562 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 68.14 



that for which they were shaped, rarely have 

 sizable regions of reduced velocity or separation 

 around them. They create few eddies in which a 

 person can stand, as is possible in the lee of a 

 square corner on a deckhouse, and fewer calm 

 areas in which a passenger can sit in comfort. 



"The wind velocity over the open decks (with an 

 excessively streamlined superstructure), even in quite 

 mild conditions, can be such as to render intolerable 

 any attempts to walk or sit out!" [Hind, J. A., SBSR 

 14 Jul 1955, p. 37]. 



To pass around a streamlined structure in a 

 gale requires bucking a wind of ferocious velocity; 

 this becomes a struggle if one is heavily clothed. 

 Nevertheless, certain things can be done to reduce 

 the air drag without sacrificing any functional 

 features of the upper works. 



First, it is possible, even in rather small vessels, 

 to provide complete internal access from hving 

 quarters to operating stations for all officers and 

 crew. Indeed, this is now general practice on 

 large vessels and may be taken for granted in 

 any modern new design. If the same provision is 

 made for unusual operating conditions and for 

 manning emergency stations it is possible to 

 eliminate the necessity for crew members to move 

 around outside the upper works in bad weather. 

 The diagram for composition of air velocities 

 around a ship, indicated at C in Fig. 26.H, shows 

 that the ratio between the true-wind velocity 

 Wt and the ship velocity V has an appreciable 

 effect upon the bearing angle of the relative-wind 

 velocity W r at the ship. For winds normally en- 

 countered in good weather at sea, with velocities 

 not exceeding 20 kt, this relative-wind angle 

 increases, for a wind nominally on the beam, 

 from 45 deg abaft the bow at 20 kt to about 59 

 deg abaft the bow for 12 kt. For winds of strong- 

 gale force, say 60 kt, the difference between the 

 relative-wind angles for a nominal beam wind is 

 still appreciable, of the order of 71.5 to 80.5 deg 

 abaft the bow for 20 and 12 kt, respectively. 

 Streamlining a deckhouse which can not swivel 

 into the wind like a weathervane but which is 

 shaped to give minimum air drag with the true 

 and relative winds both dead ahead appears 

 somewhat absurd. 



A deckhouse not extending all the way to the 

 sides or to the ends of a surface ship hull and not 

 having any overhanging deck at its top level, 

 may be considered as relatively sheltered from 

 the wind if it has a height not exceeding 0.12 or 

 0.15 times the beam of the ship and if it hes an 



equal distance back of the side of the ship. This 

 is because of the separation zones behind the 

 sharp corners at the deck edges and the reduced 

 velocities in way of the miscellaneous small 

 obstructions on the deck. Unit air pressures on 

 the exposed sides of deck erections increase with 

 their height above the main hull and Ukewise 

 with the absolute size of the areas subjected to 

 ram pressure from the wind. For a wind velocity 

 of 60 kt, or 101.33 ft per sec, with an air tempera- 

 ture of 59 deg F, the ram pressure q due to wind 

 in a stagnation area is (0.5) (0.00238) (101.33)' or 

 12.22 lb per ft'. For a 30-kt ship steaming into 

 a 60-kt wind, developing a relative velocity of 

 90 kt, or 152.00 ft per sec, the corresponding ram 

 pressure is 27.49 lb per ft'. 



Shaping the deck erections for least wind drag 

 is based upon a relative-wind direction of about 

 30 deg on either bow, because it is at about this 

 angle that the wind blows separately on the 

 several structures spread along the length of the 

 vessel. This is also the angle, indicated by the 

 diagrams of Sec. 54.9, at which the fore-and-aft 

 wind resistance i2wind becomes a maximum. The 

 beneficial effects of housing uptakes, ventilators, 

 mast and instrument foundations, and the hke, 

 within deck erections necessary for some other 

 purpose, are not to be overlooked. Objects which 

 can not be so enclosed often lie in the lee of larger 

 objects (inside the separation and eddying 

 region behind them), at the 30-deg relative-wind 

 angle, and so do not require any streamlining for 

 themselves. It is not to be forgotten, however, 

 that swirling backflows into these regions, where 

 — Ap's exist, may take with them smoke, soot, 

 and foul gases discharged from poorly placed 

 openings. 



As an indication of what may be expected in 

 the way of air flow about a great variety of deck 

 erections and upward projections from the hull 

 and superstructure, for a relative wind from ahead, 

 Fig. 68. L shows the velocity vectors in both 

 elevation and plan view around the hull and upper 

 works of a large ship. Diagrams 1 and 2 of this 

 drawing were adapted from a series of five 

 detailed diagrams pubUshed by H. N. Prins, in 

 an article describing the hull features of the Dutch 

 passenger liner Oranje [De Ingenieur, The Hague, 

 Holland, 23 Jun 1939, PI. II and p.W. 56]. The 

 comprehensive data were taken from wind- 

 tunnel tests, made on a specially constructed 

 model of the vessel, complete to the last detail. 

 Unfortunately, they cover only the wind-ahead 



