376 HYDRODYNAMICS IN SHIP DESIGN Sec. 60.11 



Figs. 34.M and 34. N give some not-too-recent (3) A variation is to be expected with type of 



values of tjo for several types; Figs. 59.A and 59. B vessel because of the characteristically different 



present more recent data on a few different types, hull shape, relative hull and propulsion-device 



In both series of diagrams the propeller efficiencies position, and nature of flow at the propulsion 



are based upon the thrust-load factor Ctl ■ device. A free-running, single-screw tug with a 



For a given ship resistance to be overcome, or short, chubby hull and a propeller abaft it would 



a given propeller thrust to be produced, with be expected to have a different rjp than a long, 



constant wake and thrust-deduction fractions, slender, high-speed, single-screw patrol vessel 



the thrust-load factor Ctl increases and the with its propeller more or less under the hull, 



actual propeller efficiency ijReM diminishes with (4) Within a single category as to number of 



decreasing diameter, while the rate of rotation n propulsion devices and within a single type of 



increases. This is because the thrust-load factor device, assuming a constant thrust T and a 



Ctl = T/{Q.5pA^Vl) increases as Ao diminishes, constant wake fraction w, the value of j?^ dimin- 



the real or working efficiency tjo decreases as ishes with a decrease in the thrust-producing 



Ctl increases, and t/p decreases with tjo . Since area of the device, corresponding to the disc area 



the advance coefficient J usually decreases as ^o of a screw propeller. The reasons for this are 



r/o decreases, indicated by Fig. 78. H, and since explained in a preceding paragraph, 



n = y^/(Ji)), a reduction in the thrust-producing (5) For a good hydrodynamic design of both 



area causes both J and D to diminish, and ship and screw propeller, based upon data such 



results in an appreciable increase in the rate of as set forth in this book, the following values of 



rotation n. The foregoing accounts for the moder- propulsive coefficient rip should be achieved, on 



ate falling off of propulsive efficiency with increase the basis of a clean, new hull, at the designed 



of rpm, revealed by W. P. A. van Lammeren, speed: 



L. Troost, and J. G. Koning [RPSS, 1948, Figs, (i) Single-screw vessels of the merchant and 



193 and 194, pp. 285-286]. generally similar types, with speed-length quo- 



The effect of the wake and thrust-deduction tients or fatness ratios in or near the design lane 



fractions, singly or in combination, is exceedingly of Fig. 66.A 0.82 to 0.72 



complex, so much so that no general or detail (ii) Twin-screw vessels of modern (1955) mer- 



rules have been formulated to predict their effect chant and similar types, having fatness ratios as 



upon propulsive efficiency. Characteristics of the in (i) preceding 0.73 to 0.65 



flow at the propulsion-device positions are (iii) Triple-screw vessels; no adequate systematic 



related to the relative positions of the device and data for vessels having three propellers nearly 



the hull. Not enough is known of these effects, alike, or for vessels having larger center wheels 



both physically and analytically, to predict absorbing more power than each of the wing 



reliable and precise values of -qp in the pre- wheels 



liminary- or contract-design stage. (iv) Quadruple-screw vessels of the liner 



Nevertheless, based upon the reasoning in the type 0.65 to 0.60 



foregoing, upon data derived from the trials of (v) Single-screw tunnel or arch-stern vessels 



many ships, and upon experience, a few prediction 0.68 to 0.55 



guides are set down: (vi) Double, triple, and quadruple-screw tunnel- 



r-,\ -r^ u- J- u 11 ii stern vessels for operation in shallow and re- 



(1) For ships driven by screw propellers, the , . , , ^ ^^^. ^ .. 



number and consequently the position(s) of the f'}^''^^'^'^ \\i ■■.■.- ^-^^ to 0.45 



wheels carried by each puts them in different i^"^ ^°'' ^^*; '^"^^^ ^f^^ '^'^. ^' ^^^^ ^"^ 



„ , •„„ f 1 • ic • i nshmg craft, propelled by single screws 



categories so far as propulsive coefficients are * iff j n79t nrc 



concerned, indicated both by W. P. A. van .'■,■■ ' 



Lammeren and by K. C. Barnaby in the latter's averaging about 0.65. 



latest publication. This is elaborated upon in For speeds other than the designed value, the 



(5) following. propulsive coefficient may vary rather widely 



(2) For equally good hydrodynamic designs there [Barnaby, K. C, INA, 1943, pp. 118-141]. 

 is no reason why the propulsive coefficient rip Below the designed speed, the value of rip is 

 should vary with absolute ship or propeller sizes, usually greater than at the designed speed; at 

 provided the sizes are adequate to avoid scale effect higher speeds, it is usually less. For the transom- 



