Idle question re secondary ports

[Twister_Ken]

Throughout the country eager students in RYA-approved nightclasses are learning how to calculate the depth of water to the nearest centimetre at 1758 in Mudhole-on-Ooze, by reference to tidal curves, secondary port differences and inside leg measurement. Ignoring the fact that none of them will ask what the barometric pressure is expected to be at 1758, has anyone ever done this in real life, or do you just look at the chart and then do a rough mental sum involving the 12ths rule, take off half-a-metre as a safety measure and then watch the echosounder like a hawk as you come in?



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[Sybarite]

Yes

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(Idle answer)<P ID="edit"><FONT SIZE=-1>Edited by Sybarite on 01/10/2004 08:37 (server time).</FONT></P>

 

[ChrisE]

Yes and the point of the question is? Working on a new article are we?

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[graham]

Yes I work it out to the nearest millimetre then apply this to a chart last surveyed in 1898.

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[Evadne]

Ditto. I got disillusioned in the exam when the correct answer allowed you to pass over a mudbank and under a power line with about 0.5m clearance on each. Hands up who would then proceed?
I once did the calculation for real, to see if I could get into Newport one weekend. The first time it appeared that there was at least 10m of water at all times, the next time I moved the decimal point and got 10cm. So I went to the Folly and had a couple of pints of Speckled Hen instead.

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[Twister_Ken]

\'fessing up

>Working on a new article are we?<

No, but it's an idea!

I met a dazed kipper theory student last night who confessed to some difficulty regarding secondary ports, amd I confessed that I never bothered. Oh, the shame of it!

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[tcm]

anything even close on a powerboat (when going aground isn't "oops" but risks several grand of damage or entire boat sinking or entire boat sinking a bit later hence life too) i wd ring up the port itself.

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[StugeronSteve]

It was 1mm when I wuz a lad yer slacker. If it's going to be tight I take a bit more care than doing it in me head, coz as soon as I think of a second number I tend to forget the first. But, generally I do just as you say and look for 0.5 to 1m comfort zone.

<hr width=100% size=1>Think I'll draw some little rabbits on my head, from a distance they might be mistaken for hairs.

 

[dralex]

I just let the chart plotter guide me- it's always right. The trick is to stay well away from any water less than 10m deep, then you can't go wrong. Works everytime, but it's getting a bit boring out here.

<hr width=100% size=1>Life's too short- do it now./forums/images/icons/wink.gif

 

[Sybarite]

If it's going to be really tight then the faster you go the more likely you will break through.

John

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[rickp]

Talking of which, I saw some interesting PROPs the other day. Its amazing what can happen...

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[jimi]

Yup I do it .. Bembridge & St Vaast (When I can't get the lock opening times off the internet) come to mind.

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[Landale]

12ths is easier for normal activity unless it's a tidal curve that is skewed. However, I do do it the long way with extremes of access particularly if a bar is involved.

IIRC the tables are for a normalised 1013mB. Pressure effects might cause me to scratch my chin and take a view on the situation, isn't the rule of thumb about 10cm water for about 30mB pressure under normal circumstances?

- In otherwords be aware of it but under normal UK conditions forget it, paticularly as we want water - low pressure possible bad condiitons and higher tides, whereas high pressure normally means settled weather

I don't think you can make an easy direct conversion between pressure and alterations in water levels because you will have to allow for i) the immediate pressure difference (the easy bit) and ii) any upstream tidal surge created as a result of the pressure changes.

Tide times are important under edge conditions and what confuses me is that the Local Council Tables here, don't agree with Reeds (even after carefull interpolation) which don't agree with the chart plotter, which doesn't agree with observation. The spread can be as much as plus/minus 30mins.

My solution under normal circumstances is that I want half a metre plus the sea, and if in doubt grope!

Too much thinking for a Friday gone to the boat......

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[DJE]

Very good; but how often do you update your Bembridge chart. We get regular briefings from friends at Brading Haven Yacht Club and over the last couple of years the depths have been up and down like the proverbial.

Mind you twin keels and 4 foot draft tend to encourage a more relaxed attitude.

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[ChrisE]

Re: \'fessing up

Sorry aout the short rejoiner was in 'bl***y hell it's early in the morning' mode.

As my home port, Keyhaven, is a drying 2dary port you have to do the sums 'cos even at HW you've probably not got more than 1.5m under your keel going in and out and it is normal, in calm weather, to go in and out with 3 to 6 inches grace otherwise you'd be waiting forever.

If I'm honest I now rely upon my TotalTide programme to do the sums for me although up until last year I did them myself.

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[Solitaire]

While knowing how to work out secondary ports seems to be a preoccupation these days why not use the internet to do it. Boating is <A target="_blank" HREF=http://www.powerboattraininguk.co.uk>Serious Fun

 

[AlexL]

er - do a rough calc using 12ths and then make sure I do it on a rising tide if any doubt. The sand banks on the east coast move about every bloody month anyway so working it out to the nearest micron is a bit pointless, you may know how much water there is but the sea bed has moved!

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[Evadne]

I always work on the Bembridge depths from 10 years ago, before it was dredged, as that's what it's trying to get back to.

Incidentally I went through the barrier gap between Horse sand fort and the shore at the weekend, at LW-2. I have always known that there are nasty rocky bits all along the barrier, but actually seeing them sticking out of the water was a little sobering. Five Brownie points to the first YM theory candidate to tell us how much water above CD there was then....

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[Twister_Ken]

\'fessing up part deux

Been into Poole lots of times, never done any sums though. Did once run aground in Salterns Marina, but the muppet in the office told me I'd be OK with 2.1m of lead dangling down.

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[snowleopard]

i have a clunky old DOS program which can produce a tidal graph for any port. saves thinking.

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