TEN THOUSAND DEADWEIGHT-TON TANKER. 323 



less course at 8 knots in still air, the engine would operate at 56.3 R. P. M. and 15.3 pounds 

 M. E. P. If a wind sprang up, in order to maintain the same speed the R. P. Ms. would 

 have to be increased to 58.8 and also the M. E. P. to 17.8 pounds in order to produce the 

 R. P. M. of 58.8. 



The difference between the I. H. Ps. for these conditions is (engine constant is 1.1316) : 



I. H. P. (against wind) = 1.1316 X 17.8 X 58.8 = 1,184.4 

 I. H. P. (still air) = 1.1316 X 15.3 X 56.3 = 974.8 



Increase in I. H. P. = 209.6 



Increase in E. H. P. = I. H. P. X propulsive coefificient = 209.6 X .52 = 109.0. 



,„. J ^ E. H. P. X 33,000 , ,,^ 



Wmd force = ^ ' ,= 4,440 pounds. 



This wind force is that part of the total air resistance due to the moving wind. The 

 assumed wind at this time was about 1 5 knots ; relative wind 23 knots. 



Froude's formula for the air resistance of a ship is given by Taylor ("Speed and Power 

 of Ships") as: 



R^ = .0043 AV^ 

 Where 



R^= Air resistance in pounds. 



V = Speed through air in knots. 



A = Area of upper works projected on an 'thwartship plane. 



For the case in hand .0043 A works out to 8.95, so that : 



R^ = 8.95 V 



The force of wind by this expression is then: 



Wind force = 8.95 (23== —8=) =4,160 pounds. 



which is approximately 6 per cent less than the experimental value derived above (4,440 

 pounds). 



Proceeding in a similar manner, going with the wind, 



R. P. M. = 55.7 at 8 knots; M. E. P. = 14.7 pounds. 

 Difference of M. E. P. = 15.3 — 14.7 = 0.6 pound. 

 Wind force = 1,023 pounds aiding. 



Wind as before 1 5 knots, then relative wind = 7 knots pushing ship : 



8.95 (49 + 64) = 1,010 pounds. 



or practically one per cent less than experimental value. 



Since these two percentages are not the same, it means that the no-wind M. E. P. line 

 is not quite correctly located, but the difference is trivial. 



Either the area for use in Froude's formula was estimated too small or the velocity of 

 wind was assumed too little. 



