532 



HYDROnVNAMTCS IN SHIP DESIGN 



Sec. 67.22 



(2) Incorporate a forebody which would be 

 employed Math a normal form of stern 



(3) Do not hesitate to use twin or multiple 

 skegs under a transom or shelf-type stern 



(4) Do not be concerned about asymmetry 

 between the inboard and outboard sides of skegs 

 carrying screw propellers if the water flow or 

 propeller performance is improved thereby 



(5) Give serious consideration to the use of 

 twin or multiple rudders. If maneuvering qualities 

 are important, twin rudders are placed abaft 

 twin propellers. 



(6) Work out an arrangement whereby the 

 rudders, propellers, and shafting can be dis- 

 assembled or removed with the least interference 

 between themselves or from other parts 



(7) For rudders mounted in propeller races, keep 

 them far enough from the propellers to permit 

 removing the propellers without disturbing the 

 rudders other than turning them to a convenient 

 angle 



(8) If the afterbody of the ship is especially wide 

 and full, and the ship is to have three or four 

 propellers, consider spreading the skegs far apart, 

 and carrying the outboard or wing propellers in 

 the skegs. The inner propeller(s) may be placed 

 in the tunnel between the skegs, with the shaft (s) 

 for the inner propeller(s) carried by double-arm 

 struts of the erect V-type. 



Study of flow phenomena since the publication 

 of the 1947 reference indicates that the former 

 limiting slopes for the tunnel roof may be approxi- 

 mately doubled, making them 16 to 18 deg, but 

 probably with an accompanying increase in 

 thrust-deduction fraction. The flow pattern 

 should be checked, however, by taking the usual 

 flowlines around the skegs and inside the tunnel 

 in a model basin, and by observation of tufts in 

 a circulating-water channel. The thrust-deduction 

 values require checking by a self-propelled model 

 test. 



If tests on a model with chemical indicators or 

 tufts show irregular flow within the tunnel or 

 water crossing underneath the skeg in the design 

 as completed, the skegs may be shifted sideways 

 until a satisfactory flow pattern is obtained. If 

 twin skegs of full depth interfere with maneuvering 

 they may be cut away in profile in the manner of 

 a clear-water single-skeg stern. 



67.22 Design Notes for the Contra-Guide 

 Skeg Ending. These design notes apply to 

 coutra-guide features in a vertical skeg termination 



ahead of a screw propeller. The unsymmetrical 

 termination usually extends both above and 

 below the propeller axis, but this is not necessary. 

 The plane of the termination passes through, or 

 lies close to the shaft axis. The purpose of the 

 contra-guide feature is, as explained in Sec. 25.16, 

 to leave as little as possible of the rotational or 

 tangential component of velocity in the outflow 

 jet of the propeller. Before proceeding with the 

 design of a contra-guide skeg ending, it should be 

 known whether a contra-rudder is to be fitted 

 abaft the propeller to accomplish part of this 

 purpose. 

 The design problem consists of: 



(a) Determination of the true deflection angle 9s 

 abaft any unsymmetrical skeg ending. This is a 

 general problem involving the flow about the 

 trailing edge of any body when circulation is not 

 present, discussed in Sec. 36.3. 



(b) Subdivision into a series of subproblems, one 

 each for a series of horizontal planes passing 

 through the termination of selected radii on the 

 propeller. It is customary to subdivide the 

 propeller radius R into tenths or twentieths, 

 indicated on the propeller drawing of Fig. 70.O. 

 If the exact or final propeller diameter D is not 

 known at the time, the skeg ending may be 

 intersected by horizontal planes passing through 

 the waterlines used for delineating the remainder 

 of the hull. 



(c) Selection of the actual angle ds to which the 

 flow into various radii of the propeller shall be 

 deflected. This flow is directed always to 7neet 

 the propeller blades when rotating in the ahead 

 direction. 



(d) Selection of the offsets of the deflector 

 terminations from the ship centerplane or from 

 the construction plane of the skeg, to give the 

 angles selected in (c) 



(e) Avoidance of separation on those sides of the 

 deflectors having the steeper waterhne slopes. 



Reference books on the design of hydraulic 

 machinery, including propeller-type pumps, 

 appear to give little or no specific information on 

 the actual design of guide vanes ahead of impellers. 



One of the early patents on this device [U. S. 

 1,500,073, by Hans Haas, 1 July 1924], states 

 that the deflector shape "has proved to be 

 specially suitable" when the product of (1) the 

 rotational velocity component imparted to the 

 water opposite any propeller radius and (2) that 

 propeller radius formed (3) a constant quantity 



