348 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 59.17 



parts whose longitudinal position is fixed by the 

 thrust bearing, depending upon whether the 

 axial weight component is directed forward or 

 aft. The axial friction effects in the various shaft 

 and machinery bearings, with the parts rotating 

 at any speed above very slow, may generally be 

 neglected. The axial weight component is usually 

 a secondary factor in the selection or design of a 

 thrust bearing, but it is a factor of importance in 

 the analysis of shipboard thrust measurements, 

 where its magnitude is often appreciable, com- 

 pared to the propeller thrust [SNAME, 1934, 

 pp. 151-152]. 



Fig. 59.L, adapted from Figs. 13 and 14 on 

 pages 151-152 of SNAME, 1934, illustrates 

 diagrammatically several types of machinery, the 

 weights of each to be included in the computation 

 for the axial component, and the manner in which 

 the various forces are combined at the thrust 

 bearing. As further refinements, not always 

 carried out in practice: 



(1) The weight of the propeller and of those 

 portions of the shafting completely surrounded by 

 water may be reduced by the buoyant forces of 

 the water on those parts 



(2) Part of the measured thrust is due to the 

 hydrostatic head over the section area of the 

 shaft where it enters the hull stuffing box 



(3) The -|-Ap's exerted over the projected axial 

 area of the hub abaft the blades, as well as the 

 — Ap's exerted over the forward exposed area 

 of the hub, outside the shaft, are measured in 

 model tests and reckoned in ship trials as part 

 of the thrust exerted by the blades. 



For high-speed vessels such as destroyers, in 

 which the attitude changes materially from the 

 at-rest to the running condition, the actual shaft 

 declivity at any .speed is a combination of that 

 built into the ship (or that imposed by the par- 

 ticular loading condition) and the running trim 

 at that speed. Assuming a change of trim from 

 zero to full speed of 1.5 deg, not uncommon in 

 these craft, the axial weight component of the 

 rotating parts of one main propelling unit is 

 changed by the sine of this angle, or some 2.6 

 per cent. This may amount to 3 per cent or more 

 of the full-speed thrust [SNAME, 1933, pp. 268, 

 275, 277]. 



59.17 Estimates of Thrust and Torque Varia- 

 tion per Revolution for Screw Propellers. The 

 reasons for the generation of thrust and torque 

 variations on the blade of a screw propeller as it 

 rotates, and for the application of offset forces and 

 bending moments on the shaft, are described in 

 Sees. 17.3 through 17.7, 17.12, and Sec. 33.13 in 

 Volume I. The equalization of the thrust and 



Jaw Couplinq with No 

 Fore- and- Aft Restraint 



Forces Zi|T and A^T are the Products of the Wcltjhts W, and 

 W2 and the Sines of the Respective ArKjIes of Declivity, 

 with Friction Effects Nei^lected 



When Calculating the Total Weiqhl 

 of the Rotating Parts, the Weiqhls 

 of the Propeller and of the Shaftino 

 Sections Surrounded b"y Water are 

 to be Diminished b'^ the Buoyancy 

 of that Water 



On Slow-Speed and Medium-Speed 

 Vessels the Chanoe of Trim and of 

 Shaft Declivity Due to Speed Through 

 the Water is Usually Neqli(jible. 



Horizontal Baseplane^^^ 



Fig. 59.L Weight and Force Diagrams Illustrating Relationship Between Propeller Thrust and 

 Measured Axial Load on Thrustmeter 



