832 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 77.14 



brake power required is about 430 horses. When 

 rigged for direct drive and fitted with hydrau- 

 lically operated chitches, with all lubricating oil 

 carried in the engine sumps, the weight is 2,650 

 lb per engine or 5,300 lb total. This includes all 

 liquids in the engines and the piping systems. It 

 is the same as was used for the first estimate. 



With an increase in the ratio of hull weight to 

 total weight of from 0.28 to 0.33, it is clear that 

 too much additional fuel can not be carried if the 

 percentage of margin is also to be increased. The 

 fuel is therefore limited to the 140 gal necessary 

 to run both engines at full power for 4 hr. Its 

 weight is 140(6.2) = 868 lb. 



Hull fittings usually account for an unreason- 

 ably large proportion of the total weight for small 

 boats because many of these parts are of standard 

 size and constant weight for a rather wide range 

 of boat sizes. A good value for a twin-screw planing 

 boat of 35-ft waterline length seems to be about 

 8 per cent of the total. 



The planing boat under design is a tender, 

 therefore many usual items of equipment are not 

 required. For example, no bunks or mattresses 

 are needed. Stores can be held to a minimum and 

 only a small quantity of drinking water need be 

 carried. A low percentage of weight assigned to 

 the stores group, say only 3.0 per cent of the total, 

 appears adequate. 



The customary amount of electrical and elec- 

 tronic equipment is called for. This is estimated 

 as 800 lb. 



To avoid overweight, the most common fault 

 of planing motorboats, an ample margin is man- 

 datory. At this stage a good 10 per cent of the 

 total of the preceding groups is none too much, 

 or about 9.5 per cent of the resulting total. 



A summary of the groups of the second weight 

 estimate follows: 



Total 



17,148 lb 



Adding a margin of 9.5 per cent of the total 

 weight the latter is 17,148/0.905 = 18,948 lb. In 

 round numbers this is 19,000 lb or 8.482 long tons. 



At no stage of the design or construction should 



the weight of the boat exceed this figure. Any 

 reduction under this limit is a definite advantage. 

 A revision of the displacement-length quotient 

 gives 



W (or A) 8.482 



(O.OIOL)' (0.35)'^ 



= 197.8 



The design limit of 200 for this quotient is ap- 

 proached closely but not exceeded. 



E. Monk publishes a weight distribution for an 

 average cruising motorboat, in percentage form 

 similar to Fig. 77. D, without giving an average 

 value or range of T„ [Yachting, Jan 1955, p. 118]. 

 He also gives, in the same reference, a breakdown 

 of the hull structural weights only, for average 

 V-bottom and round-bottom cruisers. 



D. S. Simpson gives a simple and convenient 

 method for estimating the preliminary values of 

 eleven weight groups for a trawler design 

 [SNAME, 1951, p. 561]. For the weight of the 

 hull and its joiner work, the estimate is based 

 upon the product of the ship length L, the beam 

 B, and the hull depth D, all expressed in feet, 

 times selected coefficients which give the weights 

 in long tons. For the deckhouses, their weight is 

 based upon their volume times another coefficient. 

 The weight of gear and equipment, and of ballast, 

 is based upon the length L in ft times a selected 

 coefficient to convert it into tons. The particular 

 coefficients for these dimensional expressions are 

 derived only from a rather voluminous collection 

 of data, combined with a generous amount of 

 background and experience. They work at present 

 with little in the way of method and system; they 

 would work much better if some additional time 

 and study could be devoted to them. 



When the motorboat (or small-craft) design is 

 more completely worked out an overall direct 

 estimate of weights is made, by the method 

 described in Sec. 77.39. 



77.14 First Approximation to Shaft and Brake 

 Power. The first, and perhaps the second and 

 third approximations as well, to the shaft power 

 of a surface ship are made by suitable methods 

 which give the designer an idea directly of the 

 shaft power necessary to drive a given size and 

 type of ship at the required speed. The first part 

 of Sec. 66.9 contains examples of two methods 

 employed in the preliminary design of the ABC 

 ship. 



For the designer of a motorboat, on which the 

 proportion of propelling-machinery weight is 

 much greater than on a normal type of displace- 



