852 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 11 2R 



Assuming a suitable vertical position of the 

 CG, in this case 0.5 ft above the DWL, the at-rest 

 and underway CG positions are placed on the 

 drawing, in a transverse plane 14.21 ft forward 

 of the AP. Scaling the rise of the CG due to the 

 forward speed of 24 kt from the original of Fig. 

 77.P gives 0.81 ft, as compared to the 0.67 ft 

 derived from the Murray reference earlier in 

 the section. 



Two other features now require some considera- 

 tion. The first is the question of whether or not 

 the steersman at the control station can see ahead 

 over the bow in this running attitude. Fig. 77. P 

 indicates that with the stem carried all the way 

 up to the forward deck line, vision in a horizontal 

 direction is just possible from the proposed 

 control station. In order to see clearly to a point 

 on the water surface some 90 ft ahead of the 

 steersman, his line of sight would have to be 

 depressed about 5 deg, indicated by a broken line 

 in the figure. Clear vision at this angle would 

 necessitate rounding the stem head and using 

 some reverse sheer forward. 



The second matter is one of appearance. A 

 planing boat running at full speed with a trim 

 of the order of 5 deg by the stern seems to be 

 struggling along, as though it could not quite get 

 through its hump speed. The same boat, running 

 at the same speed but with a trim of only some 

 2 deg, gives that delightful impression of smooth, 

 effortless running which is the aim of every 

 motorboat designer and builder and the hope of 

 every owner and operator. Few among those who 

 ride or watch seem to reahze, or to be concerned, 

 that the resistance and power may be less at 

 5 deg than at 2 deg trim by the stern. If level 

 running becomes of more importance than 

 efficiency of propulsion, a trim-control device of 

 some kind is clearly indicated, as described in 

 Sees. 36.26 and 37.24. 



The next step is to estimate, possibly from a 

 diagram such as Fig. 77. P, about where the 

 impact area will be under the bottom when the 

 craft runs at high speed through relatively short 

 waves. If this area is well forward of the CG the 

 boat receives an up-pitch moment as it strikes 

 every crest. If, however, the impact forces are 

 applied more or less underneath one of the heavy 

 weight groups such as the propelhng machinery, 

 the result of slamming on wave crests is limited 

 generally to a compression of the structure 

 between the engine bearers and the bottom of the 

 boat, with an effective reduction in pitching 

 moment. 



Despite the fact that this item has received little 

 attention in the technical literature, John Plum 

 and others who have produced high-speed planing 

 craft which give reasonably satisfactory behavior 

 under these severe conditions maintain that the 

 proper adjustment of the fore-and-aft location of 

 the heavy weight groups, and the control of the 

 proper trim angle, are more important factors in 

 superior wavegoing performance than any par- 

 ticular or special shape of the hull. To give the 

 naval architect a more reliable background in 

 this respect and to furnish him with better design 

 rules, an intensive and thorough study of the 

 effect of weight location and trim control should 

 be carried out at the earUest opportunity. 



77.28 First Space Layout of the 18-Knot 

 Round-Bottom Hull. For the round-bottom tjT)e 

 of semi-planing motorboat selected tentatively 

 in Sec. 77.10 as the one best suited for the 18-kt 

 speed, the requirements of Table 77. a indicate 

 that the craft must make this speed with a half- 

 load of fuel and a crew of two, as well as two 

 passengers and their personal baggage. It must, 

 however, be able to accommodate, at the slower 

 speed of 14 kt, any of the items listed in (6) of 

 Table 77. a and to run for 6 hr at full power. 

 In short, it must have power enough for the 18-kt 

 load and speed, but room enough for the 14-kt 

 load. 



Sketching a preliminary arrangement indicates, 

 as it did for the full-planing boat, that a hull 

 40 ft long is unnecessarily large and that the 

 weight hmit of 25,000 lb need not even be ap- 

 proached. A second arrangement sketch, repro- 

 duced in Fig. 77. Q, reveals that a transom-stern 

 hull of 35-ft waterline length is ample to contain 

 the necessaiy spaces and volumes without 

 crowding. 



For this length the Taylor quotient 2\ at 14 

 kt is 2.366, F,, = 0.705. For 18 kt it is 3.042, 

 F„ = 0.906. From the utihty-boat curve of Fig. 

 77. C, the total weight should be not in excess of 

 18,000 lb or 8.036 tons. Using the PhilUps-Birt 

 Eq. (77.iiia) of Sec. 77.14, the 14-kt speed, and 

 the proper value of K2 from Table 77. e. 



Pb (horses) = 



V^ (kt) W (long tons) 

 (14)^(8.036) 



= 201. 



(2.80)' 



It is to be remembered that at the 18-kt speed 

 the useful load (and the total weight) will be 

 considerably less than 8.04 t. It appears for the 

 moment that one of the HN-10 diesel engines 



