•-190 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 66.24 



other, be brought nearly parallel to the shaft 

 axis as it passes into the propeller disc. 

 (7) The same as for the side-water paths, the 

 changes in curvature along the bottom-water 

 paths should be a minimum, both in number 

 and magnitude. This means easy and gradual 

 longitudinal slopes in the actual flowplanes, 

 with small magnitudes of curvature. 



So much for the general rules, to be kept in 

 mind as sketching of the sections progresses. 



The la3'out of the maximum-area section, taken 

 from Fig. 66.G, forms the basis for the body plan. 

 The station offsets of the tentative designed 

 waterUne are laid off along the 26-ft designed 

 waterline trace on this plan and are numbered 

 accordingly. Short vertical hnes are sketched in 

 through these points in the entrance, to serve as 

 zero-flare references for the section lines where 

 they cross the designed waterline. 



The bulb-bow section at the FP is drawn first, 

 following the rules of Sec. 67.6. A tentative 

 section at Sta. 5 is then sketched in at the forward 

 quarterpoint. Its section coefficient is taken from 

 a curve of section coefficient based on ship length, 

 similar to Fig. 67.1 of Sec. 67.10. The section 

 outhne is tangent to the floor hne at the bottom 

 and to the vertical reference hne at the DWL; 

 a large radius or easy sweep is used below about 

 0.3H. This is the region where the bottom water 

 does its greatest twisting and where particular 

 care is required to insure easy flowlines. Except 

 for the designed-waterline region it is probably 

 the most important part of the hull, at least in 

 the entrance, and the part that has the greatest 

 influence upon pressure resistance. Before easing 

 the section at Sta. 5 too much, the area is meas- 

 ured and checked with the forward-quarter 

 ordinate at Sta. 5 on the preliminary section-area 

 curve. Section 5 is reshaped as necessary to give 

 the proper area and section coefficient; see Sec. 

 67.10. This may involve a possible widening of 

 the DWL. By the use of the bilge diagonal for 

 fairing, or several waterlines, or both, it is fairly 

 simple to sketch in the remaining entrance 

 sections, meeting the designed waterline along 

 the short vertical reference lines, including inside 

 them the areas given by the preliminary section- 

 area curve, and conforming to the section-coeffi- 

 cient curve. The abovewater portions of the 

 section lines in the entrance are reserved for the 

 time being. 



Turning to the afterbody a tentative transom 



outline at the AP is next sketched, following the 

 rules of Sec. 67.20. Since the afterbody is to 

 terminate in nearly horizontal shelf-like sections 

 approximating the form of the immersed portion 

 of the transom it is evident that any centerhne 

 skeg must of necessity be relatively thin. This 

 skeg will, in fact, form a sort of major appendage 

 to be added under the main hull. The next step 

 is therefore to sketch in, on a separate large-scale 

 stern profile, the centerline or half-siding buttock, 

 the one meeting the bottom of the transom and 

 representing roughly the top of the skeg. There 

 must be room at about the after quarterpoint, 

 or possibly at one-fifth of the length from the 

 stern, for a large-diameter motor or gear on the 

 main shaft, low down in the vessel. The half- 

 siding buttock therefore must start no farther 

 aft than the after quarterpoint and must rise 

 rather rapidly to meet the bottom of the transom. 

 Indeed, if reverse curvature (concave downward) 

 is to be worked into the after end of this buttock, 

 as is desirable, the latter must rise at a rather 

 steep angle forward of the concave portion. 

 One must be prepared to bring it upward at a 

 slope approaching closely the critical angle for 

 separation at a submergence of about Q.7H. It is 

 known that the owners will require one or more 

 model tests as a check on the performance of the 

 underwater hull. It appears, therefore, that a 

 centerline or half-siding buttock slope as steep 

 as 17 or 18 deg may be risked at this stage of the 

 design. Laying this buttock down to a large 

 scale on what will eventually be the stern profile 

 gives a series of heights for the termination at 

 the centerhne of all main-hull sections in the run. 



Taking the after quarterpoint at Sta. 15 as a 

 sort of midpoint in the run, an easy curve is 

 swept in between the designed-waterline intercept 

 and the half siding at the baseline. The lower or 

 inboard portion of this section is made somewhat 

 flat and the upper portion is given a slight out- 

 ward flare at the DWL. The area at Sta. 15 is 

 then measured and checked with the A-curve, 

 whereupon the section is readjusted as necessary. 



Sections between Stas. 10 and 15 are rather 

 easily drawn, following the general procedure for 

 the sections between Stas. 5 and 10. A section 

 line is drawn first to meet the centerhne buttock, 

 as if there were to be no skeg; see the broken 

 hnes at Stas. 16 through 18.5 in Fig. 66.P. The 

 skeg is then drawn in separately. However, the 

 stations abaft 15 include the skeg as a part of 

 the main hull, so some little sketching and re- 



