Anchoring/tides

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castoffandgone

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Having sailed our 31’yacht around the Clyde and the Scottish western isles for a few seasons now, I feel we have missed out in the pleasures of anchoring out overnight as much as we should have. This is largely down to our (i.e. my husband and myself) lack of confidence in working out the tides.

So, I set about discreetly trying to find out how others went about this. To my disappointment and surprise I got all sorts of gobble d gook and furrowed brow explanations as to how simple it was. Now, I am not the sharpest knife in the drawer and I still don’t understand how planes fly or cement yachts float, but surely there is a SIMPLE way of working it out.

Hypothetical: I will arrive at Puilladobhrain anchorage on the West Coast tomorrow 9Nov. At about 1400hrs. Boats draft is 1.8m depth sounder set to water under the bottom of the keel.

1) I look at the Clyde cruising club directions and it says “anchor anywhere in the pool in a suitable depth 4-6metres good holding with 20metres or more of chain” The guide also shows the tide “as Oban MHWS 4.0 MHWN 2.9 ML 2.4 MLWN1.8 MLWS 0.7”

2) I check the Oban tide tables on my laptop and note that high tide is at 1404hrs (my arrival time) is 3.4m and the lowest tide before my departure the following day is 1.5m at 0819hrs

Questions:
What does the guide mean “suitable depth” and how do I find it.
What is the relationship between the depth shown on the charts and the height shown in the tables.
I presume I need to motor around until I find the ‘suitable depth’, but what would that be in my example.
What is the easiest way of doing it?

Your help would be appreciated. Incidentally, I am posting this on my own account, he has a separate one!
Thanks
 
Yes, some of the advice can be horribly confusing - but the examples you gave are guides, and your limiting parameters are dependant on the draught of your boat. On the face of it, the 4-6m bit doesn't mean much as the actual depth will depend on the tides, but as predicted depth is the sum of the tides given in the almanac and the charted depth (which is the depth at lowest astronomic tide) it allows you to get a feel for what you're looking for. Obviously you'll be looking to have a safe depth under the keel as you go through low water, so look for a spot whose depth will achieve this, but particularly at Springs, when the scope you let out will be greater to accomodate the highest tidal range, you may just swing into shallow water on low tide and run aground! This is not uncommon in popular anchorages in the Solent which get pretty crowded at times, so people chance their arm close to the mudbanks - usually no harm done except to your dignity.

Rob.
 
If six people explain this to you you will get 6 slightly different explanations and you will end up more confused than ever

I'd, therefore, recommend that you find a suitable book, one written and illustrated in a style with which you are happy, and stick with that.

Its is explained in the Almanacs eg Reeds and the PBO one but perhaps more in style that will refresh the memory of someone who already understands it rather than in a manner for someone who does not.

Evening classes, if any are available in your area, are the best way of learning all this stuff but you are probably to late to enroll for this winter.

Maybe someone has link to a good on line explanation.
 
Many people like to make this very complicated, and it can be, if you need to know the exact depth at any minute of the day.
However, if you're not very good at mental arithmetic or are tired, I find it helpful to draw a little diagram.
Draw four parallel horizontal lines,
top one is height of tide today [your example 3.4m]
2nd one low water today [1.5m]
3rd one is the chart datum,[0m] usually lowest level the tide will normally fall to [known as Lowest Astronomical Tide. [LAT]
4th line depth of water shown on chart. This is measured below the Chart Datum / LAT.
I then draw a little picture of my boat floating at low water to show draught and depth needed.
This may seem childish and oversimplified but when tired or confused the simple tricks are best.
 
VicS said:
I'd, therefore, recommend that you find a suitable book, one written and illustrated in a style with which you are happy, and stick with that.
Click to expand...

Maybe the RYA's "An Introduction to Navigation" will be what you need.

I've not seen it but it is written by stroppy (ex??) forumite Tim Bartlett and I have found magazine articles he has written clear and easy to follow
 
Ok, fed the cats and now to try the worked example!

I'm never sure what depths the cruising guides are talking about either, but as they are suggesting 20m of chain, that suggests that at the usually recommended 3 x depth scope with chain, they are suggesting 6m depth at high tide. So in this instance, you are looking on the chart for a spot which gives you a safe swing and a depth on arrival of 6m (4.2m allowing for the offset on your instruments). As you've chosen the easiest scenario of arriving at high tide, that's about it. Just note that at low water, you charted depth can be calculated by subtracting the range, ie 6m - (3.4m - 1.5m) = 3.9m, leaving you with 2.1m under the keel.

Of course, predicted tides and actuals don't often agree, but you've enough room for error here.

Rob.
 
Get a nice app on your phone like Tidesplan.

At any time it will tell you how much more the water will go down from now to low tide.

Make sure you anchor with more than that under your keel.

That's all you need to do.
 
Thank you, I am thinking about what you have said and I'll see what I can do with it.

I know a training course would cover this topic, but I'm happy with the rest of our sailing and really not sufficiently keen to delve deeper - other than for my own ends i.e. anchoring in nice spots!
 
The echo sounder.

The instructions in the pilot book are good, anchor in 6 meters of water (according to your depth sounder) with 20 meters of chain out, that's about 3 times the depth of water.

Practicing in the Clyde would have been a good idea just to get the feel of anchoring where there is only about 1 meter of tide. For a 2 meter tide you only need another 3 feet of water under you.
that's wrong, I'm thinking 1 knot of tide, the tides in the Clyde aren't that different.
 
Last edited:
castoffandgone said:
Is that the depth shown on the chart or what the echo sounder is showing?:confused:
Click to expand...

The actual scope (amount of rode out in relation to the depth) is not a fixed amount as it will vary according to circumstances. 3* is a good minimum, and is usually related to the maximum depth - that is at high water. However more than that is common, but the actual amount can be constrained by circumstances - too much might mean swing too wide an arc and getting into shallow water at low tide.

The simple drawing method suggested earlier is really the only thing you need. The key levels are charted depth which is the lowest it can go, tidal rise to HW which gives you the maximum it can be in the cycle and LW which will give you the lowest it will be in the cycle. Most of your tides go up and down in a predictable way so it is also easy to estimate the height of tide, and therefore the depth at any time. There are various short cut methods for this, but a simple linear calculation such as in 3 hours (half a cycle) the tide will have risen or fallen by half the expected tidal range is usually adequate for anchoring in the sort of depths you are talking about.

A little bit of practice and you will soon get the hang of it - then you can come down to our part of the world and tackle our unusual tidal patterns - and learn the hard way (well actually usually soft sticky mud way) what happens when you get it wrong! Then you will realise how lucky you are sailing in an area with mostly deep water and simple tides.
 
castoffandgone said:
Boats draft is 1.8m depth sounder set to water under the bottom of the keel...
Click to expand...
Once you start messing about with tides, having your sounder set like this is very confusing. I suggest you reset it to show the actual depth, and put a label underneath to remind you. Now you can get a rough idea of the current tide by comparing your sounder with the chart.

The rest is easy, say the tide range is 1m to 3m, and you arrive mid-tide, so 1m. You now know that the lowest tide is going to be 1m below what your sounder says, and high tide 1m above. So in your case, the minimum depth to anchor in (now as shown on the sounder) is 1.8m (draft) + 1m (tide fall) + 1m (safety).
 
castoffandgone said:
Just found and downloaded 'Tidesplan' to my ipad, but it doesn't come with a user guide - typical. Oh well, maybe it's intuitive!
Click to expand...

There is a user guide. I have the ipad version as well.

Press "About" There is an FAQ, Quickguide and detailed description of every function.

RTFM! :D
 
castoffandgone said:
Hypothetical: I will arrive at Puilladobhrain anchorage on the West Coast tomorrow 9Nov. At about 1400hrs. Boats draft is 1.8m depth sounder set to water under the bottom of the keel.

1) I look at the Clyde cruising club directions and it says “anchor anywhere in the pool in a suitable depth 4-6metres good holding with 20metres or more of chain” The guide also shows the tide “as Oban MHWS 4.0 MHWN 2.9 ML 2.4 MLWN1.8 MLWS 0.7”

2) I check the Oban tide tables on my laptop and note that high tide is at 1404hrs (my arrival time) is 3.4m and the lowest tide before my departure the following day is 1.5m at 0819hrs

Questions:
What does the guide mean “suitable depth” and how do I find it.
What is the relationship between the depth shown on the charts and the height shown in the tables.
I presume I need to motor around until I find the ‘suitable depth’, but what would that be in my example.
What is the easiest way of doing it?

Your help would be appreciated. Incidentally, I am posting this on my own account, he has a separate one!
Thanks
Click to expand...

To make it simple, your draft is irrelevant since your depth guage is from the bottom of your keel (I assume you are certain of this?).

If the tide is currently at 3.4 then subtract this from the current depth reading and you will see the water under your keel at low tide. Use the Rule of twelfths to work out intermediate tide heights - you don't need to be that accurate.

It should be obvious that you're not aiming for 0m under the keel at low water - waves and rocks can cause havoc so you'll want a metre or so for safety.

Scope on your boat will be charted depth plus high water height plus 1m (for the bit between water and bow roller) times 3 for chain or 5 for rope.

Don't just multiply your depth guage reading for scope - since yours is measured below the keel this will be nearly 3m out and 9m once you multiply by 3!
 
castoffandgone said:
Hypothetical: I will arrive at Puilladobhrain anchorage on the West Coast tomorrow 9Nov. At about 1400hrs. Boats draft is 1.8m depth sounder set to water under the bottom of the keel.
Click to expand...


...........and if you do work it out and get there, here's what you can expect at sunset:

YBWpic.jpg
 
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