170 



KNOWLF.DGE. 



May. Mil. 



Therefore his designs are hmited in various 

 directions, and it is onh- b\- sliglit alterations in 

 the shape or in the flotation Hnes of the hull 

 that he can expect to lessen its resistance to the 

 water through which it is to he dri\en. The effect 

 of these alterations can be tried, of course, hv 

 building a large number of \essels : but evidenth- 

 the cheaper way, if it is equalh- trustworth)', will 



shape and resistance, a considerable effect on the 

 vessel's speed. One ma\- think of the resistance 

 offered hv a ship's hull against its forward move- 

 ment through tlu' water as being composed of 

 a frictional resistance and a wave- and eddv- 

 making rc-sistance. The first kind of resistance 

 is brought about hv the friction of the immersed 

 surface of the ship's hull w ith the water ; the 



/Vy i^i/iif pei-iitissiott of 



/'/:ysic'al Lahoratoiy. 



Figure 3. Model-niaUing Appar.atus. 



l)e to experiment with \arious types of models, 

 which being made (for example) of wax. can actuall}' 

 be altered to different shapes. 



The larger the vessel of which the most desirable 

 shape has to be ascertained, the more numerous the 

 difficulties. All sorts of new factors of resistance to 

 passage through the water are introduced. The hull 

 of an ocean liner, or an ocean tramp, or a cruiser, 

 is not smooth. It is studded with bilge-keels, pitted 

 with shaft-escapes, and has other openings and 

 projections in its sunken sides. The rudder, as it 

 moves, causes resistance: and the rudder has to be 

 protected. The propellers also exercise, by their 



second b\- the formation of waves at the bows 

 and of waves and eddies around the stern. 



These coefficients can be found empirically by the 

 aid of the wax models. The frictional resistance 

 depends on the size and nature of the immersed 

 surface, on tin- dcnsit\- ot the water, and on 

 the speed at which the ship or its model is 

 dri\en or towed. As the frictional coefficients 

 for the different kinds of surfaces of ship and 

 model are known — they varj- not only with 

 the nature but the lengths of the immersed 

 surfaces — the fric-tional resistance may be ascer- 

 tained matluinatuiilK' without difficultv. If the 



