Tidal calculations

[boomer]

I'm looking for sensible advice on this one so I should probably go to the PBO Forum. However, I'll give the Master Mariners amongst you a chance to redeem yourselves.

I'm fairly comfortable with the normal tidal calculations for Standard and Secondary Ports but am less sure about how the tides affect depths further off-shore. Imagine I am planning a passage that takes me past, say, Brightbourne, and the Almanac give a tide of 6 metres there on a particular day. Ten miles off-shore there is a sandbank with a charted depth of only 2 metres. How much extra water can I expect over the sandbank at the top of the tide and when will this occur relative to HW Brightbourne?

This must be an important consideration in heavily shoaled areas such as the Thames Estuary where inshore channels are decidedly shallow.

ps would the feeble-minded please resist the temptation of asking where Brightbourne is.

 

[paulineb]

Take all your tidal references from Kentish Knock. There's a buoy called Kentish Knock off the shores in the shallows and if you wnat any reference to the tidal clearance/flows around that buoy, contact Thames Coast Guard who will be able to work out the clearances at any time of the tide.

Pauline B

 

[Guest]

Re: Brightbourne

Found it. 51 30.27N; 000 06.24W. That's where all the bright ones are anyway.

 

[hlb]

Well besides, Im a bit pissed at the mo cos of bad back, but trying to make sence Hic!!
And not wanting to gointo great depth, (Or shallows)
The tide table gives depth and range (If you subtract one from tother).
But mainly its not worth the bother in this forum, cos the boats faster than doing the calculatoin, so just go round the iffy bit and save the sweat!!

Haydn

 

[kimhollamby]

If you are looking at shallows with a charted depth of 2m, then that is measured at chart datum, so you usually have a small measure of comfort, even on spring tide low waters in the Thames Estuary. In theory.

For starters, there can be a much greater error between the predicted tidal heights and actuals caused simply by wind and pressure. So there's the first reason for building in a safety margin.

Secondly, rather than being too worried about any anomalies between local tide heights and tide heights at the nearest standard port (usually small), I would be much more careful about charted depths in the Thames Estuary, especially on the little backwaters that have long since lost their buoyage and hence, their commercial traffic and hence, the Hydrographic Office's interest.

If you check on the chart you can see when the last survey was conducted for a given area; sometimes the dates can be more than a little eye-opening. As you'll know, the shallows of the Estuary move around a fair bit, so I'd give a generous margin on everything to stay safe, in case the water has shoalled since last surveyed.

Incidentally, not too keen on the 'call the Coastguard' advice elsewhere here. There are plenty of safe routes around the Estuary that involve no risk of grounding (given reasonably accurate nav) and so if you are not sure about using the short cuts you would be better placed to avoid them. Heard a (by definition) 'qualified' skipper of a small Dutch yacht spend over 30 minutes asking panicky questions of Thames Coastguard before tackling(!) the S Edinburgh at half tide on a rising tide...painful to listen to but hats off to the men back at base who were patient in the extreme.

For my money, calculate your depths on standard ports for the hours ahead (12ths rule works fine if allowing a good margin), keep a weather eye on the depth sounder and have fun if you think your abilities are up to it.

 

[boomer]

Thanks for that Kim. All received and understood