consisting of two plane surfaces fitted in the counter, inclined at about 

 an angle of .30 deg., the outer edges flush with the counter when not required, but 

 let down by suitable machinery when wanted for use, was tried on board PI.M. 

 ship ' Sultan ' and immediately condemned as useless, needs no further comment. 

 Other forms which have not been received at all favourable need no description. 



Another form, however, invented by Mr. Gumpel, seems very promisino-. Its 

 peculiarity is that the tiller-power applied in the ordinary way gives motion to a 

 crank working on a vertical axis moving freely on the rudder-blade at or about the 

 centre of water effort. The fore end of the blade is directed by a pintle moving in 

 a fore-and-aft slot, so that it always remains in a line with the keel. When put 

 hard over to an angle of 38 deg. 40 min. the crank is at right angles to the blade, 

 which requires the length of the crank to be to the distance of the vertical axis 

 from the fore edges as 4 to 5. The ratio of the forces required at the tiller of this 

 rudder to that of the ordinary rudder, to keep it at angles of 5 deg., 10 deg., 15 deg., 

 up to .3.5 deg., and 38 deg. 40 min., is given in the following Table, and is in- 

 dependent of the laws of resistance. 



