RESISTANCE, PROPULSION AND SPEED OF SHIPS 



253 



tions are less reliable and because several wave-height 

 codes are in use. By interpolation, the points can be 

 placed on the blank chart similar to Fig. 12. A separate 

 sheet for each wind strength can be used and a mean 

 curve can be drawn through the points corresponding to 

 a certain wind strength for all ships. This will indicate 

 the mean speed-length ratio used by ships at a particular 

 wind strength. The relati\'e positions of the plotted 

 points above or below the mean curve will indicate the 

 relative seakindliness of .ships. The objective of the 

 research in this case is to develop ship forms and propm- 

 tions, which would permit the maintenance of higher 

 speed in adver.se weather without dangers connected 

 with excessive ship motions. The necessary reser\-e 

 power can be evaluated after the maximum safe speed is 

 established in \'arious degrees of severity of adverse 

 weather. 



The published research on the ship speed in adverse 

 weather has been limited to full or nearly full-load condi- 

 tion. A similar re.search for ships in ballast is suggested. 

 In this case the permissible ship speed appears to be 

 limited by slamming. 



5 Suggested Research Topics 



1 The Project on Evaluation of a Ship's Resistance in 

 Waves by a Strip Theory is reconunended. This is vis- 

 ualized as following Hax'clock's approach, Section 2.2, but 

 u.sing a more complete evaluation of hydrodynamic 

 pressures. In this evaluation cognizance should be 

 taken of the .ship-wave interaction and of the coupling 

 of heaving and pitching oscillations; the coupling has a 

 strong effect on phase relationships which prunarily 

 govern the resistance. The e\'aluation of hydrodynamic 

 pressures used by Kor^'in-Kroukovsky and Jacobs 

 (3-1957) may be useful in this comiection. 



2 The Evaluation of the Resistance Caused by Waves 

 and Ship Motions of Large Amplitude is suggested. The 

 solution of the problem in Project 1 was based on in- 

 finitely small ship motions so that pre.ssure integrations 

 were made o\'er the still-water wetted area. The integra- 

 tion over true instantaneous wetted areas may be ex- 

 pected to yield greater resistance in case of ships of 

 pronounced V-form. The problem may be solved 

 theoretically by expressing the pressure and wetted-area 

 fluctuations by a series of harmonics. It also can be 

 solved numerically by a suitable calculation procedure 

 based on a series of instantaneous ship-wave positions. 

 Harmonic ship oscillations can be assumed. 



3 A Search for Methods of Extending the Resistance 

 Data obtained in regidar wa\'es to the irregular ones is 

 recommended. The work of Pershin and Voznessensky 

 (3-1957) appears to be the only attempt made 

 to date. Section 2.5. The linear superposition, .so 

 succe,s.sful in the analysis of .ship motions, is not applic- 

 able in the present case, since the resistance appears to be 

 proportional to the square of the wave height. 



4 Model Resistance Tests on Geosim Series in Wide 

 Tanks are needed. Test data obtained in narrow tanks 



appear to be of questionable validity because of simul- 

 taneously occurringscale effect and tank-wall interference. 



5 Theoretical and Semi-Empirical Evaluation of the 

 Resistance Added by Wave Reflection, Section 2. 1 , is st ill 

 in a rudimentary stage and further de\('lopment is needed. 

 This component of the resistance can be expected to be 

 significant for large tankers and bulk cargo .ships. These 

 ships ha\-e a large angle of entrance and, because of their 

 length, ]iitch but little in moderate weatliei'. 



6 The Evaluation of the Total Ship Resistance at Sea 

 due to all causes requires further attention. The pri- 

 mary references in this connection are Kent (3-1951, 

 3-1957) and Per.shin and Voznessensky (.3-1957). 

 These publications contain also fiu'ther references 

 on the subject. It appears that the prolilem can 

 be .solved best by proper blending of theoretical and 

 empirical methods. 



7 Further Statistical Observations on shij) resistance 

 and powering at sea are needed. A general pattern of 

 such research has been well formulated by Bonebakker, 

 Clements, and Aertssen, and the primary need is in a 

 greater volume of data. 



8 An Investigation of Statistical Averaging of the 

 data of Projects 1 to (> is needed in order to provide 

 typical .ship-voyage data. This is a nece.s.sary pre- 

 re(|uisite to correlating theoretical and towing tank data 

 with sea obser\';tt ions of jiroject (7). 



9 Detailed Investigations of Ship Powering under 

 specific weather conditions are needed. This will provide 

 a A'erification of the data obtained under Project 6. 

 Past work of Kent and others suffered from inability 

 to ])ro\'ide reliable (|uantitative data on sea waves. 

 It should be emphasized that wave conditions and ship 

 motions must he recorded simultaneously with power 

 measurements. 



10 Detailed Investigations of a Ship's Powering in 

 head swell and light wind is needed for a relatively simple 

 coi-relation with theoretical and experimental work 

 imder Projects 1, 2, 3 and 5. In this case the resistance 

 caused by wa\'es can be assumed to predominate strongly 

 over that caused by all other influences. 



1 1 Application of Bonebakker's ( 1951, 1954) Method 

 of Power Evaluation to Model Tests in oblique waves is 

 suggested. Head waves and towed models were visual- 

 ized in Project 4. The towing is not practical in oblique 

 waves since it would impose I'estraints and affect model 

 motions. The resistance and powering of a free self- 

 propelled model can be estimated, however, on the basis 

 of the apparent propeller slip, following Telfer, Bone- 

 bakker, and Clements. The ca.se of oblique waves is 

 important because the components of resistance due 

 to rolling, yawing and rudder motions are added to the 

 wave resistance. 



12 Log Analysis Data, patterned after Lewis and 

 Morrison (1954, 1955), are needed for a large number of 

 ships and operating conditions. The immediate objec- 

 ti\-e of this research is to provide a sufficiently large 

 number of points for Lewis' type plot. Figs. 12, 13 and 

 14, in order to permit: 



