Yachtmaster theory question

[Prasutigus]

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

HA HA

 

[East Cardinal]

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.

Oh dear!

 

[East Cardinal]

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

I agree with you. What are the RYA teaching?

 

[ex-Gladys]

I agree with you. What are the RYA teaching?

I am a theory instructor, and the RYA aren't teaching anything odd.. what's odd is interpretation of posts. The RYA approach is based around draft, "clearance" (being south coast based they usually want 1-2 metres!), charted depth, and height of tide...

 

[Skylark]

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

I agree with you. What are the RYA teaching?

I'm intrigued. Do tell, what weird way does RYA teach to pick a spot to anchor? I'm also a Shorebased Instructor.

 

[capnsensible]

Oh dear!

You seem to have mis spelt 'absolutely correct'.

 

[GHA]

You seem to have mis spelt 'absolutely correct'.

:encouragement:

 

[Momac]

: "There is no need even to consider such matters as charted depth." Why not? .

Calculating the depth at next low water starting from a depth measured now could be more accurate than the depth taken from the chart . The depth on the chart could have been surveyed years before and may have changed.

 

[WFA]

The following slide explains the issue and answer to RYA Essential Navigation & Seamanship course students who are two levels away from Yachtmaster.............

 

[lw395]

Calculating the depth at next low water starting from a depth measured now could be more accurate than the depth taken from the chart . The depth on the chart could have been surveyed years before and may have changed.

It is an exercise in knowing what you need to know.

To anchor over the next low tide, all you need to know is that the tide won't drop by more than you currently have under our keel.

Bringing charts into it shows a failure to get to the point.
To use a chart to tell you if you have enough water, you first have to know precisely where you are. Then your chart has to be totally accurate and have the necessary information on it in sufficient detail.
Your position could have errors.
The chart could have errors.
It is a tidal height exercise.

In reality one would look at the chart for obstructions and probably the almanac or pilot book.
But it is a tidal heights exercise and in that context, drifting off into displacement activity is just a refuge for those who haven't had tidal calcs battered into their skulls yet.

 

[Heckler]

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.

Exactly, some still havent got it!
Stu

 

[East Cardinal]

The problem with all this is the assumption we are discussing anchoring. The OP does not mention anchoring.
Relying on an echo sounder without using a chart is bonkers.
I know nothing of what the RYA teach on anchoring, hence my question.
I have anchored ships many times but never without a chart.

 

[laika]

To fully answer we need more information.

Perhaps we could consider this to be a meta-question. As yacht skippers we don't necessarily have a complete view of what is going on but we learn to make inferences based on the information we do have and be aware of the margin of uncertainty involved. Our yachtmaster question becomes "In what context could the given statement be true?". I agree with the majority of other respondents: the most likely context for the quoted Cunliffe statement is when choosing a place to park (not necessarily on the hook, maybe on a buoy) for a duration which crosses a single low tide. You don't need to know the charted depth, only how much less water there will be under your keel than there is "now" at the lowest point during your stay.

Consider this "tactics" as opposed to "strategy". Choosing your destination (i.e. picking somewhere that there will be *somewhere* to park) and noting any obstructions (i.e. big rocks or a steep sandbank as opposed to the gently shelving shoreline) is something you do before you go, yes with reference to the chart and available depths, and make pilotage notes accordingly. Once you get there and have an idea of a relatively benign topology (with reference to your pre-prepared pilotage notes), you base your decisions regarding your *exact* spot (where to drop your hook, which buoy you can pick up without toppling over at 3am) on the echo sounder and your notes as to how much further the tide will fall as people have been describing.

Although there are some fine explanations of this here, I think the last part of the question might imply that the OP may have some potential uncertainty around the concept of chart datum (though not relevant to the main part of the question).

 

[East Cardinal]

Absolutely correect��
Makes sense. Where is the OP? Has that answered your question?
Understanding the clearance below AND above your vessel is very important. Ask the guys who sailed HMS Queen Elizabeth under the Forth Bridge on their way to Portsmouth.

 

[RichardS]

Absolutely correect��
Makes sense. Where is the OP? Has that answered your question?
Understanding the clearance below AND above your vessel is very important. Ask the guys who sailed HMS Queen Elizabeth under the Forth Bridge on their way to Portsmouth.

You seem to have gone from Oh Dear to Absolutely Correct.

John Morris, as others have said, has nailed it. We now just have to wait for everyone else to latch on.

Richard

 

[WFA]

The chart helps the navigator determin the vessels position and what can be expected to be found in the close vicinity.
Soundings, tidal information and weather conditions help the skipper determine if the vessel is (and will be) safe from grounding or an overhead obsruction.
And that is the essence of all theory and practical courses that I am aware of - including those of the RYA.

 

[Mike k]

Correct me if I am doing this wrong but when I am anchoring up for the day I look at the tide height expected that day then use the rule of 12ths but normally as I know that the largest tide is 10m so will only anchor if there is 33 feet of water showing in the sounder 2 hours before or after HT .Its rough but hasn't failed me yet .Probably would fail the yachtmaster exam!!

 

[alant]

Correct me if I am doing this wrong but when I am anchoring up for the day I look at the tide height expected that day then use the rule of 12ths but normally as I know that the largest tide is 10m so will only anchor if there is 33 feet of water showing in the sounder 2 hours before or after HT .Its rough but hasn't failed me yet .Probably would fail the yachtmaster exam!!

Really impressed when you use metres & feet in the same calculation.
When are you converting both to fathoms?

 

[john_morris_uk]

The following slide explains the issue and answer to RYA Essential Navigation & Seamanship course students who are two levels away from Yachtmaster............. View attachment 68665

Agreed. Another version of wot I sed....

It is an exercise in knowing what you need to know.

To anchor over the next low tide, all you need to know is that the tide won't drop by more than you currently have under our keel.

Bringing charts into it shows a failure to get to the point.
To use a chart to tell you if you have enough water, you first have to know precisely where you are. Then your chart has to be totally accurate and have the necessary information on it in sufficient detail.
Your position could have errors.
The chart could have errors.
It is a tidal height exercise.

In reality one would look at the chart for obstructions and probably the almanac or pilot book.
But it is a tidal heights exercise and in that context, drifting off into displacement activity is just a refuge for those who haven't had tidal calcs battered into their skulls yet.

Agreed. (I was trying to be polite though...)

The problem with all this is the assumption we are discussing anchoring. The OP does not mention anchoring.
Relying on an echo sounder without using a chart is bonkers.
I know nothing of what the RYA teach on anchoring, hence my question.
I have anchored ships many times but never without a chart.

The chart tells you what the nature of the seabed is, hopefully it also gives you an idea of the topography of the seabed. (Am I anchoring on the edge of an underwater trench that I might drag into? Are there charted dangers nearby? Ships anchor in deeper waters and don’t generally worry about grinding at LW. I maintain CD is less important for a yacht than what the echo sounder reads in your chosen spot.

The chart helps the navigator determin the vessels position and what can be expected to be found in the close vicinity.
Soundings, tidal information and weather conditions help the skipper determine if the vessel is (and will be) safe from grounding or an overhead obsruction.
And that is the essence of all theory and practical courses that I am aware of - including those of the RYA.

Exactly so.

 

[solent clown]

John Morris, do the RYA still teach "proper sounding" with a line and weight? And taking samples of the bottom with a dob of blu tack wrapped around the weight?
We have used a lead and line from a dinghy tiptoeing up a creek or two, as much for fun as anything else but never had a massive amount of luck sampling the substrate. Is this another skill that is consigned to the past, or is it still taught as a back up?