Sec. 78.19 



MODEL-TESTING PROGRAM FOR A SHIP 



899 



(i) Furthei' studies should be made of the internal 

 arrangements, supplementing the discussion in 

 Sec. 66.32, to enable the vessel to float at the 

 mtermediate-load waterlines indicated in Fig. 

 66. T. These should be combined with studies of 

 the effect of the variable weights, hi each of the 

 proposed arrangements, upon the bending mo- 

 ments imposed on the hull structure, 

 (ii) Estimates should be made of the effective 

 and shaft powers and probable speeds in the 

 variable-load conditions, for both deep and shallow 

 water 



(iii) The sinkage of the bow at designed speed, 

 indicated on Figs. 58. B and 66. R, is rather large. 

 The widening of the designed waterline forward, 

 as proposed in the first part of Sec. 78.18, should 

 improve this situation. 



(iv) A study, much more comprehensive than 

 that of Sec. 60.15, should be made of the effect of 

 fouling on the total resistance, on the wake and 

 thrust-deduction fi'actions, the propeller thrust, 

 and the propeller (or shaft) power. 



There was no time or opportunity to design, lay 

 out, and test a third alternative stem for twin- 

 screw propulsion of the ABC ship, as mentioned 

 in Sec. 69.5 on page 571. 



With respect to the preliminary hydrodynamic 

 designs of the two small motor-driven tenders for 

 the ABC ship, serving as the illustrative designs 

 in Chap. 77, the treatment given there is likewise 

 abbreviated. For a well-rounded project, even of 

 this limited scope, a number of additional items 

 should be considered: 



(a) A determination, by the best methods avail- 



able, of the lowest speed at which the 24-kt 

 V-bottom hull would actually plane, when carry- 

 ing the specified load. This would indicate 

 whether the craft could still run in the planing 

 range if slowed by wind and sea, as mentioned 

 in (3) of Sec. 77.6. 



(b) For the examples cited, involving motorboats 

 which are to be in the water only for the hours in 

 which they are required to run, the additional 

 weight due to water soakage is not a problem. 

 Nevertheless, this item must be considered for 

 motorboat designs in general. 



(c) It is possible that for a motorboat, on which 

 the propelling machinery receives much less care 

 than on a large vessel, the reduction from maxi- 

 mum rated power to maximum sustained usable 

 power should be considerably larger than 5 per 

 cent of the former, as recommended for large 

 vessels 



(d) For the particular illustrative examples 

 described, a comprehensive preUminary design 

 should include a check on the fore-and-aft trim 

 with full fuel and crew but with no cargo or 

 passengers 



(e) An adequate preKminary design for a fuU- 

 planing boat should include an estimate of the 

 vertical forces and pitching moments to be 

 expected from propulsion devices carried at the 

 stern 



(f) Definite steps should be taken to reduce the 

 running trim of a 35- or 40-ft motorboat which, 

 as indicated in the early stage of the preliminary 

 design for the ABC tender, is as great as 5 deg. 

 The trim hmit in this respect probably should 

 diminish as the absolute size of the boat increases. 



