234 NOTES ON FUEL ECONOMY. 



seen that at 12 knots and at 2 feet trim by the stern, the normal running con- 

 ditions, a saving of 9 per cent results in effective horse-power ; careful calcu- 

 lations show that 6 per cent of this is due to the finer model, beam and 

 draught both being more in the bulb-nosed ship, leaving 3 per cent due to 

 the bulb. 



EXAMPLE NO. 7. 



It is well known that river steamers at certain speeds and in certain 

 draughts of water create excessive waves, causing bad erosion of the banks, 

 besides damage to shipping. 



A firm of owners recently had conducted for them experiments with 

 several models to see what could be done to reduce this wave under their 

 operating conditions. The tank finally showed them the best lines on which 

 to lay down the vessel for minimum wave-making, the speed being obtained 

 with less power in addition. This also reduced the slowing up necessary to 

 prevent damage to shipping along the banks. The top speed was obtained 

 in deep, smooth water for less power with the model that gave best results in 

 shoal water. The vessel is 410 feet long and of 8 feet draught. 



In 20 feet of water a saving of some 5 per cent resulted at working speeds. 



At low and moderate speeds up to about 18 knots, the best model for shoal 



water was the worst in deep water, but this could well be ignored in a river 



steamer. 



EXAMPLE NO. 8. 



A sound steamer of wide beam relative to draught, having flared guards. 

 The plans submitted for bids had a midship section shaped as in ^ , Fig. i , 

 Plate 94 ; whilst this is good for stabihty, it seems to be capable of considerable 

 improvement from the point of view of the coal pile. Recognizing the neces- 

 sity of keeping the same stability, section B was designed. At the expense 

 of a 2 per cent increase in the shell and framing of the vessel, an 18 per cent 

 saving can be made in the coal bill ; in other words the improved fines would 

 cut the power from about 2,750 to 2,250, indicated, at 16 knots on a deep- 

 water mile trial. The vessel was 270 feet long and of some 2,250 tons dis- 

 placement. 



Anyone who keeps up to date with the transactions of this and kindred 

 societies will know what valuable original research work has been placed on 

 record during the last few years. These notes make no claim to originality, 

 but merely attempt to show, from the point of view of the practical designer 

 of ships, the results presented in another form by our fellow designers work- 

 ing at experimental tanks and in colleges along more purely scientific lines 

 than are possible at a shipyard, where the contract delivery date generally 

 places a strict limit on the amount of preliminary investigation possible 

 before the design has to take concrete form in the mold loft and the shops. 



