A point that I have often wondered is whether an official speed limit in a river, of lets say 5 knots, is meant to be 5 knots through the water as per the boat's log or an actual 5 knots over the ground as shown by a GPS?
Often wondered myself and also found that its imposible to keep to a 5 knot speed limit safely in some conditions.
Especialy with a high sided boat with no keel.
High cross wind+3 knot current, leaves 2 knots folward and about 4 knots sideways!!
Rules all made up by a trafic warden I'm afraid, who would not understand the question anyway, or the problem.
On the R Exe, the navigation authority (a complete misnomer in their case) have issued a speeding ready reckoner. For example, a vessel travelling between approach buoy 3 and 7 and taking less than than 137 secs is exceeding the speed limit. (Honestly! they really have produced a chart listing to the second the legal minimum time between 13 sets of buoys!!!!) I thought of passing this fascinating thread to them but decided that any navigation authority who could produce such a document probably doesn't realise there is a tide anyway! If any one really wants a laugh (or to weep) I will post the whole document.
on the basis that all enforcement is done 'over the ground' I have always assumed that to be the basis of any limit.
Where very low limits are in place for 'wash' they tend to have an additional wording in the byelaw which states that a catchall ' or some such lower speed that avoids wake etc...' so you can be 'prosecuted' for either exceeding the speed (over the ground) or making excessive wake/wash.
Interestingly non tidal boaters tend to have a method of measuring speed throught the water but tidal and sea boats are more likely to have GPS.
I would think it depends on where the laser gun is located. If located on shore, speed over ground would apply. If the laser gun is located in a Police launch, speed through water would apply since the launch is moving with the tidal currents.
Which poses the wonderful question: If there is sufficient tidal or current flow would a moored boat be exceeding the speed limit?
There are parts of the Humber and Ouse where tide speed exceeds 6 knots, I don't think there is a speed limit on these sections however. If there is it is certainly routinely broken by commercial vessels
Speed over ground must be useful in some case ( ignoring humber type tides) Imagine you'r doing 5 knots through the water with 2 knot tide and hit a moored boat - lots of damage. That must be why most speed restrictions are in place.
If you are caught on the road for speeding you are entitled to ask for the most recent certificate of testing of the instrument or speedometer. If the police can't produce one you have a reasonable defence - or so I have been told.
Try it on the Marine police or whoever and see if it works!
Trouble with speed limits is that - like on the roads - most people driving boats that can maintain the 8 knot speed limit in Chi harbour see that as the only speed to go - regardless of the fact that a planing hull will still be running at displacement speeds and making a wash like ww2 Destroyer.
Many power boats - RIBs particularly - make far less wash on the plane than at 6-8 kts.
AFAIK it is usually thru the water, not over the ground, but I do not have a definitive source. Re the point about exceeding speed at anchor in a fast current, the getout there is that the speed limit generally applies to boats underway.
I spect the real answer is that it is generally in harbours etc that you find these limits, so the rules are embodied in local bye laws, so they could be inconsistent from place to place.....
A laser gun can only measure speed relative to the police boat. So if the rules say speed=speed over ground, the police boat wd need to have his laser gun interfaced to his GPS with an algorithm to overcome cosine error, and if the rules say speed=speed thru water the police boat wd need his laser gun interfaced to his log and again cosine error would need to be taken out. None of which happens I'm sure.
Has anyone been pulled for speeding and actually "done" as opposed to told off.
And while on the subject, I'm in Lymington and when going down the river a few eeks back at 6knot limit (axshully I had 6.8 on GPS, it was hard to steer at less due to F4-5 crosswind) the ferry Cenred came up behind me, 5-hooted me, and eventually overtook at about 8kts. He didn't give me any "I intend to overtake" hoots. Is there a special ruke for them??