For example, (c) interpolates halfway between 0900 

 and 1000. Read up day 21 to this point and note that 

 the value lies between two 11 contour lines, but 

 less than 12, Read answer as +1 1 . 



b. Other uses of chart 



1 . Boat clearances under low bridges. 



Example: boat passing under bridge. 



Problem (d): A small craft is passing under a low 

 bridge, at or near this station, at 0400 on 21 July. 

 The boat captain visually estimates his vertical 

 clearance between bottom of bridge and top of 

 craft as 3 feet. During what hours of the day will 

 the craft not be able to pass back under the bridge? 



Solution: At the intersection of the 4-hour line and 

 21 -day line read height as +6 feet. Add vertical 

 clearance (+3 feet) to this value giving +9 feet. 

 This means that any time during the day when the 

 height is +9 or more the craft will not be able to 

 go under the bridge. The dashed arrows point 

 towards the times when passage under the bridge 

 would be restricted. These times are: 



Day Hours 



21 0600-1230 



1815-2245 



Example: Boat waiting to pass under bridge. 



Problem (e): A small craft is unable to pass under a 

 low bridge because of insufficient vertical clearance, 

 at or near this station, at 0800, 13 July. The boat 

 captain visually estimates he needs 3 feet less water 

 to clear bridge. How long must he wait? 



Solution: At the intersection of the 8-hour lines and 

 13-day line read height as +10.5 feet. The water 

 must fall 3 feet or to +7.5 feet before craft can 



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