Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERING 



these resistances requires the expenditure of 

 definite, calculable amounts of energy; hence 

 each kind of resistance must be considered in 

 connection with efforts to increase the speed of 

 surface ships. 



Among the interesting hull forms that have 

 been developed (or at least considered) with a 

 view to increasing surface ship speed by de- 

 creasing one or both of these resistances are the 

 bulbous-bow form, the slender hull form, the 

 semi- submarine form, the hydrofoil, and the 

 various forms devised to utilize an air cushion 

 or an air bubble. Each of these forms has speci- 

 fic advantages and disadvantages; the search is 

 not for one perfect hull form but rather for a 

 variety of hull forms suitable for a variety of 

 functions. 



Bulbous-Bow Forms 



The bulbous-bow configuration is currently 

 used on most large naval ships. 2 The theory of 

 the bulbous-bow form is that the bulb will gen- 

 erate its own system of waves which will inter- 

 fere with the systems of waves formed by the 

 ship, thus reducing the resistance that results 

 from the wave-making of the ship. 



Although the bulbous-bow configuration is not 

 so very new either in theory or in application, 

 the concept of using much larger bulbs is a fairly 

 recent development. In theory, the wave- making 

 resistance of a ship could be substantially re- 

 duced by locating a large bulb just below the sur- 

 face and just forwardof the bow. In reality, there 

 are enormous design difficulties involved, since 

 the bulb must be specifically designed to inter- 

 fere with complex wave trains generated by the 

 ship. 



Some idea of the complexity of the problem 

 may be obtained by tossing pebbles into a pond 

 and observing the waves that are formed. When 

 two pebbles are tossed in together, each pebble 

 generates its own systemsof waves. Under some 

 conditions, the systems of waves tend to cancel 

 each other out; under other conditions, they tend 

 to enhance or amplify each other; and under still 

 other conditions, they interfere with each other 

 in a chaotic, unpredictable manner which is es- 

 sentially useless as far as achieving any cancel- 

 lation of waves is concerned. 



See the discussion of stem and bow structure in 

 chapter 2 of this text. 



In spite of design difficulties, the use of large 

 bulbs is of value in the design of certain tjrpes of 

 ships. Tests of models have shown that a ship 

 with a large bulb at the bow generates smaller 

 and smoother systems of waves than a ship with- 

 out such a bulb. However, this gain is not all free, 

 since the bulb adds frictional resistance to the 

 ship. 



Slender Ships 



A slender hull ship is basically similar to a 

 destroyer except that, for any given displace- 

 ment, the slender ship is about 30 percent longer 

 than the destroyer. The extra length is designed 

 to increase speed by decreasing the wave-making 

 resistance of the ship. A slender ship has less 

 static stability than a comparable destroyer, due 

 to the narrower beam in relation to length. Also, 

 the structural weight of the slender ship must be 

 considerably greater than the structural weight 

 of a comparable destroyer. 



Some slender ships have been designed utiliz- 

 ing a large bulb at the bow and another one at the 

 stern. Such ships are of interest because they 

 have improved longitudinal stability charac- 

 teristics as well as decreased wave- making re- 

 sistance. 



Semi-Submarine 



The semi- submarine is still another ap- 

 proach to the problem of increasing speed by 

 using a special hull form. The semi- submarine 

 is shaped somewhat like avery streamlined sub- 

 marine. The main hull of the semi- submarine 

 runs submerged, while surface-piercing struc- 

 tures or fins (hydrofoils) at the stern increase 

 the dynamic stability characteristics of the ves- 

 sel and provide a means for handling engine air 

 intake and exhaust. Because the semi-submarine 

 runs submerged, except for the fins (hydrofoils), 

 the craft avoids both storm waves and the self- 

 generated wave- making resistance of surface 

 ships. 



Hydrofoils 



The hydrofoil has been described as a cross 

 between a high speed boat and an airplane. The 

 craft has two modes of operation; it may run on 

 the surface of the water, as a conventional sur- 

 face ship, or it may fly on the foils with the hull 

 clear of the water. When flying, the hydrofoil is 

 supported clear of the water by the dynamic lift 

 of the foils. 



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