Weird Experience - Boat just stopped

[Hydrozoan]

Whilst irrelevant to the situation described, it must have been quite a surprise to Fridtjof Nansen to discover that the phenomenon of 'dead water', previously considered to be a bit of a sailors' yarn, could reduce the speed of the Fram from 6-7 knots to a mere 1.5 knots - and that when encountered it could be rather difficult to escape. RMetS Journals

 

[dolabriform]

Whilst irrelevant to the situation described, it must have been quite a surprise to Fridtjof Nansen to discover that the phenomenon of 'dead water', previously considered to be a bit of a sailors' yarn, could reduce the speed of the Fram from 6-7 knots to a mere 1.5 knots - and that when encountered it could be rather difficult to escape. RMetS Journals

Really interesting, thank you.

 

[AntarcticPilot]

Whilst irrelevant to the situation described, it must have been quite a surprise to Fridtjof Nansen to discover that the phenomenon of 'dead water', previously considered to be a bit of a sailors' yarn, could reduce the speed of the Fram from 6-7 knots to a mere 1.5 knots - and that when encountered it could be rather difficult to escape. RMetS Journals

Yes - caused by fresh water floating on top of salt water, resulting in the boat making internal waves that take a lot of energy; it's the same as "hull speed", but at an internal interface!

 

[Hydrozoan]

Yes - caused by fresh water floating on top of salt water, resulting in the boat making internal waves that take a lot of energy; it's the same as "hull speed", but at an internal interface!

Indeed so - several wave tank demonstratons (of varying quality) on YouTube.

 

[AntarcticPilot]

And as you noted, mainly seen in the polar regions where the surface water can be almost fresh as a result of melting ice.

Indeed so - several wave tank demonstrations (of varying quality) on YouTube.

 

[Hydrozoan]

And as you noted, mainly seen in the polar regions where the surface water can be almost fresh as a result of melting ice.

But in principle possible whenever two (or more) layers of water of different densities (S and/or T) occur. I didn't get into the further details, but it has been suggested - and is apparently being investigated by the French CNRS/Université de Poitiers - that the fleet of Cleopatra and Mark Anthony might have been trapped by the phenomenon in the Batttle of Actium, the bay there being fjord-like: article of July this year at Behind the dead-water phenomenon.

 

[Wansworth]

Grounded on a whale?

 

[dolabriform]

Following TG's advice above I sent the info to HHA. They replied today and I am quite amazed to be honest.

"Hello David
I am contacting you in reply to the message you sent Harwich Haven Authority on the 29th of October.

Our hydrographic survey team have conducted a full multibeam seafloor survey of the area in question (please see attached). The least depth in the area surveyed was 3.9m chart datum, further more there were no marks in the seabed to indicate any recent grounding."



So it must have either been a Pot, or our imagination !

 

[tillergirl]

I congratulate you on sending in the information. The maritime authorities are very interested in reports. There are a couple of interesting red spots: I guess red is shallow.

 

[dolabriform]

I congratulate you on sending in the information. The maritime authorities are very interested in reports. There are a couple of interesting red spots: I guess red is shallow.

Thanks Roger, I agree it would seem to indicate that red is shallow.
I'll reply to them tomorrow and ask if they have a scale for the colours.

 

[tillergirl]

If they are using the same colour coding in that pdf I referred to in the other thread it would be 0.0 to 4.9. I am guessing that isolated red is where that shallow point was that was subject to that old NtM.

 

[dolabriform]

Hi Roger
I contacted HHA this morning and they sent me this:

 

[tillergirl]

I think that red blob is/was the NtM years back. They had found 1.8 or something like that and scoured it down to 3.9. Now it is at 3.5. I think, though, we can be certain you didn't touch bottom! There is, of course, a well used nautical expression - "it's one of those things".

 

[AntarcticPilot]

Following TG's advice above I sent the info to HHA. They replied today and I am quite amazed to be honest.

"Hello David
I am contacting you in reply to the message you sent Harwich Haven Authority on the 29th of October.

Our hydrographic survey team have conducted a full multibeam seafloor survey of the area in question (please see attached). The least depth in the area surveyed was 3.9m chart datum, further more there were no marks in the seabed to indicate any recent grounding."

View attachment 102681

So it must have either been a Pot, or our imagination !

Just a thought - I wonder if the configuration of the sea-bed concentrates the tidal flow so that you get a local eddy or similar? Being caught in a tidal eddy is very similar to what you describe - I have encountered them near Rathlin Island and Corryvreckan. I notice that you're on the edge of a deeper trough that might have a local effect on the current.

 

[Leighb]

Just a thought - I wonder if the configuration of the sea-bed concentrates the tidal flow so that you get a local eddy or similar? Being caught in a tidal eddy is very similar to what you describe - I have encountered them near Rathlin Island and Corryvreckan. I notice that you're on the edge of a deeper trough that might have a local effect on the current.

I see that it is close to Outer Ridge PHB. Anyone who has raced out there will (should) know that the tidal streams change direction quite sharply in that area, especially , in my experience, on the ebb. The outflow from Harwich going SE ish meets the NNE going flow from the Wallet and if you don’t allow for that you will overstand the mark by miles! I suppose this could give rise to marked eddys?

 

[dolabriform]

I see that it is close to Outer Ridge PHB. Anyone who has raced out there will (should) know that the tidal streams change direction quite sharply in that area, especially , in my experience, on the ebb. The outflow from Harwich going SE ish meets the NNE going flow from the Wallet and if you don’t allow for that you will overstand the mark by miles! I suppose this could give rise to marked eddys?

Thanks Leigh and AP, this happened about an hour before LW so it's another possibility!

I'm aware of the tidal movement there, but hadn't considered this.

 

[Bob Stays]

If the HHA hydrographic survey team can conducted a full multibeam seafloor survey in response to a query like this, then is it too much to ask if they could do the same over the Deben and Ald entrances once a year? I know its only for the benefit of us yachties but aren't we all seafarers? just asking

 

[tillergirl]

I think the short answer could be that their survey vessels are generally a little big to do the Ore and Deben. I am not sure but I think they 'buy in' survey services (they were using Titan Discovery in the last work). At least Dola's oddity was in the area that they keep an eye on. I might be wrong though.