12 



e . Thrust deduction or suction force due to the propeller ; Generally 

 this force is not considered when dealing with the resistance, since it de- 

 pends on the interaction between propeller and hull. However, it seems to be 

 appropriate to mention It, as it influences decisively the form of the after- 

 body, especially for single-screw ships, by requiring the creation of a wake 

 axially sjnnmetrlcal with respect to the propeller shaft. 



f . Resistance in a seaway : The Increase of resistance In a seaway is 

 mainly due to the oscillatory motions (pitching and heaving),*^ and. Judging 

 from results for a fixed shlpj* the reflection of waves is responsible for 

 only a small part of the resistance Increase. The damping of oscillations is 

 primarily caused by wave making. 



Older experiments led to the belief that ships with good resistance 

 qualities in calm water keep their superiority in regular waves. There are, 

 however, some indications that ships with a steep rise of the calm-water re- 

 sistance curve near the service speed are liable to high resistance in rough 

 water. This remark applies especially to hulls with high prismatic coeffi- 

 cients. 



S\unmarlzlng the state of knowledge on some problems of ship resist- 

 ance — so far as it is needed for analyzing wave resistance — we may say: 



a. The residual resistance derived by Froude's method is highly arti- 

 ficial, Including as it does items which do not follow Froude's law of simili- 

 tude. For small Froude numbers the "residual" resistance may not yield even 

 an approximate estimate of wave resistance. 



b. No reliable results are known for the viscous pressure resistance; 

 even contradictory statements are to be found as to its range. ^' ^^ Prom 

 measurements on bodies of revolution fully submerged, we can guess that the 

 total viscous drag of models up to moderate prismatics corresponds closely to 

 the frlctlonal drag including the frictional form effect.* 



c. An accurate estimate of the total resistance of slow ships depends 

 more on the reliable computation of frictional resistance including the rough- 

 ness and form effect than on accurate values of the residual resistance de- 

 rived from model results. Thus the routine towing experiments would not seem 

 to be indispensable for this most Important class of ships as long as the com- 

 putation of absolute value of resistance is the main problem; nevertheless, 



♦Special Investigations should be made as to the dependence of the eddy resistance (viscous pressure 

 resistance) upon the form of the ship, especially the afterbody (see Appendix 5)- Various rules have 

 been made for the avoidance of appreciable eddymaking; unfortunately they are more a natter of opinion 

 than of real knowledge. Some deductions are derived from inadequate experiments, for instance as to 

 bodies of revolution where the Influence of the free surface was not eliminated, etc. 



