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Old 13-08-2011, 12:31   #1
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Navigation In the 0-3 Fathoms Zone

Should a mariner be able to trust that enough navigable water exist between the “Open Sea” and a spot sounding of 2 fathoms (3.7 meter) in a charted 0 to 3 fathoms zone? (0 to 5.5 meter zone)
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Old 13-08-2011, 13:17   #2
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Re: Navigation in the 0 to 3 fathoms zone.

I'm not following here. You're in 0-3 fathoms charted and picking up 2 fathoms on the sonar?
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Old 13-08-2011, 13:52   #3
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Re: Navigation in the 0 to 3 fathoms zone.

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Should a mariner be able to trust that enough navigable water exist between the “Open Sea” and a spot sounding of 2 fathoms (3.7 meter) in a charted 0 to 3 fathoms zone? (0 to 5.5 meter zone)
No, based on your very limited info, there is no way to trust that enough water exist. If you want more "help" than that, you have to be way more verbal about what's going on. Position, what chart, draft of your boat etc, etc.

Thomas
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Old 13-08-2011, 14:13   #4
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Re: Navigation in the 0 to 3 fathoms zone.

I work on a "Shopping Cart Principle"

If sailing in an area so shallow that a shopping cart thrown overboard by some louts (Eric, for example) is going to ruin the keels paint then only sail there with eyes apeeled....

If you don't like the Shopping Cart senario, try coral head, or rock etc.

If there is only 1 meter under the keel I am going dead slow. Dunno what those in the Bahamas do....


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Old 13-08-2011, 15:16   #5
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Re: Navigation in the 0 to 3 fathoms zone.

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Originally Posted by cagney View Post
No, based on your very limited info, there is no way to trust that enough water exist. If you want more "help" than that, you have to be way more verbal about what's going on. Position, what chart, draft of your boat etc, etc.

Thomas
Still not following what the question is, but if I'm unsure about depth the two things that flash in my head is tide and bottom type. A rising tide and a sandy bottom is really no big deal. Rocky stuff and/or a falling tide is a bit worse.

If you have reason to doubt (or simply not enough reasons to be confident) with the water depth I'd focus on the things that are under your control. As Mark said, big difference between hitting a sand bar at 1 knot or 5.
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Old 13-08-2011, 15:21   #6
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Re: Navigation in the 0 to 3 fathoms zone.

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hopping Cart senario, try coral head, or rock etc.

If there is only 1 meter under the keel I am going dead slow. Dunno what those in the Bahamas do....


Mark
In the Bahamas, in certain areas, the bottom is dead flat, about 7 to 10 feet. We run as fast as we can. You get used to a foot or two under the keel. Haven't found any shopping carts there yet (no supermarkets). Now I would not do this in Miami, lots of carts!
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Old 14-08-2011, 00:37   #7
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Re: Navigation in the 0 to 3 fathoms zone.

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I'm not following here. You're in 0-3 fathoms charted and picking up 2 fathoms on the sonar?
Lets say I am seeking to take shelter in a bay with a few charted spot soundings of 2 fathoms. I am at the 3 fathoms (5.5 meter) contour and have surveyed it with the sounder and it is in accordance with my Navionics and CM 93 electronic charts and it looks good. I intend to proceed toward the closest 2 fathoms spot sounding after surveying the 4.5 meter contour.
I have found the following on ‘spot sounding’ in Google but this definition does not agree with my electronic charts that call every sounding a spot sounding. No matter the depth in that region. For now I have to go back to the boat. Will get in touch tomorrow with more detail.
4.4 Spot Sounding
Spot soundings are depths taken by manual means (a meter stick or weighted measured line) or by using sonar (if possible) in places where it is impractical, unsafe, or impossible to run a transect (Figures 5 and 6). For example, spot sounding will be conducted in a shallow area around a small island that has a shoal area covered by water, but the depth is too shallow to use a boat or sonar. Occasionally, spot soundings are used in densely vegetated aquatic areas because the motor cannot push the boat along at a constant speed.
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Old 14-08-2011, 01:14   #8
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Re: Navigation in the 0-3 Fathoms Zone

Chala,

The only real answer here is no. If you must proceed through an area that has been "Spot Sounded" then do so at a "Dead Slow" speed maintain your own soundings and be prepared to stop, reverse course and try again else where. In such areas a forward looking sonar would be a big help. Our forward looking sonar is SWM on the bow with a lead line.

Areas that are listed as Spot Sounded cannot be trusted. Use your own judgement.

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Old 14-08-2011, 01:15   #9
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Re: Navigation in the 0-3 Fathoms Zone

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Originally Posted by chala View Post
Should a mariner be able to trust that enough navigable water exist between the “Open Sea” and a spot sounding of 2 fathoms (3.7 meter) in a charted 0 to 3 fathoms zone? (0 to 5.5 meter zone)
In short, NO.
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Old 15-08-2011, 00:17   #10
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Re: Navigation in the 0-3 Fathoms Zone

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Chala,
Areas that are listed as Spot Sounded cannot be trusted. Use your own judgement.
Simes
Looking at that region of the Isle of Wight with CM93, every soundings are spot soundings are you saying that none can be trusted?
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Old 15-08-2011, 00:39   #11
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Re: Navigation in the 0-3 Fathoms Zone

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In short, NO.
So if they cannot be trusted why bother marking them on a chart? These unapproachable soundings may be more the cause of a stranding than anything else.
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Old 15-08-2011, 00:40   #12
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Re: Navigation in the 0-3 Fathoms Zone

I have a cheap fishing pole I use for this. Weight with a float attached exactly the draft +1 foot. If you throw the line out and see the float, STOP. Just make sure the weight can sink the float.
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Old 15-08-2011, 01:17   #13
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Re: Navigation in the 0 to 3 fathoms zone.

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I work on a "Shopping Cart Principle"
If sailing in an area so shallow that a shopping cart thrown overboard by some louts (Eric, for example) is going to ruin the keels paint then only sail there with eyes apeeled....
If you don't like the Shopping Cart senario, try coral head, or rock etc.
If there is only 1 meter under the keel I am going dead slow. Dunno what those in the Bahamas do....
Mark
I normally leave 2 meter under the keel when I anchor this allow for quite a tall trolley unless there is large tide (7m plus) and I know well the anchorage I will leave less. In unknown ground I check for a 6.5 meter under the keel contour and if satisfactory I check for the 4.5 meter under the keel contour and back track to the deepest spot then proceed upwind at the smallest steerable speed (0.5 Kt up). Favourable tide movement, tidal streams and moon cycle are more difficult to get at the right time. Now getting into contact with a 4.5 meter trolley is not frequent but can happen very suddenly.
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Old 15-08-2011, 01:23   #14
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Re: Navigation in the 0-3 Fathoms Zone

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I have a cheap fishing pole I use for this. Weight with a float attached exactly the draft +1 foot. If you throw the line out and see the float, STOP. Just make sure the weight can sink the float.
Good idea, does it still work in 22+ Kts wind in a short and steepy swell?
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Old 15-08-2011, 01:38   #15
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Re: Navigation in the 0 to 3 fathoms zone.

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A rising tide and a sandy bottom is really no big deal. Rocky stuff and/or a falling tide is a bit worse.
A cruising guide describes the place, as “comfortable anchorages exist close to the shore with sand bottom”.
As for the tide is what you get after a day sailing.
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