278 



HYDRODYNAMICS IN SHIP DESIGN 



Sec. 54.5 



with each. G. S. Baker iUustrates and uses this 

 method in his book "Ship Efficiency and Econ- 

 omy" [1942, pp. 14-16], but for a slightly different 

 purpose. 



In still air, the relative-wind velocity W ^ is 

 that caused by the motion of the ship itself 

 through the water, so that W r = V. The still-air 

 resistance is then 



UsA — C/D(Air)o^'l ' 



(54 .vi) 



As for the magnitude of this resistance, E. F. 

 Eggert estimates it as from 2 to 4 per cent of the 

 water resistance [TMB Rep. 264, Aug 1930, p. 1]. 

 The first example of Sec. 54.10 indicates that for 

 the ABC ship designed in Part 4 it is estimated as 

 about 2.6 per cent of the bare-hull water resistance. 

 Although it is not general practice to allow for 

 still-air resistance in ship-powering estimates, 

 this resistance is always with the ship, so to speak, 

 unless the latter happens to run in a following 

 natural wind having a velocity equal to its own 



For a relative wind compounded of ship 

 motion and an ahead wind of gale force, the wind 

 resistance can form a large percentage of the 

 water resistance. For the second example of 

 Sec. 54.10, assuming an ABC ship speed of 18.5 

 kt and a strong breeze of only 23 kt (true velocity), 

 the wind resistance is 10.8 per cent of the bare- 

 hull hydrodynamic resistance at that speed. 



54.5 Notes on Wind-Resistance Models and 

 Testing Techniques. The values of wind-drag 

 coefficient Cd( Air) now available for the estima- 

 tion of the wind resistance of the hull and upper 

 works of a ship of any size and type are derived 

 almost exclusively from tests of special wind- 

 resistance models, similar to that pictured in 

 Fig. 54. B. These have been run in water and in 

 air, sometimes in both. Despite the limitations in 

 scope of the book imposed in Sec. 1.5 of the 

 Introduction to Volume I, a few notes are given 

 here concerning the rather unusual techniques 

 employed with these models. 



When making tests with abovewater models 

 to determine wind-drag coefficients, the models 

 are: 



(1) Mounted on the under side of a flat board or 

 platform representing the water surface, and then 

 towed inverted in a model basin. The under 

 surface of the board is held at the water surface, 

 with the model upside down in the water. The 

 board, with the model, may be oriented in azimuth 



to represent relative ^vind impinging on the model 

 at any bearing from right ahead to right astern. 



(2) Made double, symmetrical about the designed 

 waterline, embod3ang two abovewater hulls which 

 are then towed submerged 



(3) Mounted on the under surface of a flat plat- 

 form and suspended, in inverted position, in a 

 circulating-water channel. 



When the test is conducted by procedure 

 (3) it is possible to look up from underneath and 

 watch the position and action of tufts attached to 

 the abovewater hull and the upperworks. This 

 is also possible in a wind tunnel but streamers of 

 dye and ink injected into the slow-moving water 

 of the channel show the flow to much better 

 advantage than jets of gas or smoke injected into 

 the fast-moving air of the wind tunnel. 



There is little to represent boundary-layer 

 development over the full-scale water surface 

 under these conditions, causing a natural variation 

 of velocity with vertical distance, because the 

 mounting board is only several times as large as 

 the planform area of the model. For the model 

 test, therefore, the water (or wind) velocity is 

 nearly constant over all parts of the model, from 

 the lower part of the abovewater hull to the 

 mast trucks. 



Fig. 54. B illustrates what is known as a drawing- 

 room model of a destroyer but it shows well the 

 amount of detail which is customarily reproduced 

 on wind-resistance models. The mounting board 

 shown there is greatly enlarged for a wind- 

 resistance test in water. 



The experimental techniques developed to date 

 do not simulate fully the actual ship conditions. 

 This is not surprising because the full-scale 

 conditions are not yet adequately known. How- 

 ever, the wind drags are usually not-too-large 

 fractions of the total resistance to motion, and 

 the available data appear to serve well for 

 estimating purposes in the preliminary-design 

 stage and for analysis and reduction of full-scale 

 trial data. 



54.6 Bibliography of Model Wind-Resistance 

 Tests. There is given hereunder a partial list of 

 published data relating to model \vind-resistance 

 tests. Other data undoubtedly exist but they 

 have not yet been collected. In addition, the list 

 contains references embodying shipboard observa- 

 tions on air drag and wind resistance and other 

 references describing the use of these data in 

 analyzing ship trials. The list follows: 



