ON STEAMSHIP PERFORMANCE. 331 



In the following Table this has been rectified by taking a length of bow 

 belonging to the speed and also to the displacement. For example, the speed 

 of a ship is 11 -22 knots, or 18-96 feet per second ; to this belongs a bow of 

 70 feet, and an afterbody of 47 feet ; the ship is 336 feet long, hence there 

 remains for middlebody 219 feet. IS'ow the question arises, what is the 

 corresponding area of midship section ? because if the area of midship section 

 as given in the Tables were used, it would result in too large a displacement 

 with the above dimensions of fore, after, and middlebody. Eor this piirpose 

 we have as follows : — 



Area of midship section X{-ol+-5l' + 1" + -19G35B)='D x35. 



Substituting 7=70, ?'=47, l"=219, B=40-92, and D=3979, the result is 

 an area of midship section=487'7 square feet. By the Tables it is given as 

 653; and the calculated section is therefore 165-6 Q' too small, or the 

 vessel under the above dimensions sits lighter on the water. It is the lighter 

 midship section which has been used in the following Table. To find the 

 corresponding draught, taking the sides of the ship at the midship section as 

 nearly vertical, the difference is divided by the beam of the ship and the 

 quotient subtracted from the given draught. This results in a lighter 

 draught of 14-3 feet instead of 18-35 as given by the Table — a difference 

 of 4-04 feet. The difference is not always so large ; in some vessels it does 

 not amount to a foot. 



Twice this difference in draught has been deducted from the actual girth 

 of the midship section in order to find a girth suited for cplculating the 

 wetted surface ; in the example this gives G=54-72 instead of 62-8. 



The coefficient of diminished resistance has been taken to belong to the 

 bow as assumed above, and has been calculated by taking the square of the 

 sine of the angle formed by half the beam and the length. 



The skin has been calculated by the following formiila, 



(•58G + •84c?)(? + r) + l"G= skin or wetted surface, 



in which formula the lighter girth at the midship section and the lighter 

 draught must be substituted. 



The calculations of resistance due to ship's way, skin, Sec. have been made 

 in the manner explained in the second Keport. It may happen that the length 

 of bow belonging to speed is greater than the ship itself as given in the Tables, 

 as in the case of the ' Midge ' aud ' Penelope.' In such cases an afterbody 

 is added to the forcbody belonging to that speed, and with the two lengths 

 and the given displacement the area of midship section, draught, and girth 

 have been calculated. 



It will be seen that in the following revised Table most of the negative 

 quantities which appeared in the former Table have disappeared. The quan- 

 tities in the following Table belong to ships of the same displacement, same 

 speed, same length, same beam, and same indicated horse-power with the 

 actual ships ; whilst those in the former Table belong to ships of the same 

 displacement, same length, same breadth, same draught, and same indicated 

 power, but not the same speed, although that speed was introduced in the 

 calculations resulting for the most part in the negative quantities as found 

 for slip or engines. 



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