Sec. T6.4 



DESIGN OF SPECIAL-PURPOSE CRAFT 



757 



The following SNAME RD sheets apply to 

 vessels of the Great Lakes bulk ore-carrier type, 

 and refer to the loaded condition: 



Sheet Length, Beam, Draft, Displ., Speed, 



130 647.2 67.0 24.0 25,430 12.0 



There are a number of additional references, 

 covering the years 1897 to the present, to be 

 found under the subject headings "Great Lakes" 

 and "Lake Vessels" in the SNAME Index to 

 Transactions, 1893 to 1943, and in the latest 

 editions of the SNAME Yearbook. 



Discussing design features, the transverse 

 limitations determine the dimensions and possibly 

 also the shape of the midsection. This usually is 

 wall-sided and flat-bottomed, with Cx values of 

 0.99 or more. If the vessels are to run on inland 

 waters, as is generally the case when such severe 

 beam and draft limitations exist, roll-resisting 

 keels are dispensed with. The lower-corner radii 

 of the midsection are made as small as the 

 designer and operator dare to use, or as the canal 

 locks will permit. Small-radius corners give a ship 

 section rather good inherent roll-damping charac- 

 teristics, so the bilge keels may not be missed. 



With 50 per cent or more of the length in parallel 

 middlebody, as is the case on most of these 

 vessels, there is usually an appreciable saving in 

 construction costs because of this factor. However, 

 to achieve the greatest benefit from this feature, 

 the constant-area section amidships should have 

 a straight sheer line, with constant depth. For 

 the average inland or protected waters, it should 

 be sufficient to add sheer at the ends only, as in 

 the alternative straight-element profile for tankers 

 of Fig. 68. C. 



With the beam and draft fixed arbitrarily, and 

 the length established approximately by the 

 carrying capacity, it appears at first sight that 

 the relatively large wetted surface resulting from 

 this combination must be accepted. This is likely 

 to be up to 10 per cent greater than for a ship of 

 normal form. The ratio of friction resistance to 

 total resistance is therefore large, of the order 

 of 0.6 or more. Because of their length and the 

 extent of shallow water they must generally 

 traverse, these vessels run at relatively low values 

 of r, , of the order of 0.4 to 0.55, F„ of 0.12 to 

 0.164. 



It must also be recognized in the design of these 

 vessels that many of them run in shallow-water 

 regions, in the form of dredged channels or rock 

 cuts, in which the nominal bed clearance is 

 reckoned in small fractions of a foot. On a large 

 Great Lakes freighter, for example, the square- 

 draft to depth-of-water ratio V^Z^/Zi may reach 

 the value of V 1700/25.2 or 1.62. The sinkage at 

 even a relatively low speed may be equal to this 

 nominal clearance so that the ship actually 

 scrapes along over the bottom. It seems not 

 possible to relieve this situation by any drastic 

 reduction in Cx without cutting into the useful 

 load. The alternative operating solutions ar- to 

 load to a lesser draft, or to slow down in +" areas 

 of extreme shallow water. 



In the past the trend of desig or vessels of 

 this type has been to keep the block coefficient 

 high and to carry the maximum amount of useful 

 load within the limiting length, beam, and draft. 

 High resistance and high power were not frowned 

 upon by the operators if large cargoes could be 

 carried. However, for vessels like Great Lakes 

 ore carriers which run in deep as well as shallow 

 water, and where the total amount of cargo 

 carried per vessel during the ice-free season is the 

 principal operating criterion, there may be a 

 better solution. Increasing the speed by fining 

 the ends and carrying several more whole cargoes 

 per season may more than make up for less 

 tonnage per cargo [Telfer, E. V., SNAME, 1951, 

 p. 222]. It is possible, as Telfer points out, that 

 a modified form of bulb bow might have a useful 

 application at the low T ^ values at which these 

 ships usually run. It is certain — indeed it is 

 proved by several conversions — that fining the 

 stern and paying attention to the flow to the 

 propeller in the run will produce an appreciable 

 gain in average speed with little or no increase in 

 engine or propeller power. In fact, if the con- 

 tinuous development of the Great Lakes bulk 

 carrier over the past century [Baier, L. A., 

 SNAME, 1947, pp. 385-390; SNAME, HT, 1943, 

 pp. 365-375] is projected into the future, it 

 indicates an improvement in form and an increase 

 in speed with no reduction in long-time carrying 

 capacity, that will continue to improve its useful- 

 ness. 



Fig. 76. B pictures the stern of a recent (1954) 

 Great Lakes ore carrier, designed by the American 

 Ship Building Company of Cleveland, Ohio, in 

 which a definite improvement was made in the 

 form of the run ahead of the propeller. 



