680 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 73.6 



Both Dioqrams are 

 Projected Forward on 

 Transverse Plane 



Circle Tbni^ent to the 

 Inner Sides of Both 

 Strut Arms and to 

 the. Strut Hub 

 Should Hove a 

 Radius Not Less 

 Than That of the Outside 

 of the Hub 



Strut VeeAn(5le Should Be 



Greater Than 50 dea for 

 Good Flow and Ricjidit'y 



Fig. 73. D Hydrodynamic Requirements for Strut- 

 AjiM Positions at the Strut Hub 



with the strut hub are lengthened with large- 

 radius fillets forward and aft, principally to give 

 stability of position to the strut hub. 



At the hull ends of the strut arms, provided 

 these go through into the hull, no fairing is neces- 

 sary if they stand normal to the shell or nearly so. 

 When the transverse reentrant angle at the shell 

 becomes 70 deg or less, a fillet is introduced by 

 some convenient method. The fillet radius in- 

 creases as the reentrant angle becomes smaller. 

 It is often possible to bend the strut arm and have 

 it enter the hull nearly normal to the shell. 



Struts are attached to the hull by external 

 palms only when no other method can be used. 

 The palm endings are sloped and faired so that a 

 section through them, in the direction of flow, 

 approximates that of one side of a standard (or 

 acceptable) strut section. 



Normally it is not necessary, for hydrodynamic 

 reasons only, to mcrease the length or the fineness 

 of a strut-arm section where it attaches to the 

 strut hub or to the shell. If fairing is required on 

 the sides of the section, for structural or other 

 reasons, a longer termination is automatically 

 necessary if the section fineness is to be main- 

 tained. 



Under no circumstances should an upper strut 

 arm join the hull at a point above the free-water 

 surface if propulsion performance at a displace- 

 ment and trim corresponding to the position of 

 that surface is considered of major importance. 



A hole in the water is certain to form at the free 

 surface abaft the strut, because of the low hydro- 

 static pressure there. The minutest degree of 

 separation abaft the tail of the section is almost 

 certain to provide a reduced-pressure passage for 

 air, extending from the hole at the surface all 

 the way down to the propeller hub. Here the air 

 passes into the propeller disc in the manner 

 illustrated by Fig. 2.3. D, reduces the thrust, and 

 creates noise and vibration. 



If it ever becomes necessary to carry or to rig 

 a strut section which must come to the surface, 

 a flat subsurface plate resembling those in Figs. 

 7.E and 36.0 is attached to it just below the 

 surface and in the line of flow. This plate prevents 

 air from leaking down through the bottom of the 

 hole in the water, just abaft the strut. An extra- 

 long fairing is necessary above this plate if the 

 strut is not to create excessively large waves at 

 the surface or to throw inordinate amounts of 

 spray. 



It is customary to place the strut-arm axes in 

 transverse planes, that is, square to the baseplane. 

 It may often be better to rake or tilt them out 

 of these planes if they can be shortened thereby, 

 if their axes can be placed more nearly normal to 

 the direction of water flow, or if better attach- 

 ments can be made to the hull structure. The angle 

 of rake or sweep-back can be as large as 30 deg 

 with the transverse plane. If piercing the free 

 surface is unavoidable it is best to rake the strut 

 down and forward. This creates less separation 

 than when raked down and aft, as explained in 

 Sec. 36.17 and illustrated at 5 in Fig. 36.0. 



73.6 Strut-Arm Section Shapes for Ultra- 

 High Speeds. For planing craft which operate 

 at high speeds and for ultra-high-speed racing 

 motorboats the exposed propeller shafts are in- 

 variably carried by single-arm or V-type struts. 

 The submergence of these struts is small because 

 of the relatively small draft, hence the cavitation 

 index is correspondingly low. At the high speeds 

 at which they travel it is impossible to make a 

 strut section sufficiently long to be free of cavita- 

 tion over its after portion. It is the practice 

 therefore to utilize only the forward portions of 

 these sections, making the entrances fine and 

 narrow, and terminating them in square or boat- 

 tail endings in the manner illustrated in Fig. 73. E. 

 They then resemble the transom stern on a fast 

 motorboat. Single struts having blunt ends may 

 be placed forward of the propeller provided there 

 is an exposed sloping shaft ahead of the strut. 



