248 



THEORY OF SEAKEEPING 



18 



n 



16 

 15 

 14 

 13 

 IE 



o II 



c 



D- 



CO 



<u 



CD 

 O 



c 



o 

 ^ 6 



5_ 

 4_ 

 3_ 



Following 



Quartering 



Beam 



ead 



Noie; 



Cases where Sea and Wind Directions 

 differed Radically are Excluded 



I 234SG789 

 Sea Condi+ion 



Fig. 10 Reduction of speed of Victory-type ships in a rough sea 

 (from Lewis and Morrison, 1954) 



weather scale can be obtained by ^ 2 (sea disturbance 

 iiuml)er)= and plotting this against the corresponding 

 Beaufort wind force. The effects of winds on the beam 

 are taken to be one half, and folknving winds one fifth 

 this amount for any given Beaufort wind force, this 

 again being established from BSRA'^ data." 



The effect of the weather factor on shaft h(jrsepower is 

 then estimated using the coefficient b from statistical 

 analysis. The relation.ship derived by Clements is, 



SHP, Ti + bW 



SHPi 



i'l 



Y 



Yr 



I'l - bW 



(24) 



where the symbol }' denotes the ratio shp/N', and sub- 

 scripts denote: 



'•■ British Shipbuilders Research Association. 



1 — Mean voyage data under calm weather conditions 

 2 — Any gi\en condition of weather 

 100 — condition when apparent slip is 100 per cent 

 Shaft hor.sepower is related to apparent slip by equa- 

 tion (21). 



Additional data on the increase of the ship resistance 

 due to weather will be found in Aertssen (1959). Aerts- 

 son investigated the propulsive characteristics of two 

 ships, the particulars of which are shown in Table 5. 

 'i'ho relationship between the delivered horsepower and 

 ship speed is shown in Fig. 15 for ditferent wind velocities 

 up to Beaufort (5. Quoting Aertssen: "A mean value 

 of the increase of power due to weather effect for two 

 winter A'oyages of the Jadotrille between Antwerp and 

 the Canary Islands is 11 and 19 per cent, and for the 

 whole voyage of the Elisabeth 15 per cent. Thus 15 

 per cent is the mean power allowance for weather effect 

 for both ships on their usual routes." Attention should 

 be called to the fact that these data refer to the route of 

 predominantly fair weather and to the ships of large 

 length. 



3 Ship Speeds in Storm Seas 



When wind strength and wa\'e height exceed a certain 

 limit, the maximum speed of a ship depends on its 

 motions and not on the resistance and the available 

 horsepower. Excessive pitching and heaving may cause 

 slamming and shipping of water. The latter may 

 damage the deck gear and the deck cargo which is often 

 carried on fully loaded ships Excessive accelerations 

 may cause shifting of the cargo or, on liners, they cause 

 excessive discomfort to passengers and difficulties in hotel 

 ser\ace. It becomes necessary to reduce the propeller 

 RPM and ship speed in order to ease its motions. 

 Weather conditions, at which the reduction of power 

 must begin, \'ary with the type of a ship and its power. 

 On the tanker, investigated by Bonebakker, this occurred 

 at wind strength of 5 on Beaufort scale. Aertssen (1957, 

 NS^IB Symp.) showed that a modern general cargo 

 ship can be expected to maintain the full power up to 

 wind strength of 6. 



The characteristics of a Victorij type ship are shown in 

 Fig. 1. This simplified figure was constructed on the 

 basis of a preliminaiy analj'sis of log books. The results 

 of a more complete analysis are shown in Fig. 10. Ex- 

 planation of this figure can best be given in words of 

 Lewis and ^Morrison (1954, 1955): "In this study daily 

 a\'erage data were used from tiiree round \'o.vages during 

 winter months. In order to eliminate extraneous effects, 

 figures were excluded in any of the following cases: 



1 Speed reduced due to other causes than wind and 

 sea (i.e., fog, machinery trouble, etc.). 



2 Mean drafts other than full load (i.e. less than 26 

 feet). 



3 Wind or sea changed more than 90° during the day. 



4 Average wind differed more than 55° from the sea 

 direction. (The whid force and sea condition never 



