TRANSACTIONS OF THE SECTIONS. 239 



puted, because there are no satisfactory experiments on the subject. It is an esti- 

 mate, and may be in excess. 



The next element of the pressure to move the machinery, viz. 1*5 lb. per square 

 inch, is a little over the mean of certain trials made by my direction, of which the 

 diagrams are before me, the maximum being 1"63 lb., and the minimum 1*14 lb. 



To this no reasonable exception can be taken. 



The resistance to the rotation of the blades, 2*23 lbs., is calculated upon the basis 

 of such experiments as I have access to on the friction of water on iron, and on the 

 effective periphery of the screw. 



For this element, also, more precise experiments are needed, and it must be con- 

 sidered an estimate only. 



The slip, l - 22lb., requires no elucidation, except that it is what remains after 

 deducting the effect of the resistance to the rotation of the blades. The specific re- 

 sistance, equal to 20"78 lbs. per square inch on the piston, or in the convertible terms 

 of 1896 lbs. of effective power, may therefore be dealt with as a probable result, and 

 if so, the power of the screw is needlessly in excess of any resistance the ' Erminia' 

 is likely to offer ; and I have explained why it is so, viz. because it was designed to 

 produce a thrust at about 21 per cent, of slip, about 50 per cent, greater than the 

 resistance of the vessel in smooth water, which resistance, viz. 2763'5 lbs., turns out 

 to be 45 per cent, greater than the actual resistance. 



Hence the screw is capable of a thrust nearly double of what the ' Erminia ' re- 

 quires in smooth water. 



What, then, is the most suitable size and proportion of screw for this yacht ? 



I believe it will be found that the diameter should be as large as is consistent with 

 its being sufficiently immersed, but no larger ; and that the pitch should then be such 

 as to produce a thrust to balance the resistance under ordinary conditions at sea 

 with a moderate slip. 



If the screw be 8 feet diameter, then, to produce a thrust of 1896 lbs. at 6 knots, 

 the pitch must be 9'18 feet, and the slip about 13i per cent. 



But, under ordinary conditions at sea, the resistance will be increased, and it is 

 expedient to have a coarser pitch, in order that the thrust may balance the resistance 

 without excessive slip. 



If we assume the specific resistance at a mean between the two calculated results 

 before described, or 2329'75 lbs., the pitch must be 1T27 feet; and when the thrust 

 works up to this resistance, the slip will be about 20 per cent. 



The next vessel to which I must invite attention is the yacht of the Duke of Suther- 

 land, mentioned in my former paper. 



Full particulars of the performance of the ' Undine,' at the measured mile in the 

 Thames, on the 6th July, 1858, in Loch-Lochy on the 27th October, and in Loch- 

 Ness on the preceding day, have been laid before the " Steam-ship Performance 

 Committee" by Mr. M'Connell. 



The particulars of this vessel are : — 



Length of water-line 125 feet. 



Breadth, extreme 25 feet. 



n_. 'i.' « f F. 8'6 inch. 



Draught of water | A n . 10incll . 



Displacement about 294 tons. 



Area of greatest transverse section 154 - 33 sq. feet. 



Diameter of screw 7'10 inch. 



Pitch, &c 11-4 inch. 



Length 1*4 inch. 



Extreme breadth of blade 2'8 inch. 



Area of blade about 13 sq. feet. 



Immersion of periphery 1"8 inch. 



Diameter of cylinder 0*24 inch. 



Stroke 015 inch. 



Area of fire-grate 45 sq. feet. 



Plate surface 206 sq. feet. 



Tubes 820 sq. feet. 



