TRANSVERSE SECTIONS UPON RESISTANCE. 313 



like to enter the discussion on the method of presenting it. Having to make resistance calcu- 

 lation very frequently, I find that Taylor's method is very satisfactory, giving comparative 

 results, and is a yard stick by which we measure the resistance of vessels, taking into account 

 length, breadth, draught, longitudinal coefficient and the displacement-length coefficient. 

 These are the factors which enter into his calculations, but not the refinements which are the 

 results of Doctor Sadler's present investigations. 



In most of the vessels built and designed nowadays — that is, slow-speed vessels — the 

 skin resistance is probably two-thirds of the total resistance, and, therefore, we have attempted 

 to devise methods of calculating the wetted surface very accurately; in other words, we at- 

 tempted to determine two-thirds of the resistance very accurately, which allows great lati- 

 tude for the one-third remaining. 



By calculating by Taylor's method, resistance of models for which we have actual re- 

 sistance curves, we obtain coefficients showing the relation of our own models to Taylor's 

 series, and, applying Taylor's method and these coefficients, we can predict results with 

 accuracy. 



The Chairman : — If there is no further discussion, I will call on Professor Bragg to 

 close the discussion. 



Professor Bragg : — As there seems to be some misunderstanding regarding the use of 

 the formulae given in the appendix I will dispose of that question first. 



The effective horse-power necessary to drive a ship at a certain speed can be divided 

 into friction horse-power and residuary, or wave, horse-power. Given the data in this di- 

 vided form, it is possible to find the horse-power necessary to drive a larger or smaller, simi- 

 lar ship at a corresponding speed by simply multiplying the wave horse-power by a quantity 

 equal to the length-ratio raised to the 3.5 power, and the friction horse-power by a quantity 

 equal to the length-ratio raised to the 3.415 power. This friction horse-power should also 

 be multiplied by a factor involving the frictional coefficients of the two lengths. 



If one does not have the effective horse-power divided into these two parts, but merely 

 has the total of the two parts, the formulea given in the appendix can be used if the wetted 

 surface of the model ship is known. 



If one does not have the wetted surface of the model ship given, then a close approxi- 

 mation to the true E.H.P. can be obtained by using the simplified expressions successively. 

 This is illustrated by the following example where we start with a ship 425 feet in length and 

 work up by increasing the length 10 per cent in successive steps to a ship 624 feet long. The 

 speed-length ratio is 0.727, or equal to 15 knots for the 425-foot ship. The wetted surface 

 of the 425-foot ship is 36,350 square feet. The total E.H.P. in the table can be obtained by 

 either of the first two methods mentioned. 



Length of ship, feet 425 468 515 567 624 



Ratio of length to preceding length .... 1.1 1.1 1.1 1.1 



Friction coefficient 00908 .00905 .00904 .00903 .00902 



Wave horse-power 1,140 1,592 2,222 3,100 4,328 



Friction horse-power 2,160 2,990 4,148 5,755 7,990 



Total E.H.P ; 3,300 4,582 6,360 8,855 12,318 



E.H.P. obtained by using l.V' = 1.395 4,605 6,427 8,970 12,510 



When factor 1.39 is used 4,590 6,385 8,880 12,340 



