Passing over wrecks question

[Boo2]

Hi,

When I was doing my DS practical I plotted a route over the top of a wreck. The symbol on the chart was as per the attached snippet at the bottom to the right of the middle, but the depth was around 14m instead of the 11.7 shown in the attachment.

The instructor pulled me up on this and when I said "but the depth is 14m, we draw 2.2 and there's a couple of m worth of tide" he replied "yes but it is shown on the chart as a danger to surface navigation".

But there are places on our coastline where passing over the top of wrecks is very likely to occur despite ones best efforts, so how worried should I be if a long leg does cross a wreck symbol with plenty of spare depth ? My feeling is that I can be sanguine about that but the instructors' words do ring rather...

Thanks,

Boo2

 

[Ric]

Hi,

When I was doing my DS practical I plotted a route over the top of a wreck. The symbol on the chart was as per the attached snippet at the bottom to the right of the middle, but the depth was around 14m instead of the 11.7 shown in the attachment.

The instructor pulled me up on this and when I said "but the depth is 14m, we draw 2.2 and there's a couple of m worth of tide" he replied "yes but it is shown on the chart as a danger to surface navigation".

But there are places on our coastline where passing over the top of wrecks is very likely to occur despite ones best efforts, so how worried should I be if a long leg does cross a wreck symbol with plenty of spare depth ? My feeling is that I can be sanguine about that but the instructors' words do ring rather...

Thanks,

Boo2

The only risk - and it is vanishingly small - is that there could be nets fouled on the wreck, which could in turn foul your gear. I think the instructor was being fairly pedantic - but then that is his job and he probably also insists that you use your motoring cone!

 

[sailor211]

I would do as you did and think that 10m clear is fine, no worse than in 5m with rocks.

As long as you were aware that was a wreck then no reason not to pass over it.

 

[Robin]

The instructor needs to have some instruction or you need a can of bullshit repellent.

 

[omega2]

good job he does'nt come with us we actively seek wrecks and anchor to them.

 

[johnalison]

Real life and exams exist on different planets. In real life I tend to avoid wrecks of less than 5m or so unless they are shown as being swept but ignore others. I used to be successful at taking exams because I would say what the examiner wanted.

 

[AntarcticPilot]

I guess that there is a tiny danger of material attached to the wreck floating up and being an obstruction at the surface. The older the wreck, the less likely, but I suppose it is a potential danger.

Wrecks are not stable objects on the bottom of the sea; they degrade with time and so I suppose that a wreck that was once safe could become unsafe as parts of the structure collapse and release floating material.

Is the depth noted a swept depth, or a depth measured to what is thought to be the highest point of the wreck?

Not really an issue where I sail - wrecks are either in water too shallow to consider or too deep to worry about!

 

[Fire99]

Playing Devil's advocate here, if the charts state that it's dangerous to surface navigation then the course skipper is doing the right thing.

He's following the facts of the chart.

OK, once we are away from the course and doing our own thing then it's up to us to interpret these things and make our own judgement call as to the 'actual' risk of going over it.

 

[Mariner69]

The wreck mark you have shown is not indicated as dangerous to surface navigation so the depth shown 11.7 metres is a reasonable indication to the water available at chart datum (low water springs; as near as dammit) with a confidence factor depending on the source data. So 0.5 metres plus 1 percent of depth at the best, to 2.0 metres plus 5 percent at the worst where assessed. Check the chart for the source data.

For safe navigation you would assume the depth available is 10 metres over the wreck. Then allow for the tide height minus your draught plus a safety factor say 0.5 metre and something for a heavy swell.

Where the wreck is marked dangerous to surface navigation (different symbol) that meant 14.6 metres of depth clearance up to 1960 then increased to 28.0 metres from 1968 as the ship's draughts increased with the VLCC and ULCC being built.

It would have to be a deep draught yacht to have a problem.

This is broad brush, there are very occasionally wrecks marked as dangerous where the depths are more, but then you might as well try the lottery when your sailing a yacht.

In a nutshell, your instructor was wrong; but right, to draw your attention to the need to check carefully the planned track. IMHO

 

[Ruffles]

A while ago I was sailing near the IOW, I think between Newtown and Cowes. Anyway I was close in to avoid some tide. I spotted a wreck on the chart ahead of us with a low CD and made a mental note to check where I was in relation to it when we got closer. I forgot of course. And saw the depth go briefly from something like 4m to 1.?m.

I think this is the only time I've ever seen any evidence that these wrecks exist.

 

[Dabhand]

The main danger, especially on inshore wrecks is likely to be static fishing gear marker buoys, either pots or nets.

 

[Fire99]

The wreck mark you have shown is not indicated as dangerous to surface navigation

That paints a different picture and the Instructor is being over cautious.

 

[Poignard]

Surely what the chart is indicating is simply that :

• at that spot, the depth of water at LAT is 3.3m
  
• the reduction in depth is due to the presence of a wreck
  
• the wreck has been swept by wire and the depth confirmed as 3.3m.

Your instructor would have done better to explain this to the candidates (although it seems that you knew this already!) and what implications it might have, if any, for them when skippering a yacht. He should also have given you credit for having seen it and for having taken into consideration in your plan.

 

[VicS]

the wreck has been swept by wire and the depth confirmed as 3.3m.

No that's not the symbol for a wreck swept by wire. It's the symbol for a wreck the depth over which has been obtained by sounding!

Swept by wire symbol is

EDIT
Ah I see you were looking at the wrong wreck. OP was referring to the one near the bottom of the extract with 11.7m of water

 

[Deleted member 36384]

No that's not the symbol for a wreck swept by wire. It's the symbol for a wreck the depth over which has been obtained by sounding! ....

Oh yes it is - assuming parsifal means the wreck, wire swept, depth 3.3 m LAT roughly in the top right quadrant of the chartlet.
_______________________________________________________

Boo2 the depth within the dotted line of a wreck is a depth confirmed by sounding but the soundings may not cover all of the structure, hence the sounding may not represent the shallowest point over the wreck. Only verified depths by wire sweeping or safe clearance depths give the navigator an assurance of the minimum navigable depth.

Your instructor is correct in that the symbol can not be fully interpreted as having a safe clearance over it and therefore should not be crossed directly over the top of.

 

[VicS]

Oh yes it is - assuming parsifal means the wreck, wire swept, depth 3.3 m LAT roughly in the top right quadrant of the chartlet.

Yes I guess he was but that was not the one that was the subject of the original post! That has 11.7 m obtained by sounding Not by wire sweep!

 

[nigel1]

Where the wreck is marked dangerous to surface navigation (different symbol) that meant 14.6 metres of depth clearance up to 1960 then increased to 28.0 metres from 1968 as the ship's draughts increased with the VLCC and ULCC being built.

It appears things have changed again, in the 2011 edition of chart 5011, a wreck NOT considered dangerous to surface navigation is described as being in a water depth of over 200m, or depth unknown
In the case of a wreck considered dangerous to surface navigation (as described by Mariner69, the new description is Wreck, depth unknown, which is considered dangerous to navigation.
Seems a case of CYA

 

[Poignard]

Yes I guess he was but that was not the one that was the subject of the original post! That has 11.7 m obtained by sounding Not by wire sweep!

Ooops! as they used to tell us at school - "always read the question first"

Still - it's the right answer to the wrong question.

 

[electrosys]

The wreck mark you have shown is not indicated as dangerous to surface navigation ...

Well - there's some ambiguity here (which may be at the root of the controversy):

According to Chart 5011, 2008 (the only copy I have) - the symbol shown [Section K: Rocks, Wrecks, Obstructions - item 26] represents "a wreck over which the depth has been obtained by sounder, but not by wire sweep." No remark is made for that symbol, concerning any 'safety of surface navigation'.

However, under [Section K: Rocks, Wrecks, Obstructions - item 1], there is an entry which states:
"Dangerline: A danger line draws attention to a danger which would not stand out clearly enough if represented soley by its symbol, or delimits an area containing numerous dangers, through which it is unsafe to navigate."

Now the symbol for item 26 does contain a 'danger line' - so whether the chart indicates 'a danger to surface navigation' is ambiguous.

Naughty UKHO ...

 

[Skylark]

There are a number of teaching opportunities around this point and most have already been mentioned. You did seem to have a predantic instructor but to be fair we weren't there so don't have the whole picture. It may also be worthy of discussion that charts, generally are not created for the benefit of small boat users. What may present a risk to a commercial vessel may not be an issue to a yacht.

It's always good practice to pay particular attention to any underwater features around the vicinity of your track, however, common sense and experience should prevail. It's common to sail outside of a buoyed channel to keep clear of commercial vessels using it, if the chart shows sufficient water for your draft and your depth gauge concurs.

I've just had a weekend on a UK charter boat, first time in a very long time. We had a discussion about the depth gauge, was it water or below the keel. I duly tied a line to the whinch handle and despatched it to the sea bed. The gauge was reading exactly in the middle between water and under keel. The moral is that it took no more than a minute to check and recalibarte the gauge.