Sec. 73 J 



"15 5cQ l e of Ordin ates 



FIXED-APPENDAGE DESIGN 



Chord C 



679 



x-Distonce from Leodinq Edqe in Per Cent of Chord c 

 Fig. 73.C Half-Sections of Five Strut Shapes 



straight-ahead motion on a given course. It 

 seems reasonable to assume that the ship is 

 running in this fashion for about 98 per cent of its 

 operating time. 



The proper angle can be estimated after a 

 fashion, using data for flow around the stern as 

 given in various chapters. Nevertheless, good 

 design requires an experimental determination on 

 a model with special apparatus. It is preferred, 

 because of the influence of induced velocity in 

 the water passing into a propeller disc, that the 

 strut-arm section angles be determined while the 

 adjacent propeller is delivering normal thrust. 

 This can be done, along the lengths of the two 

 struts of a pair, by apparatus described and 

 illustrated by H. F. Nordstrom [SSPA Rep. 32, 

 1954, Figs. 22 and 23, pp. 30-31]. 



Twisting the strut arms to suit the angle of 

 flow is usually an inconvenience when the struts 

 are built, but failure to align the strut sections 

 with the flow, especially ahead of a propeller 

 disc, only invites trouble by setting up disturb- 

 ances in the inflow jet. 



Whether a strut with two or more arms lies 

 ahead of or abaft a screw propeller, it is well to 

 avoid a strut-vee angle (see Fig. 36. B) which is 

 nearly or exactly the same as the angle between 

 two blades which may be passing the arms simul- 

 taneously. A convenient example in this respect 

 is the four-arm strut abaft the 4-bladed propeller 

 of the arch-stern ABC ship, described in Sees. 

 73.7 and 73.8 and illustrated in Fig. 73.F of 

 Sec. 73.8. When looking forward on this vessel, 

 consider the radial position of the lower port 

 strut arm as zero angle. With spacings of 75, 60, 

 and 75 deg between the three pairs of arms, 

 reckoned in a clockwise direction, the angular 

 positions are tabulated as follows: 



Should a 5-bladed wheel be fltted at a later time 

 on this vessel, the angular "stagger" is rather 

 small. With one blade opposite the port lower 

 strut arm the next blade is only 3 deg ahead of 

 the port upper arm, the second blade 9 deg 

 behind the starboard upper arm, and the fourth 

 blade 6 deg behind the lower starboard arm. 



The matter of whether the strut-arm axes lie 

 close to the propeller-shaft axis (radial type) or 

 whether they are spread so as to pass close to the 

 mean radius of the strut hub (tangential type), 

 is often determined from a structural rather than a 

 hydrodynamic standpoint [Roop, W. P., "The 

 Strength of Propeller Shaft Struts," SNAME, 

 1926, pp. 119-137 and Pis. 44-52]. Considering 

 the flow of water through the strut position there 

 is probably httle to choose between the two. The 

 tangential arms are well spread at the hub but 

 they usually involve reentrant angles alongside 

 the barrel considerably smaller than 90 deg. 

 Radial arms give good attachments for supporting 

 the shaft but the passage between the arms at 

 the hub surface may be somewhat constricted if 

 the strut-vee angle is small. To avoid blocking 

 effects due to closeness of the arms at the hub 

 there should be an open space at the barrel 

 surface equal to at least 3 times the maximum 

 strut-arm thickness, indicated at 2 in Fig. 73. D. 

 Better still, a clearance circle may be drawn 

 between the arms equal to the maximum outside 

 diameter of the hub, as at 1 in the figure. Strut-vee 

 angles larger than 60 deg represent good solutions 

 of all requirements but it frequently becomes 

 necessary to decrease this angle to the order of 

 45 deg. This should be a minimum value, to 

 afford adequate rigidity to the assembly. 



Fairing the hub ends of the strut arms into the 

 strut hub or barrel is probably more a matter of 

 structural continuity than of easing the flow 

 around these junctions. A fillet radius at the 

 barrel equal to or exceeding the maximum strut- 

 arm thickness appears to be adequate for good 

 flow, even at the apex of a transverse reentrant 

 angle. However, the strut arms at their juncture 



