ECONOMICAL CARGO SHIPS. 93 



nomical length. These figures, of course, assume the ship to have a capacity cargo, 

 and even a small reduction in this cargo would tell heavily against the larger ships. 



These curves completely justify the building of tramp ships between 350 feet 

 and 400 feet in length, with a sea speed of 10 knots. 



(d) Form of Ship. — To compare the effect of a single design run at various 

 speeds, with designs worked up for the individual speeds, calculations were made 

 which are plotted on Fig. 5, Plate 40. The figures for these calculations are shown 

 in columns 32 to 39, Plate 39. 



They show that a vessel designed for a speed of 13 knots, and powered for that 

 speed, will earn more for her owner running up to full speed than at a lower run- 

 ing rate. On the other hand, a vessel designed for 10 knots, with power good for 

 a speed of 13 knots, shows a falling off in earning at speeds over about 11 knots. 

 This diagram clearly shows the advantage of having a ship carefully designed to 

 work under specific conditions. 



{e) Value of Model Experiments. — Fig. 20, Plate 41, shows the actual loss in 

 profit for the 500- foot ship where the power to drive it is 10 per cent above the 

 minimum. This 10 per cent seems large, but I assure you it is a very modest esti- 

 mate of the increase of power necessary in a model designed without the use of ex- 

 perimental data. Even careful students of ship resistance might produce a ship 

 design of this kind. This year, for example, we were able to lower the resistance 

 of a 400-foot ship at 10 knots by 8 per cent through increasing the displacement by 

 i^ per cent ; and in another case, with two ships of identical size and fullness, a 

 slight change in the curve of areas of afterbody made 6j4 per cent difference in 

 resistance without change of displacement. 



The cost to the shipowner of this extra 10 per cent is one of the astonishing 

 features of the figures compiled. 



At 10 knots the extra power for the 500- foot ship is 325 horse-power, and this 

 gives an actual loss in earning capacity of $16,636.00 per annum, which, at 5 per 

 cent compound interest, represents in twenty-five years 79 per cent of the cost of 

 the ship, or, in this particular case, $1,610,250.00. At 13 knots the cost is even 

 greater ; the 583 additional horse-power costing the ship operator a loss in profit of 

 $24,789.00 per annum, equal to $2,782,230 for the 500-foot 13-knot ship in a life of 

 twenty-five years. 



(/) Detention in Port. — Fig. 10, Plate 40, shows for the 500- foot and 350-foot 

 ships the difference in realized profits if these ships are detained twice as long in 

 port as the ship with which they are compared. Every shipowner is aware that the 

 earning capacity of the ship is measured by its time at sea, but these figures will 

 help to emphasize the necessity for greater care being given to the improvement 

 of the methods of loading and unloading ships. 



In Fig. 19, Plate 41, the time at sea is expressed as a percentage of the year, and 

 it is shown that the slow-speed ship can remain at sea for a higher percentage of its 

 life, and to this largely is accountable the superior earning capacity of this type of 

 ship. 



