HYDRODYNAMICS IN SHIP DESIGN 



Sec. 60.17 



even though no additional thrust is necessary to 

 propel the ship at the increased speed. 



Skeg and stern endings that are too bkint, 

 ahead of a screw propeller, and insufficient 

 clearances, may always be expected to generate 

 vibratory forces and moments. At low speeds 

 these may be of small magnitude and hence not 

 objectionable. At higher speeds, however, a 

 propeller developing increased thrust and absorb- 

 ing greater power may generate vibratory forces 

 and moments that are by no means acceptable. 

 Air leakage from the surface may be initiated or 

 augmented because of the greater — Ap's in the 

 blade fields. Cavitation may become a factor, at 

 least in the region of the upper blades, because 

 of the greater blade-section speeds, possible 

 greater propeller diameter, and diminished depth 

 of submergence of the blade sections in the upper 

 blade positions. Fitting a propeller with wider 

 blades, in an old aperture, actually diminishes 

 the clearances, when they should be increased. 



60.17 Powering for Two or More Distinct 

 Operating Conditions. Exerting the maximum 

 thrust at low towing speeds, combined with 

 developing the greatest practicable free-running 

 speed for shifting quickly from one operating 

 area to the next, is mandatory for any tug 

 worthy of the name. Economical propulsion at 

 cruising speed, combined with efficient propulsion 

 at high or top speed, is a design problem for any 

 patrol vessel. Both economical and efficient pro- 

 pulsion, on the surface as well as submerged, is 

 practically a "must" for every type of sub- 

 mersible, as well as some types of pure submarine. 



Each of the foregoing is perhaps more of a 

 design than a calculation problem, or perhaps 

 more a problem of operation than of design. The 

 operator and owner usually must decide how much 

 one condition is to be favored over the other. 



Sec. 67.15 mentions the proposals and actual 

 installations of the past in which designers have 

 attempted to meet the problem of driving a ship 

 with one or two propellers under one operating 

 condition and with two or more under another 

 condition. This still involves running one set of 

 wheels in both ranges, usually at different ship 

 speeds and rates of rotation. It may be done by 

 varying the pitch mechanically or accepting a 

 reduction of efficiency in one or both conditions. 



Permitting one or more propulsion devices to 

 free-wheel while the others are driving means 

 some added resistance due to the windmilling 

 action of the free wheels. Furthermore, each 



shaft of a windmilling propeller, plus all those 

 parts of the machinery which can not be un- 

 clutched from it, have to be lubricated continu- 

 ally during such an operation. 



60.18 Backing Power from Self-Propelled 

 Model Tests. A discussion of reversing and 

 backing is included under maneuvering in Part 5 

 of Volume III. However, it is stated here that 

 model-basin establishments equipped to conduct 

 self-propulsion tests can carry out steady-state 

 tests of this kind in the astern direction. Fig. 60. V 

 embodies the results of such a test on EMB 

 model 3594, representing the minelayer U. S. S. 

 Terror. Fig. 60. T contains data for the ahead 

 self-propulsion tests of this model at the same 

 displacement and trim. It is to be noted that the 

 thrust-deduction fraction in the backing con- 

 dition is very large, as might be expected, and 

 that the wake fraction is still positive. The pro- 

 pulsive coefficient is less than 0.40. 



H. F. Nordstrom presents the test results and 

 an analysis of the self-propelled experiments on 

 models of fishing boats, in which astern thrust 

 was achieved (1) by reversing the direction of 

 rotation of the 2-bladed propellers when set for 

 normal ahead running and (2) by angling the 

 blades to give reverse pitch [SSPA Rep. 2, 1943; 

 summary and some figure legends in English]. 



9 10 II IE 15 14 15 16 17 

 Ship Speed , kt 



Fig. 60.V Backing-Test Dat,\ for a Twin-Screw 

 Naval Vessel, from a Self-Propelled Model 



