Principles of naval engineering - Historic Naval Ships Association

Chapter 25. -NEW DEVELOPMENTS IN NAVAL ENGINEERINGthese resistances requires the expenditure ofdefinite, calculable amounts of energy; henceeach kind of resistance must be considered inconnection with efforts to increase the speed ofsurface ships.Among the interesting hull forms that havebeen developed (or at least considered) with aview to increasing surface ship speed by decreasingone or both of these resistances are thebulbous-bow form, the slender hull form, thesemi- submarine form, the hydrofoil, and thevarious forms devised to utilize an air cushionor an air bubble. Each of these forms has specificadvantages and disadvantages; the search isnot for one perfect hull form but rather for avariety of hull forms suitable for a variety offunctions.Bulbous-Bow FormsThe bulbous-bow configuration is currentlyused on most large naval ships. 2 The theory ofthe bulbous-bow form is that the bulb will generateits own system of waves which will interferewith the systems of waves formed by theship, thus reducing the resistance that resultsfrom the wave-making of the ship.Although the bulbous-bow configuration is notso very new either in theory or in application,the concept of using much larger bulbs is a fairlyrecent development. In theory, the wave- makingresistance of a ship could be substantially reducedby locating a large bulb just below the surfaceand just forwardof the bow. In reality, thereare enormous design difficulties involved, sincethe bulb must be specifically designed to interferewith complex wave trains generated by theship.Some idea of the complexity of the problemmay be obtained by tossing pebbles into a pondand observing the waves that are formed. Whentwo pebbles are tossed in together, each pebblegenerates its own systemsof waves. Under someconditions, the systems of waves tend to canceleach other out; under other conditions, they tendto enhance or amplify each other; and under stillother conditions, they interfere with each otherin a chaotic, unpredictable manner which is essentiallyuseless as far as achieving any cancellationof waves is concerned.See the discussion of stem and bow structure inchapter 2 of this text.In spite of design difficulties, the use of largebulbs is of value in the design of certain tjrpes ofships. Tests of models have shown that a shipwith a large bulb at the bow generates smallerand smoother systems of waves than a ship withoutsuch a bulb. However, this gain is not all free,since the bulb adds frictional resistance to theship.Slender ShipsA slender hull ship is basically similar to adestroyer except that, for any given displacement,the slender ship is about 30 percent longerthan the destroyer. The extra length is designedto increase speed by decreasing the wave-makingresistance of the ship. A slender ship has lessstatic stability than a comparable destroyer, dueto the narrower beam in relation to length. Also,the structural weight of the slender ship must beconsiderably greater than the structural weightof a comparable destroyer.Some slender ships have been designed utilizinga large bulb at the bow and another one at thestern. Such ships are of interest because theyhave improved longitudinal stability characteristicsas well as decreased wave- making resistance.Semi-SubmarineThe semi- submarine is still another approachto the problem of increasing speed byusing a special hull form. The semi- submarineis shaped somewhat like avery streamlined submarine.The main hull of the semi- submarineruns submerged, while surface-piercing structuresor fins (hydrofoils) at the stern increasethe dynamic stability characteristics of the vesseland provide a means for handling engine airintake and exhaust. Because the semi-submarineruns submerged, except for the fins (hydrofoils),the craft avoids both storm waves and the selfgeneratedwave- making resistance of surfaceships.HydrofoilsThe hydrofoil has been described as a crossbetween a high speed boat and an airplane. Thecraft has two modes of operation; it may run onthe surface of the water, as a conventional surfaceship, or it may fly on the foils with the hullclear of the water. When flying, the hydrofoil issupported clear of the water by the dynamic liftof the foils.629