538 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 67.24 



greater than the e.rpanded chord length of the widest 

 element or section. It is pointed out in Sec. 33.3, 

 and diagrammed in Fig. 33. E, that the clearance 

 abaft the wheel, at the 0.772, may be less than 

 that ahead. However, it should not be small 

 enough to interfere with the circulation flow 

 around the blade elements, or to bring the trailing 

 edge of the blade through a region of large +Ap 

 ahead of a blunt rudder post or equivalent. A 

 good rule is to make the after edge clearances, 

 both upper and lower, as marked on Fig. 33. D, 

 not less than the maximimi thickness of whatever 

 hull element or fixed or movable appendage may 

 lie abaft the propeller. 



It is well to note that the application of the 

 rules given here require prior knowledge of the 

 maximum blade width of the propeller which it 

 to run in the aperture being designed. Also that 

 most of the propeller charts employed to work 

 out the preliminary design of a wheel, following 

 Sec. 70.6, do not give the ship designer the 

 blade-width data for the optimum propeller. 

 He is then required to use the maximum expanded- 

 chord width for that series propeller which best 

 meets the needs for the preliminary design. 



Designed WQterline~-|^ 



Arch Clearance to 

 Nearest Point 'of Hull 



|T,p 

 Submerqence 



O.ZT) or C0.7R, 

 chord length at 

 0.7 radius, which- 

 ever is qreater 



weep Line s, 

 Aft and Ford. 



0.05 to 0.10 ft for\ 



Boseline^ 



Small Circles Indicate 

 Minimum Normol Clearances 



Small Croft 

 0.E5 to 0.5 ft for 



Lorqe Vessels 



NOTE a - Not Less Than Rudder Post or Rudder 

 Thickness Aboft Propeller 



NOTE 2- If Leodinq Edge of Rudder Post or 

 Rudder is Fine and Well-Shaped 



NOTE 3- Profile Shown is That of Transom- 

 Stern ABC Hull 



Fig. 67.U Elevation of Rudder Horn, Propeller 

 Aperture, and Skeg Ending for ABC Transom Stern 



Taking the 4-bladed Wageningen B.4.40 series, 

 for example, the note in small print at the right 

 of Table 8 on page 204 of the Dutch book "Re- 

 sistance, Propulsion, and Steering of Ships" 

 [RPSS, 1948] states that the (expanded maximum) 

 blade-element length at 0.6/2m«. is 0.2187Z). For 

 the after aperture clearance, specified as not less 

 than the thickness of the fixed or movable append- 

 age lying abaft it, the designer is required to 

 rough out these parts; this is, in fact, part of the 

 preliminary stern design. 



A general guide at this point as to the loading 

 on the individual blades and the aperture clear- 

 ances necessary is the value of the thrust-load 

 factor Ctl on the propeller. If of the order of 1, 

 the clearances may be somewhat on the low side. 

 If 2 or greater, the clearances must be larger to 

 avoid vibration. 



Fig. 67. U is a diagram which indicates, by the 

 small circles and the rules set down, aperture 

 clearances which are in general acceptable, for a 

 screw propeller not too heavily loaded. The 

 contours give the actual clearances worked into 

 the preliminary transom-stern design for the 

 ABC ship. 



Rules similar to those for single-screw propeller- 

 aperture clearances govern for the edge clearances 

 at the termination of bossings, multiple skegs, 

 and the like, indicated in Fig. 33.B of Sec. 33.3, 

 with the proviso that this clearance, at any 

 propeller radius, should be not less than the 

 expanded chord length of the blade at that radius. 



It is difficult, with present knowledge, to 

 formulate a rule for determining the hull tip 

 clearance of a screw propeller, illustrated in 

 Figs. 33. B and 33. C. In fact, probably no one rule 

 or group of rules could cover all cases to be en- 

 countered in ship design. Two features, not 

 entirely independent, are involved here. First, 

 the propeller blade tip should not be subjected to 

 brief passage through a region of high wake 

 velocity where the lift and drag forces on its 

 elements are suddenly increased. Second, assuming 

 uniform, non-axial flow, a blade tip should not 

 swing close enough to the hull to cause a sudden 

 large force on the shell plating or an adjacent 

 appendage due to the pressure field around the 

 blade or beyond the tip. 



Keeping the blade tips clear of high-wake 

 regions is a matter of: 



(a) Boundary-layer thickness, a function prin- 

 cipally of absolute ship speed, of fore-and-aft 



