Sec. 69.8 



GENERAL DESIGN OF PROPULSION DEVICES 



573 



Assuming that the latter is the case, a syste- 

 matic variation in the flow at the position of the 

 propulsion device is looked for, one which will 

 give a superior efficiency or perhaps a larger 

 absolute thrust for a particular direction of 

 rotation. This matter is discussed in Sees. 33.4 

 and 33.6. If the upward component of flow on 

 the outboard side of a twin skeg is larger than 

 on the inboard side, outward-turning screw 

 propellers are indicated so that the outer blades 

 moving downward may "meet" the water flowing 

 upward to them. If for some special reason the 

 reverse is the case, as with the outside water 

 flowing horizontally and the inside water upward 

 through a tunnel, inward-turning propellers ■ are 

 found more efficient. If circumstances limit the 

 design of long deflection-type bossings or asym- 

 metrical skegs to a particular diversion of the 

 surrounding water the screw-propeller rotation is 

 selected to take advantage of it, provided of 

 course that other design requirements are met. 



Considering hydrodynamics only there are 

 very few reasons why any propulsion device of a 

 single- or multiple-unit installation may not 

 rotate in the direction which best produces 

 the desired performance of that unit by itself. 

 This is on the basis that the outflow jet from 

 any one device does not pass through the disc 

 or thrust-producing area of another device, and 

 that the resulting unbalanced torque applied by 

 the propelling plants to the hull, discussed in 

 Sec. 69.13, lies within acceptable limits. 



It is customary for large vessels, but by no 

 means necessary for all vessels, that screw pro- 

 pellers be rotated in the following directions: 



(a) Twin screws, in opposite directions, especially 

 if the flow patterns are decidedly of opposite 

 hands, symmetrical with the centerplane 



(b) Surface propellers or those which run for 

 much of the time with part or all of their upper 

 blades out of water, must rotate in opposite 

 directions in pairs if the large lateral forces pro- 

 duced by them are to be balanced 



(c) Triple screws embody wing propellers rotating 

 in opposite directions. The center propeller 

 rotates in the most suitable and convenient 

 direction, especially if under some conditions the 

 vessel is to be propelled entirely by the center 

 propeller, with the wing propellers free-wheeling. 



(d) Quadruple screws rotate, in pairs, in opposite 

 directions on opposite sides of the ship. 



69.8 Design to Equalize or to Apportion the 



Powers of Multiple Propellers. As soon as it is 

 decided to use multiple propulsion devices, two 

 or more in number, the designer begins to think 

 about the problem of equahzing the powers 

 absorbed by all of them. This corresponds to the 

 proper proportioning of the powers, regardless 

 of the number of propulsion devices, to the de- 

 signed powers of the propelling units which are to 

 drive them. The large quadruple-screw liner 

 Empress of Britain of the early 1930's was pro- 

 pelled by two large inboard screws plus two smaller 

 outboard ones. All four were to be used in trans- 

 ocean service but for around-the-world cruising 

 the outboard screws were to be removed entirely, 

 leaving the ship to be driven by the inboard 

 propellers only. These were capable of absorbing 

 two-thirds of the total power and were the only 

 ones which could be reversed. 



Proper design procedure involves consideration 

 and, if possible, control of the following items at 

 each propulsion-device position: 



(a) Boundary-layer thickness and velocity profile, 

 for both clean- and foul-bottom conditions 



(b) Retardation or possible reversal of flow behind 

 blunt bossing or skeg endings, involving wake 

 fractions with large positive values, possibly 

 exceeding LO 



(c) General and local direction of flow through 

 propeller discs or other thrust-producing areas, 

 plus wake-survey data. This is a case where 

 consideration of only the axial component of 

 flow is definitely not adequate. 



(d) Non-axiality of flow, due not only to the 

 shape of the adjacent hull but to the necessity 

 for placing shafts to suit the engines inside and 

 the propulsion devices outside 



(e) Possibility of an outflow jet from a propulsion 

 device ahead finding its way into the inflow jet 

 of one abaft it 



(f) Rate of rotation of the various propulsion 

 devices when absorbing the designed powers. 

 The propeller torque may be so large that the 

 engine delivers rated torque at less than the 

 designed rate of rotation, preventing the develop- 

 ment of full rated power. On the other hand, the 

 speed of advance may be so high that the engine 

 reaches its rated rpm when developing less than 

 the rated torque. 



(g) Necessity for accurate correlation of torque 

 and rate of rotation to achieve full rated powers 

 for internal-combustion engines driving (single or) 

 multiple propellers. 



