600 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 70.13 



consideration of the natural frequencies of vibra- 

 tion of the hull, the machinery foundations, the 

 propelling plant, and the propulsion system 

 [Kane, J. R., and McGoldrick, R. T., SNAME, 

 1949, Vol. 57, pp. 193-252]. The number of 

 blades which best avoids these frequencies or their 

 major harmonics in the operating speed range is 

 chosen [Brehme, H., Schiff und Hafen, Nov 1954; 

 abstracted in English in MENA, Aug 1955, pp. 

 318-321]. 



In some cases restrictions on propeller diameter, 

 or the need for large blade area, coupled with 

 high power requirements, indicate the use of 5 or 

 even 6 blades. Also this number of blades may be 

 necessary to keep resonant frequencies outside of 

 the operating speed range. Two-bladed propellers 

 are used on sailing ships with auxiliary power, as 

 they offer the least resistance when housed abaft 

 the skeg in the saiUng condition. 



For twin- or multiple-screw ships, a 4-bladed 

 propeller can be of smaller diameter than a 

 3-bladed propeller for the same power. This means 

 that smaller bossings and struts can be used to 

 maintain the same hull tip clearance. Usually, the 

 reduction in appendage resistance more than 

 compensates for the small loss in propeller 

 efficiency when using four blades. As in single- 

 screw ships, propeller efficiency is usually a 

 secondary consideration in the choice of the 

 number of blades. First, there is little variation 

 in efficiency between 3- and 4-bladed propellers 

 and second, vibration considerations will again 

 be the controlling factor, especially for ships of 

 moderate to high power. In many cases other 

 factors, such as type of engine, number of cyl- 

 inders, and the like, enter into the choice of the 

 number of blades. 



Further discussions of the proper number of 

 blades to be used for a screw propeller are given by: 



(a) G. S. Baker, SD, 1933, Vol. II, pp. 49-50 



(b) R. H. Tingey, ME, 1942, Vol. I, pp. 277-278 



(c) D. W. Taylor, S and P, 1943, pp. 143-144 



(d) W. P. A. van Lammeren, RPSS, 1948, pp. 225-226. 



Although it appears absurd from the point of 

 balancing and of reduction of bending moments 

 on the propeller shaft, there are some appi'eciable 

 advantages in the use of a single-bladed propeller. 

 These are discussed by S. Sassi [Ann. Rep. Rome 

 Model Basin, 1938, Vol. VII, pp. 95-99]. There is 

 reported the case of a sliip which normally 

 traveled at 10.5 kt at 70 rpm but which, with all 

 blades except one broken off, made 7 kt at about 

 85 rpm [SBSR, 6 Mar 1924, p. 289]. 



It is interesting to note that screw-propeller 

 designers, even in the earliest days of develop- 

 ment of this device, considered that they had 

 some latitude in the number of blades and usually 

 exercised it. 



70.13 Use of Raked Blades. Due to the 

 contraction of the inflow jet ahead of the propeller, 

 the water enters the propeller disc at a slight 

 inward angle, depicted in Fig. 32. M. A slight 

 rake aft places the blades normal to the flow, 

 with a gain in efficiency. On the other hand, rake 

 causes an increase in the bending stresses in the 

 blade due to the fact that the centrifugal force is 

 offset from the blade root. In heavily loaded or 

 high-speed propellers this latter factor is usually 

 the controlling one in limiting rake. Rakes of 

 over 15 deg are seldom used in any ship or propeller 

 design. 



Forward rakes are rarely seen. Sir Charles 

 Parsons at one time advocated a forward rake 

 of 1 in 10 for very thin propeller blades on the 

 high-speed experimental vessel Turbinia. His idea 

 was that an axial component of the centrifugal 

 force on them would act in an after direction and 

 help balance the thrust forces acting forward 

 [Burrill, L. C, IME, 1951, Vol. LXIII, p. 15]. 



In some ship designs, where the maximum 

 volume must be crowded into a given length, 

 the hull profile is not only carried well aft toward 

 the propeller position (s) but the waterlines have 

 large slopes in this region. Blades may then be 

 raked aft to augment the fore-and-aft clearances 

 between the propeller sweep line and the forward 

 edge of the aperture opening, if the vessel has 

 a single screw, or between the sweep line and 

 the hull, bossings, skegs, or struts if it has multiple 

 screws. W. P. A. van Lammeren [RPSS, 1948, 

 p. 229] gives the following limits (not design or 

 optimum values) for rake: 



(a) With moderately loaded screws for merchant 

 vessels, 6 to 10 deg for single-screw ships, and 8 

 to 12 deg for twin-screw ships 



(b) With heavily loaded, fast-running screws for 

 warships generally no rake is used. 



When running under load a propeller blade 

 actually bends forward. This may be as much as 

 0.10 or 0.12 ft at the tip of a destroyer propeller. 

 Thus if it is desired to run at the designed speed 

 with no rake it would be wise to design the pro- 

 peller with a little rake aft to allow for this 

 bending effect. 



If proper attention is given to the details of 



