Cole rock Bembridge, IOW, UK.

[Channel Sailor]

I had a maybe near miss with Cole rock(s) recently. The Imray chart and Navonics both say the highest points are dries 0.2 and awash respectively . I sailed over it when the tidal predictions say there was 3.9m raise of tide above CD (2nd June 12:35hrs BST). I expected to see on my sounder 3.7m depth. But I actually saw 1.8m on the sounder! So what could be going on here? Is Cole rock actually much higher dries that charted, or could my depth sounder reflect/measure inaccurately over rock. Are there any locals here who know the rock(s) well?

With sufficient rise of tide I was thinking I would like to go the same way again one day, because it cuts the corner nicely by Bembridge Ledge buoy.

BTW I have navigated in plenty of rocky places where one only goes with sufficient rise of tide (Channel Islands for example), so this rock appeared to present no particular concern.

 

[Sticky Fingers]

Is your sounder calibration correct? And to what datum… Depth of water? Under keel? Under the sounder transducer?

 

[Channel Sailor]

I am confident with the sounder as I reference it very often in shallow water and it always appears to be spot on. I did calibrate it with a lead line a year ago. But good point that I should check it again with the lead line, because something could have changed very recently such as the offset has been changed by crew accidentally or an engine start low voltage dip could have screwed it up. I doubt it though, because I used it the next day to anchor for lunch in 3.6m metres over a drying mud/sand location off Hill Head.

The depth reading in the post above was simplified for clarity. The vessel is 1.55m draft. The set up is the sensor is located under rather flat bow sections at about 0.2m and has a a 1.4m offset. So in effect the display actually reads from ”below the keel” assumed at 1.6m. So what I actually saw on the display was 0.2m, but I added on the 1.6 offset in the post above,

 

[Chiara’s slave]

I am confident with the sounder as I reference it very often in shallow water and it always appears to be spot on. I did calibrate it with a lead line a year ago. But good point that I should check it again with the lead line, because something could have changed very recently such as the offset has been changed by crew accidentally or an engine start low voltage dip could have screwed it up. I doubt it though, because I used it the next day to anchor for lunch in 3.6m metres over a drying mud/sand location off Hill Head.

The depth reading in the post above was simplified for clarity. The vessel is 1.55m draft. The set up is the sensor is located under rather flat bow sections at about 0.2m and has a a 1.4m offset. So in effect the display actually reads from ”below the keel” assumed at 1.6m. So what I actually saw on the display was 0.2m, but I added on the 1.6 offset in the post above,

This sort of confusion is why my sender is offset to surface.

 

[shortjohnsilver]

At present mines set to water depth (surface) but only recently. I had had it set to depth below the keel, for years!
TBH I’m thinking of returning to depth below keel. I sail on the east coast and, well, it’s shallow around here mostly and it’s good to know immediately what you actually have rather than having to conduct a quick mental calculation to arrive at what depth there is, right then, right now! Interesting though.

 

[Chiara’s slave]

At present mines set to water depth (surface) but only recently. I had had it set to depth below the keel, for years!
TBH I’m thinking of returning to depth below keel. I sail on the east coast and, well, it’s shallow around here mostly and it’s good to know immediately what you actually have rather than having to conduct a quick mental calculation to arrive at what depth there is, right then, right now! Interesting though.

You get used to wherever you set your offset I guess. And realising you can’t sail in less than 2m in flat water isn’t that taxing. I like it to correspond to chart plus tidal rise, I find that simple. Rather than chart plus tidal rise minus depth of keel, or was that bottom of hull, I don't remember.

 

[benjenbav]

This sort of confusion is why my sender is offset to surface.

That works well until someone else helms who expects to be comfortable until they see a reading of 0.0 - don’t ask me how I know this.

 

[Chiara’s slave]

That works well until someone else helms who expects to be comfortable until they see a reading of 0.0 - don’t ask me how I know this.

Well of course. I tell any guest crew, and would always ask on a strange boat.

 

[pyrojames]

I think it is easier for casual crew to have it set to the bottom of the keel. Then they don't need to know anything about the keel depth etc.

 

[Chiara’s slave]

I think it is easier for casual crew to have it set to the bottom of the keel. Then they don't need to know anything about the keel depth etc.

My ‘keel’ moves, disappears even. Perhaps that directs my logic.

 

[mikegunn]

That works well until someone else helms who expects to be comfortable until they see a reading of 0.0 - don’t ask me how I know this.

And for that reason I have a Dymo label on my depth instrument which clearly states “depth below w/l”. I stuck it there after putting myself aground because I’d forgotten how I’d applied the offset. I don’t look as stupid as I am.
Mike

 

[Channel Sailor]

As suggested above I have checked the calibration on the depth sounder. All is Ok. Tested it over mud (in the marina) using a lead line.

But it could part of the answer to my question is the effect of the the wind in tidal heights. On the day in question the wind had been blowing for a day or so from the east or NE. The strength was F4 occasionally F5. Maybe this wind lowered the tide compared to predicted. But by 2m! contributing to the the mystery could be the depth sounder reads differently over rock/weed Compared to mud.

I am still puzzled. I might ask the nearby RNLI station if they know of any anomalies with this rock.

 

[alahol2]

Search for Real Time Tides. You can get a good idea of what affects the height, pressure, wind etc. There may even be some historical data.

 

[Thistle]

I had a maybe near miss with Cole rock(s) recently. The Imray chart and Navonics both say the highest points are dries 0.2 and awash respectively . I sailed over it when the tidal predictions say there was 3.9m raise of tide above CD (2nd June 12:35hrs BST). I expected to see on my sounder 3.7m depth. But I actually saw 1.8m on the sounder! So what could be going on here? Is Cole rock actually much higher dries that charted, or could my depth sounder reflect/measure inaccurately over rock. Are there any locals here who know the rock(s) well?

With sufficient rise of tide I was thinking I would like to go the same way again one day, because it cuts the corner nicely by Bembridge Ledge buoy.

BTW I have navigated in plenty of rocky places where one only goes with sufficient rise of tide (Channel Islands for example), so this rock appeared to present no particular concern.

Weed over the rock??