574 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 69.9 



The available data relating to power equaliza- 

 tion and proportioning and to correlation of 

 torque and rate of rotation on existing merchant 

 ships or on self-propelled models of them are 

 by no means extensive. J. M. Labberton in a 

 paper entitled "A Method for Determining 

 Proper Pitch for the Inboard and Outboard 

 Propellers on a Four-Screw Ship" [ASNE, Nov 

 1937, pp. 576-584], discusses this question and 

 gives the following data for the old Mauretania, 

 taken during the trials of 1907: 



Port Port Star. Star. 

 Outer Inner Inner Outer 



Rate of rotation, rpm 

 Shaft power, horses 



187.3 186.6 1S8.6 188.6 

 17,350 20,650 20,650 18,600 



AH four propellers had a diameter of 17 ft and 

 a pitch of 15.75 ft. In this case the rates of rotation 

 were as uniform as could be hoped for in such a 

 large new ship but the inner propellers were 

 absorbing some 53.5 per cent of the total power, 

 or about 15 per cent more than the outer pro- 

 pellers. 



The designer may find that he has only limited 

 freedom in shaping the hull and placing the 

 propulsion devices. After he has done what he 

 can in positioning these devices properly and 

 working out the adjacent appendages he is able 

 to make use of existing model-testing techniques 

 which indicate and record the flow velocities and 

 directions at the propeller positions, both at 

 the hull and appendage surfaces and at distances 

 from them. It is possible, for example, to make a 

 wake survey at an after propeller position with a 

 model propeller working in a position ahead. 

 Facilities have been developed but are not yet in 

 general use in model basins, whereby a wake 

 survey is made just ahead of a working propeller. 



From these data it is possible to estimate 

 rather closely the wake magnitude and distribu- 

 tion at each propeller position. An estimate of 

 the individual thrust-deduction fractions is, 

 however, still difficult. Before a model is self- 

 propelled it should be possible to determine what 

 variations in propeller design are necessary to 

 compensate for hull features not under the control 

 of the designer. One or more series of self-propelled 

 tests, possibly with a change in propeller design in 

 between, should insure reasonably close equaliza- 

 tion or apportioning of the full-scale shaft powers, 

 and correlation of the torque-rpm values. 



69.9 Powering Allowances. A doctrine in- 

 volving design and performance allowances. 



applying to the hydrodynamic features of a new 

 ship whose principal characteristics are being 

 formulated, is set down in Sec. 65.3. The emphasis 

 in that section, repeated subsequently in the 

 present one, is concentrated on the outstanding 

 advantages to be gained by designing a speed 

 margin rather than a power margin into the ship. 

 This means that the ship hull is shaped to be 

 driven efficiently at a speed greater than the 

 sustained speed when developing its maximum 

 power. The alternative method, practiced in 

 some quarters, is to design the ship hull for the 

 sustained speed and then to add a large power 

 allowance. This might be acceptable if the 

 problem were only one of overcoming increased 

 resistances due to heavy weather and to fouling. 

 However, it results in overdriving and poor 

 performance at the augmented speeds necessary 

 for a ship which, running on a definite schedule, 

 has to make up time after a spell of bad weather. 



Good design of the propelUng plant of any 

 water craft calls for a reserve of power (1) to 

 meet emergeircies, (2) to enable the plant to 

 keep running and to deliver a sort of average 

 power with minor casualties, and (3) to permit 

 it to run much of the time at less than maximum 

 rating. With pressures, loads, and other factors 

 reduced, wear and tear is usually diminished and 

 the periods between overhauls is increased. 



Almost every plant is capable of developing an 

 emergency overload power for a few minutes, 

 perhaps for a few hours, if it becomes a matter 

 of saving life or the ship. Since this may result 

 in slight but permanent damage to the machinery, 

 it is not considered in the customary powering 

 calculation. The maximum designed shaft power 

 is therefore that "for which the propulsion 

 machinery is designed to operate continuously" 

 [SNAME, Stand'n. Trials Code, 1949, p. 11]. 

 For powering a boat or ship, the machinery 

 reserve is reckoned below this level. 



In general, the reserve of power is a function 

 of the length of time that operation at maximum 

 designed power is required. For a racing motorboat 

 which may run at full throttle only a few hours 

 between engine overhauls, but which must then 

 do its utmost, the reserve is practically zero. If 

 the game is considered worth the candle, so to 

 speak, the emergency power is called upon, in 

 which case the reserve is negative. The other 

 extreme is a boat or sliip wliich operates under a 

 wide range of conditions and which stops for 

 repairs only when it will no longer run. The 



