Yachtmaster theory question

[cunningdavid]

I'm heading towards Yachtmaster certification and reading through Tom Cunliffe's book. It mostly makes perfect sense (so far) except...

Tidal heights and curves. Tom says: "Consult the tidal curve and see how much the tide will fall between now and the next Low Water, even if the tide is rising at the moment." Fine. Then he says: "There is no need even to consider such matters as charted depth." Why not? The tidal curve gives the height of tide above chart datum (lowest tide), and in many cases there will be water below chart datum... perhaps a lot of water. Why not consider it?

Thanks for any thoughts. If I could better post this elsewhere let me know too, cheers.

 

[Skylark]

Not sure I fully understand the context but......

Let’s say that you’re planning to anchor in a bay. The depth, according to the boat gauge, reads 6m. Take a look at the tidal curve for the bay in question. How much will the water fall between “now” and the next LW?. (Assuming that you plan to stay that long). If the water will fall enough for your keel to touch then it’s not a good Anchorage. There’s no reference to CD in this example.

 

[Prasutigus]

Greetings,

I remember asking the exact same question..' surely you must need to look at the soundings on the chart, when choosing a spot to drop the anchor?', it took me a while to get it..

Yes you need to look at the soundings on a chart, to pick a suitable area to anchor. For example, Studland Bay is more suitable than the Marianas Trench.

But when you are in, for example, Studland Bay, and you are looking for a perfect place to drop the hook, the chart won't help you.
To start with, the soundings on the chart are point soundings, not a contour map, so there won't be one exactly where that inviting gap is between all the other anchored boats.
The soundings will very likely be out of date anyway. You can fold up all your charts now, they are useless.

You only need to know if there will be a bit of water under your keel at the next low tide. You only need a tidal curve and your echo sounder.
The curve tells you the number of metres the water will go down between now and low water.
Subtract that from your echo sounder reading. If the answer is more than your boat's draught, you are ok.

I am not an instructor, but I remember trying to over-think this concept, the problem is grasping how simple it is!
( especially if you have been grappling with secondary ports etc recently..)

I hope that helps, P.

 

[fredrussell]

You only need to know if there will be a bit of water under your keel at the next low tide. You only need a tidal curve and your echo sounder.
The curve tells you the number of metres the water will go down between now and low water.
Subtract that from your echo sounder reading. If the answer is more than your boat's draught, you are ok.
.

Thats a great explanation Prasutigus, I get it now. I too have/had difficulty with this and even 'debated' it with my Day Skipper practical skipper when he asked me to see if we were good to anchor. Your explanation makes more sense than his did!

 

[Roberto]

Actually, when choosing an anchorage and not:wanting to ground, you should know the CD of the precise spot where you are dropping your anchor, well the circle all around it, which in 99% of cases you do not:have.
If you know the height of tide at the moment you arrive, and how much the water will fall in the following tidal cycle, you can calculate the minimum depth you need to measure on the sounder.
Basically, from depth measurement and height of tide you back-calculate the CD a chart would show in your exact spot.
I think one of the most useful tidal calculations, all can be done in advance, once you get there and begin to explore the place you just need to look at the sounderfor the minimum possible depth you calculated.

 

[cunningdavid]

Greetings,

I remember asking the exact same question..' surely you must need to look at the soundings on the chart, when choosing a spot to drop the anchor?', it took me a while to get it..

Yes you need to look at the soundings on a chart, to pick a suitable area to anchor. For example, Studland Bay is more suitable than the Marianas Trench.

But when you are in, for example, Studland Bay, and you are looking for a perfect place to drop the hook, the chart won't help you.
To start with, the soundings on the chart are point soundings, not a contour map, so there won't be one exactly where that inviting gap is between all the other anchored boats.
The soundings will very likely be out of date anyway. You can fold up all your charts now, they are useless.

You only need to know if there will be a bit of water under your keel at the next low tide. You only need a tidal curve and your echo sounder.
The curve tells you the number of metres the water will go down between now and low water.
Subtract that from your echo sounder reading. If the answer is more than your boat's draught, you are ok.

I am not an instructor, but I remember trying to over-think this concept, the problem is grasping how simple it is!
( especially if you have been grappling with secondary ports etc recently..)

I hope that helps, P.

Great explanation, thank you! I see that this is practical advice then, not a method to be used when practising theory at home with a chart.

 

[Mike k]

Great explanation, thank you! I see that this is practical advice then, not a method to be used when practising theory at home with a chart.

its a theory exam so you will have to quote chart examples using the CD and tidal curve etc

 

[cunningdavid]

its a theory exam so you will have to quote chart examples using the CD and tidal curve etc

Sure, but there's no echo sounder on a chart so in a theory situation you have to use the chart datum/soundings.

 

[MM5AHO]

One further possible complication....
Remember what your sounder does. There are two main possibilities.
One is that the sounder shows depth of water beneath the surface.
The other more likely is depth below the keel.

So if your depth sounder shows 6m, and the tidal chart shows the tide will drop 4m between now and low water, then...
If sounder is depth to surface, then you have that much water, a total depth of 2m at low water. Now consider what the boat draft is. Anything more than about 1.75m and you're pretty close to bottoming out. If draft is 2m, you are touching bottom, and any wave motion makes it worse.

But if sounder shows depth below keel, then at low water you have 2m beneath keel, and should be enough.

Add that snippet to the god explanations given already.

 

[East Cardinal]

What is the actual question you are trying to answer. Not considering CD is a strange view.
The CD is based on the LAT ( lowest astronomical tide ) and gives the depth of water below this level.
Tidal range is the depth above this.
So a low tide of 2.0 m may be ok for a small boat but if the Chart shows a drying height of 2.1m you could look a bit red faced at low water.
Anchoring requires you to know the range of tidal height over the period you will be there, in order to work out length of chain to use for your boat.

 

[Roberto]

What is the actual question you are trying to answer. Not considering CD is a strange view.
The CD is based on the LAT ( lowest astronomical tide ) and gives the depth of water below this level.
Tidal range is the depth above this.
So a low tide of 2.0 m may be ok for a small boat but if the Chart shows a drying height of 2.1m you could look a bit red faced at low water.
Anchoring requires you to know the range of tidal height over the period you will be there, in order to work out length of chain to use for your boat.

yes but if the chart shows a "3" in the middle of the bay, you have no idea what is the charted depth elsewhere, or if you enter an unknown/poorly charted port and people say "take that berth over there", you must have an idea if you are going to touch bottom or not at the next low tide(s)

the equation "measured depth= height of tide + charted depth" should be resolved the other way: I have no idea what the charted depth is here, I measure the depth (echosounder, lead etc), subtract the height of tide, to obtain the "virtual" charted depth that a highly detailed chart should have shown in that precise position.
With this calculated "local charted depth", plus the following low water heights, you know if you have enough water or not.

 

[East Cardinal]

But what question are we trying to answer?
Surveys can range from a lead line reading 200 years ago to side scan sonar this year. Obviously the data on the chart needs to be taken into account

 

[SimonFa]

Of course CD matters, who’d venture in to an area that’s going to dry out unless their boat can stand it? And IIRC, after 10 years, the course makes that point.

The question is about what to do when you get there, and as pointed out once in the general area you don’t need CD, but if you do want to use it then don’t forget to adjust for barometric pressure.

 

[Roberto]

But what question are we trying to answer?
Surveys can range from a lead line reading 200 years ago to side scan sonar this year. Obviously the data on the chart needs to be taken into account

the data on the chart is enough if you are exactly over that location, basic example a sill: very localised, very precise.
Say you arrive on a bay with a gently sloping sea bottom, the chart shows a "5" in the middle, depth contours are parallel. The bay is full of boats at anchor, you have a low draft boat and can anchor nearer to the shore. At the moment when you arrive, as you approach the shore line where no charted depths are available, what is the minimum measured depth you can accept without grounding at the following low water?
imho this is the op original question, indeed you do not know the charted depth of that particular spot, you derive it by subtracting height of tide from measured depth. Once you have the very local equivalent of charted depth, by adding the tidal curve you know the available water depths in the next tidal cycles, and if it is acceptable to your draught or not.

 

[Prasutigus]

the data on the chart is enough if you are exactly over that location, basic example a sill: very localised, very precise.
Say you arrive on a bay with a gently sloping sea bottom, the chart shows a "5" in the middle, depth contours are parallel. The bay is full of boats at anchor, you have a low draft boat and can anchor nearer to the shore. At the moment when you arrive, as you approach the shore line where no charted depths are available, what is the minimum measured depth you can accept without grounding at the following low water?
imho this is the op original question, indeed you do not know the charted depth of that particular spot, you derive it by subtracting height of tide from measured depth. Once you have the very local equivalent of charted depth, by adding the tidal curve you know the available water depths in the next tidal cycles, and if it is acceptable to your draught or not.

What he said^ imho.
Although being, myself, 'a bear of very little brain', I find it's easier to swap concepts in words of one syllable, or fewer if possible..bestens P

 

[zoidberg]

Greetings,

Yes you need to look at the soundings on a chart, to pick a suitable area to anchor. For example, Studland Bay is more suitable than the Marianas Trench.
You only need to know if there will be a bit of water under your keel at the next low tide.

Not just a pretty face, then, Mr P....?

 

[East Cardinal]

I'm heading towards Yachtmaster certification and reading through Tom Cunliffe's book. It mostly makes perfect sense (so far) except...

Tidal heights and curves. Tom says: "Consult the tidal curve and see how much the tide will fall between now and the next Low Water, even if the tide is rising at the moment." Fine. Then he says: "There is no need even to consider such matters as charted depth." Why not? The tidal curve gives the height of tide above chart datum (lowest tide), and in many cases there will be water below chart datum... perhaps a lot of water. Why not consider it?

Thanks for any thoughts. If I could better post this elsewhere let me know too, cheers.

Here is the original post. OP seems to be asking why not consider the CD. The book he is reading about tidal range is saying not to consider it. But, we do not know the circumstances of the question.
To fully answer we need more information.
There are many types of chart with a myriad of different surveys on them. Classified charts of west coast of Scotland will have a massive amount of data on them. Conversely a passage down the west coast of Africa may be on a chart with only a few soundings taken in the 1800’s.
Just using tidal data will tell you if the boat will ground at any state of tide but will not give the total depth of water under the keel.

What question does the Yachtmaster exam pose? It might be nothing to do with anchoring, but a question related to a yacht passing under a bridge................just a thought.
That is why I asked what the specific question we are answering was.
EC

 

[john_morris_uk]

What is the actual question you are trying to answer. Not considering CD is a strange view.
The CD is based on the LAT ( lowest astronomical tide ) and gives the depth of water below this level.
Tidal range is the depth above this.
So a low tide of 2.0 m may be ok for a small boat but if the Chart shows a drying height of 2.1m you could look a bit red faced at low water.
Anchoring requires you to know the range of tidal height over the period you will be there, in order to work out length of chain to use for your boat.

Of course CD matters, who’d venture in to an area that’s going to dry out unless their boat can stand it? And IIRC, after 10 years, the course makes that point.

The question is about what to do when you get there, and as pointed out once in the general area you don’t need CD, but if you do want to use it then don’t forget to adjust for barometric pressure.

Here is the original post. OP seems to be asking why not consider the CD. The book he is reading about tidal range is saying not to consider it. But, we do not know the circumstances of the question.
To fully answer we need more information.
There are many types of chart with a myriad of different surveys on them. Classified charts of west coast of Scotland will have a massive amount of data on them. Conversely a passage down the west coast of Africa may be on a chart with only a few soundings taken in the 1800’s.
Just using tidal data will tell you if the boat will ground at any state of tide but will not give the total depth of water under the keel.

What question does the Yachtmaster exam pose? It might be nothing to do with anchoring, but a question related to a yacht passing under a bridge................just a thought.
That is why I asked what the specific question we are answering was.
EC

You definitely don't need to know chart datum or even have a chart of where you want to anchor. All you need to know is the rise and fall of the tide, where you are on the curve at the time you are anchoring and HOW DEEP THE WATER IS WHEN YOU ANCHOR. The echo sounder tells you how much water there is now, you know how much you need to float in, and you know where you are on the tidal curve so you know the predicted rise and fall.

The easiest example would be knowing that its low water NOW you nudge your self into a cove or bay and find somewhere to anchor with a bit of water still under your keel. Just be careful that you aren't on a spring tide with the next LW being even lower than your current one and you will be fine. (You still need to know the tidal curve and the predicted rise of the tide to know how much scope to give yourself. )

Standard ports, secondary ports; it doesn't make any difference as it's the curve you need and you certainly don't need to know CD. You can calculate CD where you are, but who cares? All you need to know is whether there's still going to be enough water under you not to ground when it's LW and you've got enough scope out not to drag at next HW.

 

[simonfraser]

Def worth motoring around your anchor location, here on the east coast the depth varies considerably only a few meters sideways. Well it does using the echo sounder, its flat according to the chart

 

[Uricanejack]

Hmmm

It occurs to me there is the right way to pick a spot to anchor.
The wrong way to pick a spot to anchor.

And then thier is some weird RYA way to pick a spot to anchor,

Personally I will look at the chart.
Avoid the green bits. Unless you have twin keels.
The dark blue is best avoided if you have a deep
Fin
The light blue should be ok unless it’s a very deep fin.
White fine and dandy.
If the bay in question is dark blue, I have a hand lead an set of tide tables JIK.
Or I am in an area with a big range and I am figuring out how much scope I might need