544 



Fig. 67.x Non-Dimbnsional Cukvatuee Plots of Three Section- Area Curves 



towing power, or maneuverability, to fit a screw 

 propeller or other propulsion device which has an 

 appreciably greater diameter than the draft of 

 the hull, as worked out in the preliminary design. 

 As it is not good to have the propeller tips 

 constantly breaking surface, a projection of the 

 propeller disc below the baseplane is accepted. 

 This can be done, without protection for the 

 lower tips, on a vessel like a destroyer which 

 only rarely runs in water that is shallow relative 

 to its extreme draft. The propeller blades in 

 their lower positions can be protected against 

 fouling with ropes or cables by a drooping skeg 

 bar or a depressed extension of the keel. For 

 substantial protection by the hull, in the case of 

 ships which travel regularly in relatively shallow 

 water, the keel line is inclined downward and aft 

 until it passes below the propeller disc. At the 

 same time the keel line is usually lifted under the 

 entrance so that the mean draft remains about 

 the same as for a design with propellers of normal 

 diameter. 



It is brought out in Sec. 66.32 that any ship 

 in ballast or light-load condition should be 

 trimmed sufficiently by the stern to keep the 

 propeller well submerged. While such a vessel 

 may run for a large portion of its operating time 

 with a sizable trim aft, the situation is by no 

 means the same as for a ship which is designed 

 to run continuously in a corresponding attitude 

 and to deliver its best performance under these 

 conditions. 



The keel drag necessary for propeller protection 

 is often very large. There are cases of light- 

 displacement passenger steamers in which the 

 drag is equal to the mean draft, with a draft 

 forward of only about one-third of the maximum 

 draft aft. 



A hull with keel drag may be laid down by 

 constructing first a normal form with zero drag 

 and then tilting all the waterlines, closing them 

 up vertically at the FP and opening them out at 

 the AP. This transformation is of course accom- 

 panied by a major shift aft of both the LMA 

 and the LCB. Since the volume of the afterbody 

 increases in this operation, there is no particular 

 problem in shifting the CG aft to give the vessel 

 the desired trim. 



Another method of design is to rough out a 

 hull of essentially normal form and then to stretch 

 the centerline skeg vertically until the aftfoot 

 is as deep as desired. There is no need for cutting 

 up the forefoot to correspond unless it is required 

 for fairing or unless a long, straight keel is wanted 

 for drydocking. 



If the designed keel drag does not exceed about 

 0.1 of the mean draft, the diiTerence between the 

 performance of a vessel designed specifically for 

 this drag and of one designed for zero drag and 

 trimmed a corresponding amount by the stern 

 should be small or negligible, of the order of 2 

 or 3 per cent. The performance with drag may be 

 better, due to finer waterline slopes in the entrance, 

 increased immersion or submergence of the 



