Yachtmaster theory question

[Skylark]

Good practice on a charter yacht or any strange vessel as well if you are not absolutely sure.

- W

Couldn’t agree more. It seems pretty reckless to take a strange-to-you boat out, especially in shoul waters, without knowing and/or calibrating the depth gauge. It takes just a few minutes to check and remove any doubt as to whether it’s water depth or depth below the keel and if there’s been an additional safety factor applied.

 

[RichardS]

The odd bit, which I find weird, is the advice you don't need a chart. To me that's weird. Checking it by all means, makes perfect sense. particular checking you will have enough water at low water, entirely logical. Verifying the information on the chart is accurate entirely logical. Which can be quite embarrassing if you don't.

I suspect that you're overthinking it.

All that is being said is that you don't need a chart to determine whether the anchorage you have chosen is safe. All you need to know is how deep the water is now and how much it will fall between now and the next low tide.

Richard

 

[alant]

By the "Mark Twain"
Or
By the "Deep Six"

Always thought to "Deep Six" anything was a burial.

 

[pyrojames]

It wasn't part of the syllabus when I did it 13 years ago. Blu-tack sounds rather more vegan friendly than tallow though.

Tallow sounds better to me, I’d prefer to keep the plastics out of the sea than placate the vegans.

 

[Uricanejack]

I suspect that you're overthinking it.

All that is being said is that you don't need a chart to determine whether the anchorage you have chosen is safe. All you need to know is how deep the water is now and how much it will fall between now and the next low tide.

Richard

I am a deep thinker

 

[BelleSerene]

The RYA approach is based around draft, "clearance" (being south coast based they usually want 1-2 metres!), charted depth, and height of tide...

And the pic at post #29: 'clearance required under the keel at LW: 1m'

What on earth...?! If you can't sail in 25cm, certainly in 50cm, of depth (clearance), you shouldn't be navigating! No? In charted waters and with a calibrated depth sounder, why on earth would you need 1-2m of clearance?

 

[BelleSerene]

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.

Presumably the reason for the Forth Bridge was that the third one had insufficient clearance?

 

[lw395]

And the pic at post #29: 'clearance required under the keel at LW: 1m'

What on earth...?! If you can't sail in 25cm, certainly in 50cm, of depth (clearance), you shouldn't be navigating! No? In charted waters and with a calibrated depth sounder, why on earth would you need 1-2m of clearance?

Sometimes on the real sea we have waves.
Sometimes you have to allow for variations in the seabed surface.
If your boat is going to swing around on the end of 30m of chain, it's actually pretty amazing to think that that whole area is the exact same depth to within half a metre.
If there was a small rocky patch sticking up slightly above the level sand, how much would it have to stick up to get charted?

But it's an exam question. So you define the 'clearance' so that everyone uses the same.

 

[prv]

If you can't sail in 25cm, certainly in 50cm, of depth (clearance), you shouldn't be navigating! No? In charted waters and with a calibrated depth sounder, why on earth would you need 1-2m of clearance?

1. Waves. Even on a flat calm day some fishing boat might come past and send a bit of wash your way.
2. Difference between tidal predictions and reality. Southampton pilots regularly call VTS to ask for a tide gauge reading, and get told how much above or below prediction there is. The figures are not always negligible.

Pete

 

[BelleSerene]

1. Waves. Even on a flat calm day some fishing boat might come past and send a bit of wash your way.
2. Difference between tidal predictions and reality. Southampton pilots regularly call VTS to ask for a tide gauge reading, and get told how much above or below prediction there is. The figures are not always negligible.

Pete

Yes, you're both quite right of course. I guess I just get frustrated with the if-it's-not-white-on-the-chart-then-I-can't-sail-in-it phobia at the other end of the spectrum, a bit like driving in the middle of a road for fear that the kerb might come and bite us!

 

[corbu]

My tuppence worth to the OP.

I find it useful to draw a little thumbnail sketch when working this out. Reminds me to check where the echo sounder is reading from to understand clearance at low tide. Not to mention, height to the bow roller as part of scope calculation.

When anchoring, I don’t see any conflict between using charts and ‘tom’s method’. Charts to understand the general arrangement of the area, depth sounder and fall of the tide to understand what depth to anchor in.

That said I generally add in another meter or so for good luck; not for the theory exam though!

 

[BelleSerene]

That said I generally add in another meter or so for good luck; not for the theory exam though!

Oh I don't know - I've just been told off in this thread for not adding a metre above calculations! (And for a couple of sensible reasons, too.)

 

[SteveGorst]

You seem to have mis spelt 'absolutely correct'.

Plus 1 for that, what is actually there trumps what is theoretically there every time.