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Old 03-05-2016, 14:18   #1
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What really means "dangerous rocks?"

Does anyone know the origins of this data? What are the criteria for mapping? Why so vague and erroneous?

Who is the audience?

Fisherman? Commercial?

I'm tired of steering around them or at least feeling guilty when I don't. 😅
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Old 03-05-2016, 14:33   #2
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Exclamation Re: What really means "dangerous rocks?"

Quote:
Originally Posted by clifflyon View Post
Does anyone know the origins of this data? What are the criteria for mapping? Why so vague and erroneous?

Who is the audience?

Fisherman? Commercial?

I'm tired of steering around them or at least feeling guilty when I don't. 😅
They are rocks seen or detected by seafarers and do not have hydrographic value. The position and depth is not accurate and you shall not only avoid theme but keep a safe distance
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Old 03-05-2016, 14:34   #3
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Re: What really means "dangerous rocks?"

Hi, Clifyon,

Check on the chart when that particular rock was reported. When the boat or the poeple were lost, the survivors would report approximately where it went down. Sometimes those position approximate, details unknown datums are erroneous.

Other times, there will be the remains of an underwater reef, where, like seamounts, the winds and currents make the waves heap up and cause dangerous conditions in that area.

Perhaps the hydrographic survey office of the country where the rocks are could inform you more of the detail, or get hold of a recent paper chart: usually paper charts' info is more complete.

Another option, might be, if the fishermen fish there a lot, they are usually pretty well up on where evil rocks are: catches and lives depend on their skippers knowledge and luck. Share some beers, rum, whisky, cigarettes, or fresh fruit with them and see what they can tell you.

Alternatively, CF'er, El Penguino, who is an old Chile hand, may be familiar with the area and able to tell you more about it. If he doesn't catch your post, he might answer a PM.

Ann
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Old 03-05-2016, 14:37   #4
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pirate Re: What really means "dangerous rocks?"

The Charts are for military and commercial shipping with deep draft.. but we use them as well at our own discretion
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Old 03-05-2016, 14:57   #5
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Re: What really means "dangerous rocks?"

It is the modern equivalent to the chart note "That be dragons" on the old charts.
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Old 03-05-2016, 15:23   #6
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Re: What really means "dangerous rocks?"

Judging from the number of rocks awash on that chart? (And surely, YOU should have some idea where that chart came from, who the issuing authority is?)


That's like a map that says "MINEFIELD". You don't need to know where each mine is, or who reported the first explosion. You're already doing what needs to be done, keeping clear of the entire questionable area.
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Old 03-05-2016, 16:46   #7
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Re: What really means "dangerous rocks?"

It means someone hit a rock and their boat sank and they think the rock was near there...
And when you manage to hit it after reading the chart, you can help narrow down the exact location of the rock(s). Read that GPS quickly before abandoning.
Or you can go around and not risk the belly of your boat.

Maybe some of them are visible at low tide... maybe not.

******************

USS Enterprise conducting flight ops.
Senior Petty Officer in Combat Information center looks at chart and calls the bridge:
"Bishops rock ahead appx 50 miles." (I forgot the exact times and distances... so don't try to figure the speed from this)
Bridge acknowledges.
60 min later:
"Bishop's Rock appx 20 miles"
Bridge acknowledges
Appx 20 min later:
"recommend turn to avoid Bishop's Rock"
Appx 10 min more:
"Turn the ship to avoid undersea mountain"
Helmsman sees rock poking up and spins the wheel AWAY from the rock
They hit the other (hidden) peak...

I knew the guy that was calling the bridge telling them not to hit the rock...
They first tried to blame him, then the poor guy on the helm who tried to save the ship.

***************

BTW... they did NOT continue ops... they managed to stabilize the ship and ran the USS Carl Vinson at "EXCEED PROPULSION PLANT LIMITS" to prepare to rescue survivors. They thought the Big E was going down.
I was on the Carl Vinson.

Quote:

November 2, 1985Bishop's Rock, about 100 miles west of San Diego, Calif.USS ENTERPRISE is grounded on Bishop's Rock. Reports say the ENTERPRISE sustains a 60-foot gash in the outer hull and damages one propeller. The carrier continues planned operations, taking part in ReadiEx 86-1 exercise before going into drydock on November 27.
USS Enterprise (CVN 65)

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Old 03-05-2016, 17:26   #8
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Re: What really means "dangerous rocks?"

Depends on the country, but the IHO criterion for mapping as "dangerous" is a depth of 31m.

Quote:
However, if the sounding is potentially a danger to surface navigation (e.g. if it is 31m or less, depending on context), it should be encircled by a danger line....Other reported dangers, such as wrecks, rocks, should be treated similarly
A rock can be detected from satellite imagery or aerial photography and its depth not known, thus it shows up as dangerous with uncertain depth. The IHO actually has a working group devoted to the removal of "subjective" terms and would like "dangerous" replaced with actual depths on all charts, however, collecting that data is a bit expensive.

Nothing in the displayed chart shows the position uncertainty, on the graphical side that should show up as "PA" or "Rep" or something similar indicating the position is not certain. Maybe it does show up that way on the paper version, and your electronics vendor has moved it to the notes/info layer, but no way of telling from the information presented.
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Old 03-05-2016, 19:09   #9
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Re: What really means "dangerous rocks?"

I believe the graphic representation should be roughly in line with INT1, NO?

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Old 03-05-2016, 19:21   #10
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Re: What really means "dangerous rocks?"

I think this is 421.4 (13).

Because it is visualy very similar to the far more dangerous 421.3 (12) I would simply stick to one policy of NOT sailing over any of the above, except in local waters that you know exactly how much water there is over the rock. You must be aware that elsewhere (e.g. Sweden) this may have a slightly different meaning and you may hit the rock even with a relatively shallow craft.

BTW by INT1 only depth is unknown, NOT position. The chart quoted does not have to be INT though. You will use chart symbols as supplied with the chart.

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Old 04-05-2016, 01:46   #11
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Re: What really means "dangerous rocks?"

To us - something to avoid regardless of how it got there -- we are cowards and don't like potentially swimming to shore with our lifejackets inflated or launching the liferaft and getting into it.
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Old 04-05-2016, 08:29   #12
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Re: What really means "dangerous rocks?"

Usually refers to rocks within 12 feet of the surface at mean low water. Some inland charts will use a 6 foot depth.
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Old 04-05-2016, 08:34   #13
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Re: What really means "dangerous rocks?"

Quote:
Originally Posted by clifflyon View Post
Does anyone know the origins of this data? What are the criteria for mapping? Why so vague and erroneous?

Who is the audience?

Fisherman? Commercial?

I'm tired of steering around them or at least feeling guilty when I don't. 😅
How are those rocks marked on the official NOAA chart?
What is the sea state like in that location at low tide? When there is wind?
How does other boating traffic treat the area?
Do you have good insurance?
When you transit the area, do you see shallow depths on your depth gauge?

In the area I sail, near Salem MA there are several areas of rocks that are fully submerged at high tide, hard to detect at low tide and definitely a hazard. I strayed too close decades ago and at the last minute saw a disturbed pattern in the waves, checked my paper chart and turned away from the danger.
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Old 04-05-2016, 09:48   #14
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Re: What really means "dangerous rocks?"

Hello,
The best way to stay out of trouble is to avoid it. Any charts without controlling depth notes and few depth numbers or depth contour lines should be suspect. I sailed the Yellow Sea, when ever I have doubts about an area I would talk to a local fisherman.
Good luck
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Old 04-05-2016, 10:07   #15
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Re: What really means "dangerous rocks?"

The question has been answered well by several. The only ones missed are those cooling to much Scotch at the helm.
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