Speed Limits - An open question

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bob_tyler

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This is a hypothetical situation in an attempt to clarify the definition of a speed limit .

There is a narrow entrance to a harbour where, on the ebb, the water is running out at 4kts.

The speed limit throughout the harbour from the seaward end of the entrance inwards is 6kts.

Does this mean:

If you proceed inbound at 6kts over the ground (SOG) you will be really going through the water at 10kts but not breaking the speed limit ?

Or does the limit mean that the maximum speed permitted is through the water at 6kts and you will only be doing 2kts SOG.

! Which interpretation is correct. if either? Unless otherwise stated, does a Speed Limit mean SOG or Speed through the Water? They are rarely the same, only perhaps around high or low water.


Just interested /forums/images/graemlins/confused.gif
 
mmm - my understanding and experience is its sog - mainly cos the enforcers (harbour masters) use their radars to check the sog ......

which of course means yes you could be doing a shed load of speed through the water and still be inside the limit.

but if ppl complain about your wash look out - no excuses for acting in an unseaman like manner
 
speed limits are set for
a general safety - close proximity to many users in confined area
b wash - either to prevent bank erosion or 'cos of ill effect on moored craft
If you are punching 5kts of tide and hold good doing 6kts on your gps say, what'll your wash be like do 10 - 11 kts?
is your boat so slick that you produce little wake? - good luck to you.
are you a deep bow'ed tub, better chill out.
 
I always assumed it was SOG as this is your actual speed as checked on radar.
But it is an interesting question not least of which is "what happens if your going with the tidal flow"?
 
I get your drift, but you are not technically nuc as you are able to move out of the way of another vessel. /forums/images/graemlins/smile.gif

anyway - where do you experience 6 knts of current, its a lot of water moving very fast, from memory even the pentland firth doesnt achieve that - but maybe some will be kind enough to remind me if I am wrong.

in continuation of another post, I am pleased to see the portsmouth speed limit has changed from the 6 kts sog in the 1980's to something more reasonable through the water, though inho 10 kts is a tad too fast in such a busy waterway .... especially on a full flood tide of some 4 kts, 14 kts sog is very fast if something fails and effects the control of any vessel
/forums/images/graemlins/shocked.gif
and lastly - the easy way to resolve the small boat channel issue is to lay buoys on the northern most limit of that small boat channel - in accordance with the idea of small channel markings in the iala buoyage system, not rely on boats to judge it for themselves .... but of course its a little impractical to reduce the main channel permanantly I guess.
wonder if there is an eu directive about enforcing the standardisation of iala channel markings /forums/images/graemlins/grin.gif
 
[ QUOTE ]
where do you experience 6 knts of current, its a lot of water moving very fast

[/ QUOTE ]
Top of springs going through entrance to portsmouth it is at or very nearly 6 knots.

IIRC it approaches that speed in Gt Yarmouth as well.
 
thanks for the reminder .... having not conned a boat/vessel through the entrance to portsmouth for some 17 yrs the memory must have got a few unopened lockers tucked away - as I remember 4 1/2 kts for some reason.
as to great yarmouth, I had a commercial pilots license for that port for several years, and am suprised to learn now that the current reaches that speed ....... well, what did they say about Alzheimer’s ????????? /forums/images/graemlins/cool.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/grin.gif
 
would have thought Bob would be over the theoretical hull speed for his craft at 11knots and be sinking! /forums/images/graemlins/crazy.gif

I had always been under the impression that it was speed through the water on non tidal waters and SOG for tidal - clearly wrong in light of the Portsmouth advice here.

In Poole I believe it remains SOG but of course the no wake limit of 6knots is written up as 6knots or some such lower speed as produces no wake whilst the 10knots is a safety limit not no wake. This helps me as I don't have any way of measuring speed through the water and am either clear to do 10 knots SOG if appropriate or proceed at no wake regardless of SOG in the 6knot areas.
 
The 4.5 knots at greatest flow is more like neap flow. I think it only gets close to six on the really big springs (once or twice a year)
It is very many years since I have been in Gt Yarmouth as well, but I do remember that the cruise boats could get into trouble below the bridge from not having enough speed to stem the tide. But I would expect that it would only be the very big ones that aproach that speed.
 
This is very interesting. Lots of different opinions on a subject which should be clearly laid down.

Firstly, I agree with comments above that wash and safety are much more important than the actual Speed Limit. My initial posting, however, only concerned legal speed limits and was, in a way, intended to be a bit of a "devil's advocate".

What I do find fascinating, from two different postings above, is that one can leave Portsmouth Harbour under the rule "SOG" and, in a fast boat, 10 minutes later arrive in Cowes under the rule "Speed through water".

I must make clear that I am totally against Nanny State interference or anything like that.

Isn't it time to knock a few Harbour and River authorities heads together to standardise the method of measurement?

Should Speed be shown in Knots, mph or kmph?

Should it refer to SOG or through the water?
 
[ QUOTE ]
anyway - where do you experience 6 knts of current, its a lot of water moving very fast, from memory even the pentland firth doesnt achieve that

[/ QUOTE ]

6 knots is a quiet day at neaps in Pentland. Maximum tidal speed is 16 knots. Wouldn't like to try it, but must be impressive to watch.
 
From the Poole Harbour Commisioners website......

2aa Without prejudice to byelaws 2(a) and 2(b) within the limits of Poole Harbour (which for the purposes of this byelaw No. 2(aa) only shall be deemed to extend at the seaward approaches to Poole Harbour to an area within or to the West of an area taken as a radius of 1400 metres from South Haven Point (position 50º 40.78' North 01 º 56.91' West the Master of any power driven vessel or other water craft shall not, subject to the requirements of maintaining adequate steerage way and control, suffer or cause such vessel to be navigated or driven in the Harbour at a speed exceeding 10 knots through the water.
 
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