240 report — 1859. 



The performance on the 6th of July was : — 



Speed of vessel 9*26 knots. 



Screw 11-29 knots. 



Slip per cent 17-91 knots. 



Revolutions per minute 101*74 knots. 



Indicator horse-power by diagrams 157'09 knots. 



lbs. per sq. in. 



Mean pressure in cylinder 12 - 28 



Mean pressure from vacuum 10 - 70 



Total pressure 22'98 



Mean pressure in boiler 15*80 



Now, the specific resistance of the 'Undine' at 9*26 knots, estimated as the 

 ' Erminia's,' by the empirical rule founded on Beaufoy's experiments, is 3809*4 lbs. 

 By the synthetical method it is as under, viz. — 



lbs. per lbs. in terms of 



square inch. effective power. 



Moving friction 1*40 342*89 



Additional friction for load 1*55 38094 



Resistance to rotation of blades 2*37 58250 



Slip in terms of effective power 1*65 405*10 



Total, less specific resistance 6*97 1711*43 



Total pressure and effective power 22*51 5517 



Specific resistance 15*54 3805*57 



The difference between 3805*57 and 3809*4 is not material in estimates such as 

 this. 



It will be seen, also, that there is a difference of 0*47 lb. per square inch in the 

 aggregate pressure as compared with that given by the diagrams. 



So far the two methods are in harmony ; but now I have to show a screw that is 

 not so tractable. 



The direct thrust at 11*29 knots is 7469*3 lbs., and the slip being 17*91 per cent., 

 the resultant is 5022*3 lbs., or more than 31 per cent, greater than the resistance of 

 the vessel. 



This, however, cannot be so, and the apparent excess must be accounted for. 



1 have already said that the screw must be sufficiently immersed, in order that its 

 thrust may be that which is due to its diameter and pitch. 



What is sufficient is yet an open question. The ' Erminia's ' was 2 feet G inches, 

 the ' Undine's ' only 1 foot 8 inches. 



I believe this to be the explanation of the anomaly. 



The apparent thrust of 5022*3 lbs. was really an effective thrust of only 3805*57 lbs. 

 in consequence of the rotation of the blade breaking up the surface of the water. 



This screw would produce, if sufficiently immersed, a resultant thrust of 3805*57 lbs. 

 at 9*26 knots, with a slip of 10*32, say IO5 per cent. 



The actual slip was 17*91 per cent. 



We have now reached one of the most interesting of the investigations, which, in my 

 former memoranda, I pointed out as worthy of the attention of the British Association. 



This is an investigation by experiment not difficult to accomplish, and yet I con- 

 clude it has not had the consideration of the naval authorities, as they continue to 

 give screws to their ships, which are only immersed about one-fourth to one-eighth 

 of their diameter, whereas the ' Erminia's ' was immersed about one-third, while 

 the 'Undine's' was about one-fifth. The due proportion of immersion will, I 

 believe, be found to depend somewhat on the speed of rotation. 



On the Manoeuvring of Screiv Vessels. 

 By Admiral Paris, C.B., of the Imperial French Navy. 

 The propelling properties of the paddles and of the screw are very different accord- 

 ing to the form, mode of acting, and especially the position of the propellers iji 

 the ship. 



