Sec. 76.4 



DESIGN OF SPECIAL-PURPOSE CRAFT 



761 



%. 



^ 





Fig. 76.B Gkeat Lakes Ore Carrier George M. Humyhrey in Building Dock 



Photograph by Denny C. Harris, Cleveland, Ohio. The 5-bladed propeller is of built-up construction. Note the fine 



skeg ending up under the stern. The aperture on the centerline at the stern is the hawsepipe for the stern anchor. 



Summarizing the hull-design problem resolves 

 itself into: 



(a) Achieving the required carrying capacity with 

 the minimum length and wetted surface 



(b) Holding the sinkage down to the minimum 

 practicable value 



(c) Shaping the ends to keep down the pressure 

 resistance due to wavemaking, separation, and 

 the Uke. 



The wavemaking pressure resistance, at a 

 Hmiting T„ of, say 0.7, can never be a large part 

 of the whole unless the bow is deliberately made 

 too blunt. On the other hand, the separation 

 drag can be excessive. These hulls are therefore 

 in the class where the major part of the fining 

 needs to be done at the stern. The demands of 



the owners and operators for the highest possible 

 ratio of useful load to total displacement, in the 

 full-load condition, is generally such that the 

 stern can not be fined sufficiently to eliminate 

 all separation, even at depths as low as the 

 axis of a single propeller. Not only is the 

 separation drag kept large by this limitation but 

 the possibihties of air leakage to the propeller, 

 with its consequent loss of power, vibration, and 

 noise are aggravated. 



It appears that a not-too-wide transom stern, 

 with an immersion in the load condition of only 

 a foot or so, might be a partial answer to this 

 problem. The transom sheK can be mde enough 

 to guard against air leakage to the propellers 

 and it can be used to make up the displacement 

 volume lost by fining the skeg forward of the 



