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Depth of water?

Posted: Wed Feb 07, 2018 7:27 am
by Richj
Hi Folks,
Please excuse my 'newbie' question!
I am trying to sort out how to find how the depth of water changes with tide over a drying rock. Now, I understand the typical sinusoidal increase in depth and I can use simple approximations like 'rule of twelths'. However, where do I get the tidal range information from? I have tried using 'Standard Port' information, which seems fine for that port. But for a distant rock or reef it doesn't seem to work! The closest or secondary port information might, but seems less than precise?
Any thoughts gratefully received!

Re: Depth of water?

Posted: Thu Feb 08, 2018 12:04 am
by Jeff
Hi, welcome to the forum!

I've not planned a passage for over 3 years so I'm a bit rusty but....

Do you mean a rock that's far from a secondary or primary 'port' and so for which there's no specific tide height information?

In practice unless it's a really quirky situation, the peak tide height difference from the nearest easily calculable height is likely negligible isn't it? And timings can be estimated roughly by interpolation.

And also in practice you'd either avoid any sort of close encounter by a wide margin anyway, or give yourself plenty of margin for error if you 'had' to make a passage over the obstruction.

Don't forget that swell height can add and remove a lot of depth also, in swell affected locations.

Do you have a specific example in mind?

Re: Depth of water?

Posted: Fri Feb 09, 2018 12:40 pm
by Richj
Hi Jeff,
And thank you so much for the very valuable comments! First off a confession, and it might result in my excommunication! I am a sea kayaker. Nevertheless, we share many navigation interests and I joined the forum to learn, gain and hopefully contribute!

The question I posed related to a hypothetical feature, rock or reef away from primary or secondary port. Now, clearly the primary / secondary port will have tidal range data. Trying to find the height of water at a given time over the reef may be interpreted from this data, but which to use? For example, Liverpool may have an 8m range on a given day but a secondary port may only have 4m. Calculating the depth at a given time over a rock somewhere between the two could be awkward! I can understand the need for judgement but since most stuff on charts is so precise, I thought I must be missing something?

In a similar way, heights of land features I understand to be given above MHWS. But which HWS?

I may well be being super naive here! And most the time it is hypothetical in my boat which only draws a couple of inches. However, there are times; "Will we be able to take the short cut through the drying channel, or paddle around the headland", when it can be important!

Hoping you guys talk to folk in boats only 5m long!


Re: Depth of water?

Posted: Fri Feb 09, 2018 2:45 pm
by Jeff
Vessel size is not important, it's what you do with it that counts ;)

You ask good questions.

I suspect the height between two adjacent ports which aren't far up a river or inside some estuary are unlikely to differ by as much as 4m are they?

The heights even _at_ a port with figures available can never be considered to be super precise anyway due to wind and atmospheric pressure, and swell height, so you simply _have_ to give yourself a decent margin of error. And I suppose the more uncertainty and interpolation, the worse the conditions etc etc, the bigger a margin you give yourself.

As for charted heights on land being above MHWS... must admit I assumed they were above datum just now and went to check and you're quite right - how strange! Perhaps they're height above MHWS for the nearest major port? I don't know!