4466 HYDROGRAPHIC MANUAL PaGE 356 



4466. Distance Measurements by Taut Wire 



Any distance at sea may be measured by taut wire. The taut-wire apparatus was 

 especially designed to measure horizontal distances between buoy stations, and its 

 principal use is for the measurement of distances between stations in a buoy traverse. 



During a taut-w4re measurement only one man is required to attend the apparatus 

 constantly, but two observers are required to read the revolution counter when a "mark" 

 is given. The taut-wire attendant stands by the apparatus on the alert to see that all 

 parts function properly and to maintain the correct tension on the ware. He also gives 

 the "mark" when the station is abeam, so that the observers may keep their eyes fixed 

 on the revolution counter at this critical instant. At the "mark" both observers read 

 the revolution counter and agree on the reading before it is recorded by one of them in 

 a taut-wire'book — any type of small blank book will serve — the other observer verify- 

 ing the recorded reading. After a distance has been measured, one of the observers 

 takes the difference between the two revolution-counter readings and reduces it to 

 meters by multiplying it by the factor (see 4467), the other observer checking the re- 

 sult; after which the taut-wire readings, difference, and distance are entered and 

 checked in the appropriate spaces on Form 777, Taut-Wire Traverse Observations, 

 which is kept on the bridge. 



The officer-in-charge, who is on the bridge conning the ship and maintaining it on 

 range, is assisted by another officer who observes certain data and records all data on 

 Form 777. In addition to the taut-wire measurement. Form 777 contains columns for 

 the following information: The name of the buoy station, the time it was passed abeam, 

 the log reading when abeam, the difference in log readings between two consecutive 

 buoy stations, the depression angle to the station if one was measured, the distance 

 passed abeam, the side of the vessel on which the station was passed, the direction of 

 the current, and three columns in which the azimuth between stations is to be recorded. 

 In the colunm headed "Current Direction" the estimated velocity of the current should 

 also be entered. Some of the vertical columns are ruled with two horizontal spaces 

 opposite each buoy station, one of which is marked with a buoy symbol, the other 

 being marked with an anchor symbol. If the data have been observed to the buoy 

 structure, as they usually are, they should be entered in the horizontal space opposite 

 the buoy symbol, the space opposite the anchor symbol being reserved for the compar- 

 atively rare cases where special arrangements are made for observing the data directly 

 to a small buoy marking the anchor position (see 943). 



The officer assisting on the bridge enters all the observed data except the taut-wire 

 data, the latter being entered and checked by the observers at the taut-w'ire apparatus. 

 He is assisted in the observations by the officer-in-charge and the quartermaster on 

 watch, for all of the required observations at each buoy must be made almost simul- 

 taneously. The direction and velocity of the current can usually be observed shortly 

 before or after the station is abeam, but the time, log reading, and distance abeam must 

 be obtained simultaneously at the instant the observation station on the bridge is 

 abeam of the station. The direction of the current must be recorded as the direction 

 toward which the current is flowing. Tliis is usually the reverse of the observed bearing 

 since the ship usually passes on the down-current side of the station (see 2512). The 

 azimuth between stations is generally observed some time before the first station is 

 abeam, although occasionally it has to be observed in the reverse direction after the 

 second station has been passed. In the latter case it should be recorded as observed, 

 with the station names recorded in the observed order from the near to the far buoy. 



