Sec. 78.18 



MODEL-TESTING PROGRAM FOR A SHIP 



897 



the low side. Furthermore, the change of direction 

 of the water flomng around and under the forward 

 sections, forward of the ciuarterpoint at Sta. 5, 

 is more abrupt than it should be. The almost 

 obvious step is to widen the designed waterline 

 still further in way of Stas. 3 and 4, following an 

 earher change, and to trim away the hull lower 

 down in this region. This gives a better path to 

 the water. The change is reflected in a lowering 

 of the minimum value of the section coefficient 

 ahead of the forward quarterpoint. Sec. 67.10 

 calls attention to the fact that this is rather 

 high for good design. 



At this stage, a few details need to be watched 

 but not necessarily corrected. One such relates to 

 the midships portion of the designed waterline 

 of the ABC ship. In the final fairing of the lines 

 preparatory to making the model the maximum 

 section fell at Sta. 10.6 or 0.530L rather than at 

 Sta. 10.3 or 0.515L. The beam at this section 

 (Sta. 10.6) fair ed to a value of 73.33 ft rather than 

 73.0 ft. As the LMA of 0.530L still fell within the 

 design lane of Fig. 66.L (it hes to the left of the 

 double circle in that figure), and as the ship was 

 to have a transom stern which made it easier to 

 obtain small waterline slopes in the run, this 

 value was accepted. The shghtly larger beam was 

 also accepted as the waterline curvature plot 

 with this beam proved to have most of the de- 

 sirable features. The final dimensions and coeffi- 

 cients of the ABC ships as built into the models 

 are given in the SNAME RD sheets of Figs. 78.J 

 and 78.K. 



After the bUge-keel trace for the forebody had 

 been determined on the model it developed that 

 the combination of bilge-keel width shown on 

 Fig. 73. N and the bilge-keel trace caused the 

 outer edge of the keel to project beyond the 

 DWL in the upright position from Sta. 8 forward 

 to about Sta. 6.5. The requirements of item (48) 

 in Table 64.f call for no projections beyond the 

 fair hue of the hull for the first 150 ft from the 

 stem. Sta. 6.5 is about 166 ft from the FP. 

 However, for ease in handling, the hull shape 

 should embody one of the following modifications: 



(a) Determine whether the widenhig of the DWL 

 in the entrance, proposed earlier in this section, 

 would bring the present keels within the DWL 

 limits. If not, 



(b) Taper the forward end of the keels to a greater 

 extent and reduce the width back to at least 

 Sta. 8 



(c) Retain the same leading-end taper but shorten 

 the keels, moving the leading ends aft from Sta. 

 6.4 to about Sta. 7.5. 



The principal question is, what to do to improve 

 the hydrodynamic performance of either or both 

 of the alternative designs? The basis for this step 

 is to find out just what gives them the performance 

 they now have. What is it, for example, that 

 causes the thrust-deduction fraction t of the 

 transom-stern form to drop from a constant value 

 of 0.125 or 0.13 in the intermediate-.speed range 

 to a remarkably low value of 0.07 in the normal 

 running range of 18 to 20.5 or 21 kt? With a 

 nearly constant wake fraction and propulsive 

 coefficient the hull efficiency rises but the relative 

 rotative efficiency jj^ drops. There is a sUght 

 hump in the Cr curve of the 400-ft ship in Fig. 

 78. Jc at a T, of 0.68, probably because at this 

 speed the ship is only some 2.5 Velox wave lengths 

 long. There is a smaller but sharper hump at a 

 T, of about 0.83. For the 510-ft ABC ship the 

 corresponding speeds are 15.4 and 18.7 kt. From 

 Fig. 78.Nc it is noted that these humps occur 

 somewhat beyond the ends of the transition 

 portions in the t and rfHT curves [where r^HT = 

 ^^ = (1 - t)/{\ - w)]. Beyond 21 kt for the 

 ABC ship, at a T, of about 0.93, when the ship 

 length is approaching 1.5 wave lengths, the 

 propulsive efficiency falls off, as it does in most 

 ships above the designed speed. However, the 

 superiority over the Taylor Standard Series hull, 

 in respect to resistance and effective power, 

 increases in this range. 



A search for improvement obviously calls for 

 a careful determination and rather close study of 

 the wave profiles and flow patterns around the 

 hull and into the propeller at the 16- and 18-kt 

 speeds, perhaps also at the 21-kt speed. 



The fact that, despite an increased wetted- 

 surface area of 5.8 per cent, the resistance of the 

 arch-stern bare hull drops below that of the 

 transom-stern bare hull at 28 kt, where the T, 

 is about 1.240, is well worth looking into. The 

 remarkably low resistance of both hulls in the 

 23- to 25-kt range indicates that these forms might 

 well find application to shorter, high-speed vessels. 



On the basis that, of the two alternative sterns, 

 the transom-stern arrangement has selected itself, 

 so to speak, the owner-operator now has two 

 choices. Either he may: 



(1) Hold the speeds originally called for, adhere 

 to the schedule already set up, and reduce the 



