ASSiRACT 



This report presents the results of an experimental study to determine the 

 effect of various parameters governing speed reduction in a seaway. Models of 

 three typical ships (block coefficients varying from 0.50 to 0.75) were tested in 

 head seas in regular waves, ranging in length from approximately one-half to twice 

 ship length, for several wave heights and still water thrusts. The speed loss due 

 to the added resistance in waves was determined for each condition. The results 

 are presented in the form of graphs showing the critical wave length (wave length 

 for maximum speed loss) as a function of wave height and still water speed. A.lso 

 included are plots showing the percentage speed loss for constant wave lengths 

 as a function of the above' parameters; and contours of constant speed loss in 

 waves of various lengths and heights. Finally, graphs are presented comparing 

 the performance of the three vessels. 



INTRODUCTION 



At the present time no criteria are available from which the limiting acceptable sea 

 conditions for conducting speed trials can be determined. Consequently, it has been the 

 general practice to wait for almost perfect weather conditions so that reliable results could 

 be assured. It was the original purpose of this project to establish the threshold conditions 

 under which a ship's speed is affected by the seaway. These threshold conditions could then 

 serve as a general guide in evaluating full-scale trials. Thus, the study should have been re- 

 stricted only to mild sea conditions. However, the testing accuracy is relatively poor in these 

 conditions and since the present tendency is to go to higher sustained speeds at sea, the orig- 

 inal purpose of the study was extended to include some general information on factors govern- 

 ing speed reduction in a seaway. Accordingly, the tests were carried out under more serious sea 

 conditions than those required for establishing the threshold conditions, the latter then being 

 obtained by interpolation to the condition of small sea. The results of the present study, while 

 containing information pertinent for evaluating standardization trials, may also provide general 

 information on speed loss at sea. 



DESCRIPTION OF TESTS 



Models of three typical ships were tested in the llO-ft basin. In this facility a gravity- 

 type dynamometer is used as the towing mechanism and waves are produced by a pneumatic- 

 type generator. Wave heights are measured by a capacitance-type gage where the unbalance 

 of the bridge is a function of the immersion of the element.^ The bridge output is recorded 

 by a Sanborn-type recorder. 



References are listed on page 28. 



