THE PROGRESS IN STEAM NAVIGATION. 

 ADVANTAGES OF INCREASED DIMENSIONS. 



577 



Before passing on it may be interesting to illustrate the o-ain in 

 economy ot propulsion resulting from increase in dimension, bv 

 mean.s ot the following table, which gives particulars of a number of 

 typical cruisers, all of comparatively recent design : 



Length ,-^.^,, 



Breadth , j^^ 



Mean draft ^^^ 



Displacement t,,,,^ 



Indicated horsepower for 20 knots ! (;, ooo 



Indicated horsepower per ton of displacement 



The figures given are the results of actual trials, and embody, 

 therefore, the efficiencies of propelling machinery, propellers, and 

 forms of the individual ships. Even so they are instructive. Com- 

 paring the fir.st and last, for example, it will be seen that while the 

 displacement is increased nearly eightfold the power for 20 knots is 

 only increased about 2.() times. If the same types of engines and 

 boilers had been adopted in these two vessels — which was not the 

 case, of course — the weights of propelling apparatus and coal for a 

 given distance would have been proportional to the respective powers; 

 that is to say, the larger vessel would have been equipped with only 

 2.6 times the weight carried by the smaller. On the otiier hand, 

 roughly speaking, the disposable weights, after providing for hulls 

 and fittings in these two vessels, might be considered to be propor- 

 tional to their displacements. As a matter of fact, this assumption is 

 distinctl}' in favor of the smaller ship. Adopting it, the larger \cssel 

 would have about 8 times the disposable weight of the smaller, while 

 the demand for propelling apparatus and fuel would be only 2.«i 

 times that of the smaller vessel. There would, therefore, l)e an enor- 

 mous margin of carrying power in comparison with disjjIaccnKMit in 

 the larger vessel. This might be devoted, and, in fact, was devoted, 

 partly to the attainment of a speed considerably exceeding 20 knots— 

 which was a maximum for the smaller vessel— partly to increased 

 coal endurance, and partly to protection and armament. 



Another interesting conii)arison may l)e made between vessels Nos. 

 4: and 5 in the preceding table by tracing the growth in power neces- 

 sary to drive the vessels at speeds ranging from lo knots u|) to 22 

 knots. 



It will be noted from the table that follows that up to the spcMMl of 



18 knots there is a fairly constant ratio between the powers re(juired 



to drive the t\vo ships. As the speeds are increased the largrr .ship 



o-ains, and at 22 knots the same power is iv.iuiivd in Ix.tli ships. '1 he 



SM 99- 37 



