Sec. 68.14 



ABOVEWATER-FORM LAYOUT 



561 



practicable but not necessarily shunned. Reen- 

 trant angles in these coves, typified by the long 

 cove at the bottom of the set-back trunk of 

 diagram 1 in Fig. 68. K, and by the long junctions 

 of hull and sponsons in the ferryboat section at 

 2 in Fig. 68.K, and in SNAME RD sheet 84, can 

 approach 90 deg provided this small angle is 

 necessary for other reasons. Little extra drag is 

 encountered if these coves are under water in 

 some load condition provided the corners follow 

 the temporary flowlines reasonably well. 



Keeping abovewater discontinuities out of the 

 reach of waves at sea is Avell-nigh impossible. The 

 smaller the change in direction as the water 

 strikes the discontinuities the less spray they 

 throw and the smaller are the hydrodynamic 

 forces exerted on them. 



68.13 Transverse Discontinuities. Any lon- 

 gitudinal discontinuity which runs for a consider- 

 able distance along the ship is also a transverse 

 discontinuity. The latter is distinguished here 

 as one which involves a projection — or a recess — 

 from the fair section lines in the vicinity which is 

 large compared with its fore-and-aft length. A 

 good example is an old-fashioned gun sponson 

 on a combatant vessel, protruding from the ship's 

 side like a bay window to obtain a line of fire 

 along the side. Projections or recesses of this 

 type, if kept clear of the ship-wave profile, need 

 no special consideration from a hydrodynamic 

 standpoint for ships which travel in relatively 

 smooth water, surrounded only by their own 

 waves. 



The use of isolated transverse braces or sup- 

 ports for overhanging portions of the abovewater 

 hull or upper works, even when nominally clear 

 of the wave profile, is not encouraged. Under 

 some unusual and unlooked-for circumstances 

 these transverse members are liable to become 

 fouled by floating debris, ice, or breasting floats 

 (camels). 



Abovewater projections may be fitted, as on 

 whaling factory ships [SBSR, 5 Dec 1946, pp. 

 625-635], to increase the deck space locally, to 

 serve as large-area fenders for protection of the 

 underwater hull when other vessels lie alongside 

 in the open sea, and for other purposes. 



68.14 Shaping and Positioning of Super- 

 structure and Upper Works. Several major 

 considerations enter into the design of that portion 

 of any ship lying above the main hull. Every 

 deck erection, every spar or post, every pro- 

 tuberance of whatever kind has a utilitarian 



purpose. Even wind screens and shelters have to 

 be provided for passengers, especially on high- 

 speed ships [Currie, Sir William, SBMEB, Jul 

 1955, p. 435]. Some of these purposes are served 

 only in port, some only at sea, and some are used 

 almost all the time. Aside from the effect of 

 these upper works on weight, cost, and transverse 

 metacentric stability, practically all of them 

 involve some air drag and produce some wind 

 resistance. To overcome this, extra power must 

 be provided or exerted, or speed must be sacrificed. 



"The model of the (old) Mauretania required an in- 

 crease of 20 per cent in power to drive the structure added 

 to represent deck houses" [Barry, R. E., Mar. Eng'g., 

 Sep 1921, p. 690]. 



Regardless of the type of vessel or the service 

 expected of it, any owner and operator may be 

 expected to affirm that convenience of access, 

 availability of outside light and air, comfort of 

 the passengers, and the needs of the crew are 

 to take precedence over the reduction of wind 

 resistance of the upper works. Put in another 

 way, he will unquestionably be found reluctant 

 to sacrifice utility, passenger comfort, and other 

 factors having to do with the handling and 

 operation of the vessel for the sake solely of 

 reducing its wind resistance. Nor will he generally 

 find it a paying proposition to spend a great deal 

 of money and effort to diminish the wind resist- 

 ance, at no sacrifice in other features, unless some 

 outstanding improvement is to be gained. 



Tests at the Case School of Applied Science, 

 on 33-in wind-tunnel models of the Atlantic liner 

 Manhattan, showed that by completely stream- 

 lining the whole ship — in other words, by treating 

 both hull and upper works as a unit — the wind 

 resistance with the relative wind ahead was 

 reduced approximately 84 per cent, compared to 

 the Manhattan as built. This was for a ship speed 

 of 20 kt, a true wind speed of 23.2 kt, and a 

 relative wind speed of 43.2 kt. The modification 

 involved an entirely different concept of a pas- 

 senger ship, with everyone completely housed at 

 all times, but it indicates what can be done if all 

 other considerations are disregarded. 



This effort shows how easy it is to lose sight of 

 the fact that so-called streamlining of individual 

 deckhouses and other erections above the hull 

 by no means insures that it will be easier for the 

 crew to make their way about the upper works 

 under storm conditions. Structures of fair form, 

 when blown upon in a direction approximating 



