676 



HYDRODYNyVMICS IN SHIP DESIGN 



Sec. 73.3 



it is so constant in service that cavitation or 

 separation does not occur on one side or the 

 other of the nose 



(b) Lengthening the long nose or entrance in- 

 creases the wetted area and the friction drag 



(c) Thinning the entrance or nose renders it 

 vulnerable to damage and susceptible to corrosion. 

 There is also the possibility of cavitation if 

 the body runs at an appreciable yaw angle. 



(d) A Umber entrance is easily set in vibration by 

 periodic external disturbances. 



At a free-water surface, a leading edge can 

 rarely be too thin to reduce resistance, spray, and 

 feather, provided it is strong enough to resist 

 random side loads and has enough lateral stiffness 

 to hold itself firm against lateral vibration. A thin 

 leading edge projecting through the surface is 

 vulnerable to damage by floating debris. 



As the depth below the free surface increases, 

 the leading edge can be thickened, if there are 

 advantages to be gained thereby. However, this 

 is to be done with caution, having in mind the 

 following: 



(1) There is ample hydrostatic or pumping 

 pressure available to make the liquid close in 

 around the body abaft the nose 



(2) A semi-circular leading edge joined to the 



Section Proposed by P Mandel, 5NAME, 1953 o 



NACA Symmetricol Section, t/c-l/e / TMB EPH o 



■EM^^^NovySt'd. Strut Section / / /Section "^ip 



Ip ip 5 W^ 



Abscissa >! in per cent of Chord c ,^'V__ 



Nose Radius 0.015 c. 



Ficj. 73. A Leading .\nd Trailing Edges op Severa 

 Strut and Hydrofoil Sections 



two parallel or slightly flaring sides of a 2-diml 

 body, or a hemispherical nose on a 3-diml body, 

 is not a good form. It is susceptible to cavitation 

 and separation abaft the head portion [Rouse, H., 

 and McNown, J. S., "Cavitation and Pressure 

 Distribution," State Univ. Iowa Studies in 

 Eng'g., Bull. 32, 1948]. The head should be 

 elliptical, as diagrammed in the upper part of 

 Fig. 73. A, with diminishing slope and curvature 

 at the junction of the head and the straight or 

 nearly straight sides abaft it. 



(3) If the variation in yaw angle or angle of 

 attack is not too large, the leading edge may be 

 pointed, using separate circular arcs, somewhat 

 like the nose of a projectile. The ogival head is 

 then merged into an elliptical shoulder. 



(4) Increased wetted surface on a pointed nose 

 may cause more drag than increased pressure 

 resistance on a blunt one 



(5) Length may be a major objection, for some 

 reason or other, requiring a deliberate shortening 

 at the expense of nose shape. 



It is explained subsequently, in a discussion 

 of rudders as movable appendages, that sharp 

 leading edges should be avoided on those parts 

 of a ship which may from time to time encounter 

 appreciable components of cross flow. The latter 

 components are those developed at the stern of a 

 vessel when it is sweeping around on a turn. In 

 view of the much smaller magnitude of cross flow 

 at the bow or in the forebody of a vessel it is 

 rarely necessary to anticipate a relative flow 

 there other than from directly ahead. For this 

 reason the stem may be made as sharp as is 

 consistent with practical considerations in build- 

 ing the vessel. 



All that is said elsewhere about the trailing 

 edges of sternposts, skeg endings, bossing termina- 

 tions, and contra-guide sterns applies equally to 

 the after edges of appendages of smaller absolute 

 size. In particular, the endings should be thin 

 enough not to generate harmful vortex trails, 

 with alternating lateral forces on the appendage. 



73.3 The Stem Cutwater. On a metal ship 

 it is useful to employ a curved stem plate or 

 casting, roomy enough for internal fabrication or 

 assembly. However, if the ship runs at medium or 

 high speeds, the blunt stem throws a high feather 

 of spray, which may be objectionable, and it 

 generates some unnecessary pressure drag. Both 

 problems are solved by adding a .sharp cvitwater 

 to the stem as an appendage. This reduces the 



