I think it's important for the OP to understand the 13 thing is regional. Different VTS zones are going to have different assigned channels.
To answer the gentleman who asked why they didn't pick a channel and stick with it- it's do to volume of traffic.
Let's say you're in a busy canal
maybe bridges 1-4 may be on channel 11, bridges 5-7 are on channel 12 and up bound locks are channel 13- well you get the point. A given channel in a local area can only sustain a finite amount of radio
traffic before every body is standing on each other.
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