Sec. 70.40 



SCREW-PROPELLER DESIGN 



G3I 



70.39 Avoiding Air Leakage with Inadequate 

 Submersion. For a designed-speed and a de- 

 signed-load condition, assuming no severe limits 

 imposed by the projected service of the vessel, 

 a tip submergence so inadequate as to permit 

 drawing air is a matter of ship rather than pro- 

 peller design. Nevertheless, limitations are often 

 imposed, and ships do have to propel themselves 

 with reasonable efficiency at drafts (and tip 

 submergences) less than the designed amounts. 



The operation of propellers under these con- 

 ditions is described in the references of Sec. 7L10 

 and is well summarized by W. P. A. van Lammeren 

 [RPSS, 1948, pp. 262-263]. From this reference it 

 ". . . appears that screws having blades with wide 

 tips and circular-back sections are more likely to 

 be free from air-drawing." 



70.40 Design Comments on Propellers for the 

 Supercavitating Range. If the cavitation noise, 

 erosion, and vibration are not serious, a certain 

 amount of either bubble or sheet cavitation, or 

 both, may be tolerated on heavily loaded screw 

 propellers, provided this loading represents the 

 maximum that may be encountered under any 

 condition of service. This is somewhat analogous 

 to loading a boat to the gunwales if it is known 

 that the boat is to encounter no waves. 



When pushing screw propellers to their limit of 

 ultimate performance, involving large — Ap's, 

 large real-slip ratios, and high velocities over the 

 backs of the blades, it becomes necessary, at 

 least in the present state of the art, to accept 

 sheet cavitation in the running range, and heavy 

 cavitation at that. This is the case with high- 

 speed and ultra-high-speed planing craft, par- 

 ticularly racing motorboats. 



It is described previously, in Sec. 23.12, that 

 the practical limit of intensity of the —Ap on 

 the suction side of the blade, from which most 

 of the Uft and thrust are derived, occurs at the 

 vapor pressure of water. As the cavitation number 

 is lowered, sheet cavitation covers more and more 

 of the back of each blade. The thrust falls off 

 rapidly and the rate of rotation increases, so that 

 the propeller serves no longer as a suitable or 

 efficient driving mechanism for the ship which 

 carries it. 



However, if the propelling machinery is able 

 to turn it fast enough, a rather unexpected situa- 

 tion develops. When finally the whole back area 

 is uncovered and exposed to vapor pressure in the 

 cavity a further increase in the rate of rotation 

 usually results in a slowly increasing thrust. 



This is shown by L. P. Smith for several models 

 of ship propellers, where, after reaching low 

 points in the thrust curves due to cavitation, the 

 thrust begins to rise steadily as the rate of rota- 

 tion is increased [ASME, Jul 1937, Vol. 59, pp. 

 409-431, esp. pp. 415-419 and Figs. 6, 7, and 11]. 

 It is also shown by R. W. L. Gawn for a pair of 

 motor torpedoboat propellers [NECI, 1948-1949, 

 Vol. 65, Fig. 14, p. 370], where at an advance 

 coefficient / of about 0.62 to 0.67, the values of 

 Kt begin to increase after reaching their minimum 

 values. In fact, for one propeller, the Kt value 

 at J = 0.6 is as high as at / = 0.75 and at 

 J = 0.83, with a mimmum value at J = 0.67. 



Under the conditions described, not only is the 

 pressure over the whole suction side of the blade 

 then reduced nearly to zero absolute, representing 

 the limit for service conditions, but the friction 

 resistance on the back of the blade is eliminated, 

 because moving water no longer touches it. 

 Under these conditions the propeller is fully 

 cavitating, and is said to be running in the super- 

 cavitating range. The flow over the blade then 

 resembles that of the right-hand diagram in 

 Fig. 23.1. 



There has been some theoretical work done in 

 Russia on the supercavitating propeller but, so 

 far as known, the only published references trans- 

 lated into English at the date of writing (1955) are: 



(1) Posdunine, V. L., "On the Working of Supercavitating 



Screw Propellers," INA, 1944, pp. 138-149 



(2) Posdunine, V. L., "Problems in Ship-Propeller Design," 



Soviet Science, Feb 1941; English transl. in SBMEB, 

 Feb 1946, pp. 69-70 



(3) Epshteyn, L. A., "On the Action of the Ideal Super- 



cavitating Propeller," Inzhenerniy Sbornik, 1951, 

 Vol. IX. This paper lists five previous Russian 

 references, published in the period 1943-1945. 



Posdunine, in his 1944 paper, speaks of experi- 

 mental proof for his claim to reasonably high thrust 

 and efficiency in the supercavitating range. Despite 

 his assurance that these data would be forthcoming 

 they appear never to have been published in English. 

 In the discussion of Posdunine's 1944 paper by 

 F. H. Todd, on p. 144, there are given the results of 

 variable-pressure water-tunnel tests at the NPL, 

 Teddington, on a screw propeller, when extended 

 into the supercavitating range. 



One feature of the design problem, upon which 

 much more remains to be done, is that of pre- 

 venting the flow, when altered by the sheet 

 cavity over the back of one blade, from adversely 

 affecting the pressure on the face of the following 

 blade. This is done by: 



