Guess you're thinking of the river. In Thames Conservancy days, it used to be through the water, although in those days there was little in ways to measure. Apart from throwing a log over the side.
I doubt whether EA have thought about it /forums/images/graemlins/laugh.gif
Guess it depends on the nav authority. Since I spend almost all my time on the tidal (PLA) section of the river, this is the only one I'm sure of. This is an extract from the Byelaws, and refers to the Thames above Wandsworth Bridge and in the creeks around Canvey.
Byelaw 48 of the Port of London River Byelaws 1978 (as amended) sets out the .... restrictions:
Speed Limits
48 (1) The master of a power driven vessel navigating in a part of the Thames to which this byelaw applies shall not cause or permit the vessel to exceed a speed of eight knots through, on or over the water:
So, speed through the water on the tidal river - doubtless others will be able to quote the EA rules.
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ok-clear now-20 kph upstream on fast flow-dosnt that do damage?
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There's the problem with an 'over the ground' speed limit. 5mph over the ground against a 5mph current means 10mph thru the water and big wash. /forums/images/graemlins/grin.gif
Byron's got it right, the notices at the locks had no legal backing they would take you to court if compaints were made and danage could be proven the guide line was if your wash was breaking tou were to fast.
That would be the same as now then, we have no license to loose. You are prosecuted for dangerous navigation at excessive speed or causing a nuisense if making too much wash.