Page 349 equipment and instruments 4445 



(h) The bearing should not be taken of a station whose radio signal travels any appreciable 

 distance along the shore. 



(0 Night effect maj- be observed in bearings, particularly near sunrise and sunset. It is due to 

 periodic unequal ionization of the air and may cause serious errors in bearings. It is usually mani- 

 fested in a very wide or swinging minimum, so that the bearings seem to be changing while being 

 observed. Errors due to night effect are not usually encountered at distances less than 30 to 50 miles. 



4445. Radio Bearings on a Mercator Chart 



Radio bearings are arcs of great circles, or geodetic bearings, and. like them, must 

 be corrected for the convergence of the meridians before being^ plotted on a Mercator 

 chart. Only true north and south bearings, and true east and west bearings along 

 the Equator, do not require this correction. 



A table of corrections is published in the Coast Pilot; table! in Bowditch contains 

 similar data. For distances less than 50 miles, the corrections are negligible. 



It is important to apply the corrections with the proper sign. Wlien bearings 

 are taken from the ship and read clockwise, the sign of the correction is minus in north 

 latitude where the ship is east of the radiobeacon and plus where it is west. In south 

 latitude the signs are reversed. The signs are also reversed for bearings observed at 

 the radiobeacon and radioed to the ship. 



A radio bearing is always plotted Jrom the position of the radiobeacon or fixed 

 station since its position is known and charted. Consequently, if bearings are observed 

 from the ship, 180° must be added after other correction. 



For all practical distances radio bearings can be plotted without correction on 

 Lambert projections. 



445. Ship Logs 



Ship logs are mechanical devices for measuring the distance or speed a vessel travels 

 through the water. They are used extensively in navigation and in dead reckoning 

 in hydrographic surveying. Logs are calibrated to register distance in nautical miles, 

 or speed in knots. The revolution counter, although not a log, may be used for the 

 same purpose, when properly rated. 



4451. Patent Log 



The patent, or taffrail, log consists of a rotator towed through the water, the revolu- 

 tions of the rotator being transmitted through the towline to a registering dial. The 

 registering dial is sometimes installed on the rail at the stern quarter of the vessel, 

 hence the name taffrail log; but for convenience in reading and to tow the rotator 

 farther to one side of the wash of the propeller, it is often installed on the wing of the 

 bridge, the towline being streamed from the end of a boom. 



The rotator is a brass cone or spindle with spiral fins which cause it to rotate accord- 

 ing to the speed with which it is drawn through the water. The logline or towline is 

 a hard-laid type which does not snarl. 



The rotator must be submerged at all times and towed far enough astern to avoid 

 most of the efiect of the propeller. The required length of towline varies, depending 

 on the speed of the vessel and the height of the registermg device. In general, for 

 speeds up to 12 knots the length should be from 200 to 250 feet, and for 15 knots 

 300 to 350 feet. The best length for any vessel may be found experimentally by 



