The The Wash is a seaway and is part of the Waterways of Mainland Britain. It runs for 20 miles and 3¾ furlongs from River Witham - The Wash Junction (where it joins the River Witham (Boston to the Wash) and the River Welland (Main Line)) to Great Ouse - The Wash Junction (where it joins the River Great Ouse (Large Tidal Section)).
The exact dimensions of the largest boat that can travel on the waterway are not known. The maximum headroom is not known. The maximum draught is not known.
This waterway is excluded by default from route planning with the following explanation: "not suitable for many inland waterway boats."
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|River Witham - The Wash Junction
Junction of the River Witham with The Wash
Navigation beacon east of the River Witham mouth
|2¼ furlongs||0 locks|
Navigation beacon north of Black Buoy Sandbank
|2 miles and 6¾ furlongs||0 locks|
|Roger Sand (near port buoy Charlie)
This is the place to run aground and have a BBQ.
|4 miles and 6½ furlongs||0 locks|
The fairway buoy and entrance channel to Boston.
|8 miles and 2½ furlongs||0 locks|
Navigation beacon north of the River Great Ouse mouth
|14 miles and 7¾ furlongs||0 locks|
|Great Ouse - The Wash Junction
Junction of the Great Ouse with The Wash
|20 miles and 3¾ furlongs||0 locks|
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Wikipedia has a page about The Wash
The Wash is the square-mouthed bay and estuary on the northwest margin of East Anglia on the east coast of England, where Norfolk meets Lincolnshire. It is among the largest estuaries in the United Kingdom and is fed by the Rivers Witham, Welland, Nene and Great Ouse.