Design of Low-Resistance Hull Fornns 



0-3 



D(lba ) 0-2 

 V2 



Ol 



X BULB FORWARD 

 G BULB AFT 



D model(stern bulb) self propulsion 



V2 



^<U 



Si_ _Co_\ , bulb forward 



V2 VVVu..,,c 



LINE 

 X PROPORTIONAL 

 TO(LOG|oRn-2)- 



' ' . y. 



O 15 0-20 25 ^9 



^ 



7 V Fb/s«c 



Fig, 12 - Experimental results for the 19.5-ft-long 

 would-be low-drag hull form 



programmes (8) based on the Eggers method (9). In Fig. 12 points are shown 

 corresponding to the total resistance minus the wave-pattern resistance. For 

 the bulb-forward condition these points lie close to a line which may be regarded 

 as representing the frictional resistance. Thus in this condition the residuary 

 and wave-pattern resistances agree. For the bulb-aft condition, however, the 

 points of total minus wave -pattern resistance lie well above the frictional line. 

 Thus it appears that the wave -pattern resistance is much smaller than even the 

 reduced residuary resistance for this condition. Flow photographs taken in the 

 Ship Division circulating water channel explain this discrepancy. They show, as 

 in Figs. 13a and 13b, that at 5.66 ft/sec there is more evidence of separation 

 from the bulb at the stern that at 3 ft/sec. This is doubtless due to the stern 

 wave being higher at 5.66 ft/sec, causing an adverse pressure gradient over the 

 bulb. Thus part of the residuary resistance at the higher speeds with the bulb 

 aft can be attributed to an increase in form drag above that acting on the vessel 

 at lower speeds. 



In Figs. 13a and 13b a fairing can be seen over a propeller shaft which was 

 fitted in the bulb end after it was found that the bulb-aft condition was best. 

 When the model was run at 5.66 ft/sec, and propelled, as appropriate for a full 

 scale ship 693 ft long, at 20 knots, the quasi -propulsive coefficient was found to 

 be 0.74. When run at the model self-propulsion point the model resistance was, 

 as can be seen from Fig. 12, appreciably higher than when towed. However the 

 wave-pattern resistance was also higher then, (D/V^)^a^g being 0.038, much 

 greater than the unpropelled value of 0.009, and nearly as great as the value 

 0.045 for the towed model run bulb forward at the same speed of 5.66 ft /sec. 

 The running of the propeller suppressed the separation evident in Fig. 13b as 

 can be seen from Fig. 13c. 



All this raises some interesting questions. Does the lower wave-pattern 

 resistance with the bulb aft result mainly from the boundary layer thickness at 

 the stern being greater then than when the fine end is run aft? Is this true also 



723 



