Sec. 67.27 



UNDERWATER-HULL DESIGN 



541 



ically should be clear of the disc of the after 

 propeller. This means that if the general ship 

 flow runs parallel to the ship centerline there can 

 be nominal overlap of the propeller discs, with a 

 small negative disc clearance, because of the 

 contraction in the outflow jet of the forward 

 propeller. In the Omaha cla,ss of quadruple-screw 

 light cruisers of the U. S. Navy, designed in 

 about 1919, there was negative disc clearance of 

 this kind but the vessels ran successfully for 

 many years without vibration troubles. Similar 

 difficulties reported on other quadruple-screw 

 vessels with offset wing propellers having positive 

 disc clearances are beUeved due to excessive 

 elasticity of the thrust-bearing foundations within 

 the ship. 



When a vessel with offset propellers turns with 

 a drift angle, the propeller outflow jets change 

 shape, rather drastically if the turn is a tight one. 

 Undoubtedly in these cases the outflow jet of the 

 forward or wing propeller on the outside of the 

 turn passes through the disc of the inboard 

 propeller. Two propellers on the same side of the 

 ship can almost never be given sufficient disc 

 clearances to avoid this interference. 



67.26 Adequate Propeller-Tip Submergence. 

 As a general rule the greater the tip submergence 

 the better, until it reaches a value equal approxi- 

 mately to the propeller radius R. This is the 

 standard or minimum tip submergence used for 

 open-water propeller tests in model basins. There 

 is no need of increasing it further unless to 

 eliminate or reduce cavitation, or to insure 

 adequate submergence when the ship is pitching 

 heavily during wavegoing. 



The tip submergence required for any load or 

 operating condition, indicated in Figs. 33. B, 

 33. C, and 33.D, is a function of the: 



(a) Thrust-load factor at which the propeller is 

 intended to work. The greater the value of Ctl , 

 the greater the submergence needs to be. 



(b) Advance ratio or slip ratio, related to (a) 



(c) Radial distribution of circulation along the 

 propeller blade, particularly near the tip. Large 

 — Ap values near the tip call for a water layer of 

 appreciable thickness over the propeller. 



(d) Amount of shielding from air leakage which 

 can be expected from the hull at the running 

 attitude of the ship 



(e) Increased (or decreased) nominal tip sub- 

 mergence due to a wave crest (or trough) over 

 the propeller position. 



Numerical values or ratios are not available 

 for estimating the proper or minimum tip sub- 

 mergence as functions of (a) and (b). It is known 

 only that the greater the thrust loading, the 

 greater the advance ratio, and the greater the 

 circulation near the blade tips, the greater is the 

 — Ap on the back of the blade tips and the thicker 

 must be the superposed water layer to prevent 

 air leakage. Regions of high wake velocity 

 close to the water surface augment (a), (b), and 

 (c) locally and call for good shielding. 



If the hull shape is such as effectively to shield 

 the propeller from air leakage, say in the form of 

 a wide transom stern over a single wheel, the 

 nominal tip submergence can be small, approach- 

 ing zero. In fact, under the tunnel stern on a 

 shallow-draft vessel the tip submergence is 

 definitely negative. 



For the thrust loadings and advance ratios on 

 low- and medium-powered ships having propellers 

 of adequate diameter, the increase in water 

 depth due to the wave crest which forms at the 

 stern when running at designed speed is usually 

 sufficient to shield the wheel. Under these con- 

 ditions, the nominal tip submergence may also 

 be small. 



When maneuvering rapidly, such as during 

 crash-backs, the pressure differentials around the 

 upper blade tips become extremely large. Shield- 

 ing by the hull, as in tugs, is the only effective 

 preventive against air leakage. 



The degree of submergence — or emergence — 

 expected during wavegoing is considered sub- 

 sequently in Part 6 of Volume III, together with 

 its effect on propeller performance. 



67.27 Design for Minimum Thrust Deduction. 

 The manner in which a thrust-deduction force 

 is exerted on a hull, either inside the limits of the 

 inflow jet ahead of the propeller or inside those 

 of the outflow jet astern of it, leads to the con- 

 clusion that the transverse projected areas within 

 these jet limits should be a minimum. This is 

 accomplished for a skeg carrying a screw propeller 

 by keeping the skeg as thin as possible for at 

 least 2 diameters ahead of the wheel, within the 

 limits of an imaginary cyUndrical surface pro- 

 jected from the screw disc along the propeller 

 axis, described previously in Sec. 33.2 and 

 illustrated in Fig. 33.A. A large bossing carrying 

 a wing propeller is likewise as thin as possible 

 consistent with stiffness as a shaft support. For 

 a tunnel within which a screw propeller is mounted 

 the roof of the tunnel is not to drop too sharply 



