134 



l!^ DRODN \ WIK.s I\ Mill' 1)1S|(;\ 



Sec. -16.2 



VWave NWalerline 



Outlines> of 

 Separ-alion Zone 

 ana Estimated 



Lookinq Foi^ward 



Wave Vyaterline 

 Not Shown 



Tic. 46.A TypicAL Sktaration-Zone Areas on Ships, as Projected on a Transverse Plane 



small distances below the air-water interface, 

 the equal-pressure surfaces would lie generally 

 parallel to the free-water surface, as at 2 in Fig. 

 4G.A. This would be true whether the air-water 

 interface is essentially flat at low speeds or is 

 deformed by waves at higher speeds. 



This is a strong probability that other factors 

 enter into the formation of a .shallow separation 

 zone abaft the stern. If so, these factors will have 

 to be determined and studied before reliable 

 predictions can be made. G. Birkhoff offers the 

 suggestion that: 



"Pcrliaps, tlic rciison wliy a free nurfaoc affects flow 

 separation at the stern is, that it inhibit.s vertical turbu- 

 lence at the surface. Such a reduction in turbulence is 

 known to affect flow separation in other cases" (SN.\ME, 

 1954, p. 396|. 



It might be addcil llial tiie presence of the ship 

 hull also suppre.sstjs tho.se velocitj' components of 

 turbulence which are normal or nearly normal to 

 the solid surface. 



It may be for one or butli of I he rcji.sons oul- 

 lincd in the two i)rcccding paragraphs that .separa- 

 tion occur.-^ in the upper surface layers under and 

 Ix-hind the run of u .sjiiling yacht. Such a crall, 

 having what is considered a line, fair form iiml 

 favored with a [icrfcclly normal curve of s<'ction 

 areiw, may liave a buttock slopi! i« of only a few 

 dcgrcca, yet the watcrline slope j „- near the .snrf.uc 



ma.y be ten or more times that amount. Despite 

 the gently sloping buttocks, separation of the 

 layers close under the wafer surface oc(^urs at 

 extremely low speeds. This .sej^aration is, further- 

 more, marked by a large number of vertical-axis 

 \-ortexes, easily visible when the water surface 

 is almost glassy smooth. 



For reference purposes a separation zone in the 

 run starts at separation points V.j. and ]•>« along 

 the surface waterline, on the port and starboard 

 sides, respectively. If the wave profile in way of 

 the run is known, it is more accurate to position 

 these points along the waveline. The ,sei)aration 

 zone is usually bountled by reasonably fair 

 streamlines and stream surfaces, leaving the ship 

 hull at the locus of the separation points. The 

 zone extends far enough astern so that all (he hull 

 and the customary appendages abaft the .S('i)ara- 

 tion-point locus lie within them. On a moving 

 boat or ship the extent of the separation zone 

 on the water surface is readily indicated by 

 sjjrinkling the water in the vicinity with wocmI 

 chips. Those in the separation zone swirl around 

 and follow the .shij) while tho.se outsich^ tli(> zone 

 arc rapidly carried astern. 



( Icncrally, as plotted on a IjihIv plan .iml as 

 indicated in diagram '1 of i'"ig. •It'>..\, the width of 

 till- separation zone for a ship with ,a pciinted 

 niii is considerably greater lii.an its dcpili The 



