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Old 12 November 2013, 17:35   #1
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Perhaps I need a speed log!?

I set off down the Medina at about 4pm, thinking if I'm quick I can get out into the solent for a quick sea trial after a few RIB improvements. I didn't need to read the channel buoy to know the speed limit in this part of the world is 6 knots. I've sailed this stretch in my yacht numerous times without incident. Setting the throttle to a speed that felt very much like 6 knots I ambled up the river against the tide. To my amusement a blue flashing light made its way towards me. After a reasonably polite warning from the harbourmaster I reduced the throttle to an almost snails pace. It felt like 3 knots.

Once out to sea I managed 5 minutes of solent fun before turning back to beat the setting sun (Nav lights disconnected presently). I was almost near the chain ferry doing what seemed like less than 6 knots when, again, I was met by flashing blue lights and, to the amusement of my kids, a siren. Albeit a different harbourmaster the warning this time had a more serious tone to it.

Crawling back to my berth at a pace that seemed slower than I can swim I couldn't help thinking ... "Perhaps I need a speed log!?"
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Old 12 November 2013, 17:56   #2
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Shouldn't be that tricky, you just need to be in displacement mode, so tubes in the water?

HullSpeed[kts] = 1.34 * SQRT( Lwl [ft])

for a 21ft boat thats 6.14kts

However, a GPS would be mighty useful for you for a number of reasons, including SOG. I think technically you are supposed to be Speed Thru' Water at the Medina.

If in doubt try swimming it ;-)
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Old 12 November 2013, 17:59   #3
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I've seen them pull loads of people in the past. The speed limit is not against the water I guess it's against the land.
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Old 12 November 2013, 18:00   #4
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Speed limit is 'over ground' isn't it? So you don't want a log (which is through the water - and notoriously unreliable at low speed on a rib anyway). Any GPS will provide what you want [or a smartphone in appropriate wrapper].

Speed overground means if you are travelling against the tide, the speed through the water could be much lower.
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Old 12 November 2013, 18:01   #5
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Quote:
Originally Posted by ShinyShoe View Post
Shouldn't be that tricky, you just need to be in displacement mode, so tubes in the water?

HullSpeed[kts] = 1.34 * SQRT( Lwl [ft])

for a 21ft boat thats 6.14kts

However, a GPS would be mighty useful for you for a number of reasons, including SOG. I think technically you are supposed to be Speed Thru' Water at the Medina.

If in doubt try swimming it ;-)
I bloody knew it, this was my retort exactly (without the calcs). I was completely at displacement speed. The bow rises very quickly once she starts to lift off the tubes. I wasn't sure how fast this occurs, but I'm not arguing with those calculations!!
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Old 12 November 2013, 18:04   #6
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Speed limit is 'over ground'
Better not go out during Springs then else I might have to put it in reverse!
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Old 12 November 2013, 18:08   #7
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"Perhaps I need a speed log!?"
No! You need a bigger engine, he won't catch you then
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Old 12 November 2013, 18:09   #8
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I've seen them pull loads of people in the past. The speed limit is not against the water I guess it's against the land.
Cowes Harbour is Speed through the Water since late last year / earlier this year:

Quote:
Originally Posted by cowesharbourcommission.co.uk
3.2.1 Every vessel navigating the Inner Harbour of Cowes or within 100 metres of the Mean High Water Mark shall unless otherwise authorised by the Harbour Master in writing, subject to the requirements of maintaining adequate steerage way and control, be navigated at a speed not exceeding 6 knots through the water.
Are they being stricter in view of NTM30?
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Old 12 November 2013, 18:12   #9
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I bloody knew it, this was my retort exactly (without the calcs). I was completely at displacement speed. The bow rises very quickly once she starts to lift off the tubes. I wasn't sure how fast this occurs, but I'm not arguing with those calculations!!
Did they say you were speeding? Or creating too much wake?
Did they mention the conditions imposed by NTM30?
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Old 12 November 2013, 18:22   #10
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Did they say you were speeding? Or creating too much wake?
Did they mention the conditions imposed by NTM30?
They said I was speeding, but they did refer to my wake too. I did keep checking my wake wasn't washing up onto shore. I think for the speed my RIB creates more wash than my yacht, and I guess they were probably judging my speed by my wake.

NTM30 wasn't mentioned.

To be honest, the second guy was quite stern, but when he realised I wasn't apathetic and wasn't purposely ignoring the rules he was quite polite. I think I opened the throttle a buoy too early too on the way out (by mistake), luckily he didn't see that or I would have been for it!
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