Sec. 77.36 



PRELIMINARY DESIGN OF A MOTORBOAT 



859 



protection to the propener(s) and ruddcr(is), is 

 added to the liuU draft. 



This estimate, plus a preliminary layout of the 

 propeller(s) and rudder(s), and an estimate of 

 the sinkage at the stern, indicates roughly whether 

 the limiting-draft conditions can be met. 



Because of the small immersion of its surface 

 propellers and the small trim by the stern at 

 which it runs, a sea-sled type of motorboat may 

 be admirably suited to operation in waters of 

 limited depth. A 75-ft cruising yacht of the sea- 

 sled type, in which the extreme draft is of the 

 order of only 3.5 ft, is illustrated in the literature 

 [Yachting, Apr 1950, p. 62]. 



77.35 Estimate of Screw-Propeller Charac- 

 teristics. For the selection of preliminary screw- 

 propeller characteristics for a motorboat, use is 

 made here of a nomogram developed by W. E. 

 Fermann, issued by the Marine Division of the 

 Federal-Mogul Corporation, and used since 1943 

 by the Bureau of Ships of the U. S. Navy Depart- 

 ment. The diagram is reproduced as Fig. 77. U. 

 On the facing page there are instructions for its 

 use, supplemented by an example worked out 

 for the 24-kt planing type of ABC tender. 



Use of this Fermann nomogram requires that 

 three of the principal quantities be known: 



(a) The speed of advance Va of the propeller, 

 expressed as miles per hour, where 1 kt = 1.15 

 mph. The value of Va is reckoned as 0.90 the 

 speed V of the boat for a single-screw craft with 

 a fine run and 0.95 times that speed for twin-screw 

 craft, whether of the planing or round-bottom 

 types. P. G. Tomalin gives a special nomogram 

 for determining the factor (1 — w) [SNAME, 

 1953, p. 602]. 



(b) The shaft power Ps delivered at each pro- 

 peller. This may be taken as 0.95 times the rated 

 brake power P^ of the engine connected to that 

 propeller. The instructions accompanying the 

 Fermann chart state that the shaft power is equal 

 to the brake power times the efficiency of trans- 

 mission to the propeller times a barometric 

 modifier times a sustained-load factor. The 

 product of the last three factors is given as an 

 average of 0.90 for gasoline engines and 0.85 for 

 diesel engines. 



(c) The rate of rotation in rpm at which the 

 propeller turns. This is equal to the engine rpm 

 for direct drive. 



Taking the twin-screw 24-kt planing type ABC 

 tender as an example: 



(1) The speed of advance Va is 0.95 (21) kt. In 

 the units required for the chart. Fig. 77. U, this 

 is 0.95(24) (1.15) = 24.84 mph. 



(2) The shaft power, considered here as the power 

 delivered directly to tiie propeller, is 207 horses 

 per shaft, from the calculation at the end of 

 Sec. 77.26. On the basis that the engine is rated 

 for sea-level operation the barometric modifier 

 of (b) preceding is 1.00, for a boat to be operated 

 always at sea level. Strictly speaking, since the 

 24 kt is a maxunum trial speed and not a sus- 

 tained speed, the sustained-load factor should 

 also be 1.00. However, it seems wise, as in the 

 case of the ABC ship described in Chap. 69, to 

 limit the power at designed speed to about 0.96 

 of the maximum. Allowing for transmission losses 

 of the order of 3 per cent and taking the nearest 

 round figure, the shaft power delivered at each 

 propeller is assumed to be 210 horses. 



(3) The engines are designed to run at 2,500 rpm 

 at rated full power. 



Only when special performance justifies the 

 cost is it possible to install a custom-made motor- 

 boat propeller, conforming to a design drawn 

 up in accordance with big-ship methods such as 

 described in Chap. 70. Normally, a propeller is 

 selected from stock, having the desired number 

 of blades, diameter, pitch, and mean-width ratio. 



77.36 Propeller Tip Clearances; Hull Vibra- 

 tion. For a craft of the displacement type the 

 propeller-tip clearance need be no more than the 

 nominal turbulent boundary-layer thickness 5 

 (delta), determined for an a;-distance equal to 

 that from the stem to the propeller position and 

 for a speed V equal to the highest boat speed 

 expected. Using the ABC round-bottom tender 

 as an example, the .r-distance to the propeller is 

 about 33 ft. At a speed of say 14 kt or 23.65 ft 

 per sec in standard salt water, R^ from Table 45.b 

 is about 60 (10*^). Assuming that the flow is fully 

 turbulent and interpolating between the graphs 

 of Fig. 45. C, the value of 5 at the propeller is 

 about 0.3 ft. The data plotted there are for fresh 

 water but the values would not change materially 

 for salt water. 



For a planing craft, the R^ length is somewhat 

 shortened because of the diminished wetted length 

 along the keel. For the ABC planing tender, 

 running at 24 kt or 40.53 ft per sec, the mean 

 wetted length is just under 22 ft, giving an R^ of 

 about 66 (lO**) in salt water and a 5 at the pro- 

 peller position (from Fig. 45. C) of the order of 



