Observation and a question about how tides affect a boat at anchor.

[RichardS]

A current thread about the Capo Nord / Marano Lagoon in Italy where I've posted a couple of photos has reminded me of another strange thing that happened in the lagoon that I can't explain. Bear in mind that I'm a Med sailor and not used to tides.

I took a photo of my snubber line whist this was happening which is what reminded me but it hasn't come out well because I was trying to lean under the trampoline.

OK ... so I wake up in the morning around 8:00 and go on deck to check the state of the tide. We're anchored inside the lagoon just North of the Punta Faro entrance and I know that the tide will be flooding in for another 3 hours so I'm aiming to leave around 11:00 for the sail to Venice. I go to the bows to check that eveything is looking good. I look down over the front crossbeam expecting to see the bridle snubber stretching out in front of the boat as it was yesterday evening when the tide was coming in.

But there's no snubber line visible. I kneel down and lean over the crossbeam and look under the boat and there's the snubber bridle attached to the bows as usual but streaming backwards in a V under the trampoline. I try to pull one end of it forward and moves it a little .... but as soon as I let it go it snaps right back under the boat.

How is this possible? There is just a gentle breeze, perhaps 5 knots, we're in around 4 metres of water with 25 metres of chain out and draw 1m. The boat should surely be being pushed back by the current which is streaming past the bows as if we're sailing forwards at 2 or 3 knots but instead she's straining forwards on the snubber into the stream.

We've got no sails up of course, so surely the windage from a 5 knot breeze is not overpowering a 3 knot current? The strange thing is that as the stream begins to slow down nearer HW, the boat slowly moves backwards until the snubber bridle is hanging vertically, even though we still have the breeze.

Am I going completely doolally?

Here's a photo taken at the same time as I was standing on the bow of the Punta Faro marina and Lignano town just south of us. You can just see the lagoon entrance on the extreme left of the photo:

Richard

 

[ex-Gladys]

Happens all the time on my swinging mooring when there is enough wind for the boat to make way over the tide... but the underwater config keeps the hull(s) aligned with the tide...

 

[Leighb]

It would all depend on the relative underwater area and windage, if you look at lines of moored yachts you will often see them lying in opposite directions depending on their keel area and upper works, I guess that a catamaran has a lot of windage compared to the relatively small underwater area so would lie to even a relatively light wind?

 

[guernseyman]

I've anchored many times in Belvoir, Herm. Once, after spending some time below, I came up to settle in the cockpit. After a while I realised the boat was facing the wrong way and, like you, found the cable as taut as a bowstring along the hull.
No particular problem; but I do wonder if, at the next change of tide, I could have lost the rudder if the boat had turned again in the same sense.
As it happened, I turned the boat round with the engine and then it lay properly.

 

[markhomer]

Just to confound the non tidey folks ,

It also statisfys ie flow at different directions at different depths just like the wind but in smaller bands , id never thought about it untill one day poured a pint or two milk overboard which clearly showed the differing flows as it sank

 

[jamie N]

That's the 'halocline' illustrating the different densities of the water at the various depths. On the Greatbelt in Denmark, the current was frequently up to 3kts in a Northerly direction, down to -12 metres or so; thereafter, within 50cms. below that, it would be 3kts in a Southerly direction, purely due to the extremes of the fresh water from the Baltic at one level, with the salt water from the North Sea at the other layer. (I can't recall which the upper one though??)

 

[jwilson]

I'm not sure of the exact physics but have seen this many times with very long anchor rodes in deepish water and strong tides even with no wind - anchor holding but warp from bow runs aft. I assumed the drag of the tide on a cross-section of a really long warp was greater than the resistance of the bows-on hull and caused a big loop aft midway down the warp.

In the Bristol Channel have seen a 3-4-knot bow-wave - and yet the anchor warp goes back from the bow. And the anchor is definitely holding.

 

[RichardS]

Thank you for some interesting replies.

Surely, the idea that the gentle breeze could overcome a 2 or 3 knot current without any sails up just doesn't add up. I was waiting for the tide to stop flooding for this very reason as I didn't want to be battling against it. I seriously doubt whether even with the sails up I could have made much headway against that current in that breeze and I have a 20m mast and big sails. The idea that I could have pulled up the anchor and, without starting the engine or raising the sails, started to move forward out of the lagoon just doesn't make sense. The breeze wasn't even from directly aft but was on the port quarter.

There must be many of you who have manouvered your boats in a tidal stream of 2 or 3 knots with the sails down and have a fair idea about some sort of correlation for a "typical" boat about the wind speed needed to balance the current. I assume that the two are not equal ....... which would mean that a boat in a 2 knot current with a 2 knot wind against the current will always go with the current. The question then is how much higher does the wind have to be above 2 knots in order to balance the current.

My guess would have been much higher ..... like 10+ knots of wind on a bare hull being equivalent to 2 or 3 knots of current .... but maybe I'm way out on this? If I am then I have the answer ..... 5 knots of wind from the port quarter is more powerful than 2 knots of current and hence my boat was being driven forwards into the current .... although if this is the explanation I'm still surprised why she was in such perfect balance rather than flipping around and facing the wind as she has done in every other anchorage I've ever been in.

Richard

 

[PaulRainbow]

I doubt it was the wind, i think it would be due to one or other of the tidal anomalies mentioned. The tide does unexpected things in estuaries, rivers and harbors, especially where sea and fresh water can mix. I have often seen water flowing in a seaward direction in a river, yet the water level is rising.

 

[ex-Gladys]

I doubt it was the wind, i think it would be due to one or other of the tidal anomalies mentioned. The tide does unexpected things in estuaries, rivers and harbors, especially where sea and fresh water can mix. I have often seen water flowing in a seaward direction in a river, yet the water level is rising.

Don't think too hard, it works like that all the time... think of the Blackwater Estuary. High tide at W. Mersea is 20minutes earlier than Osea Island, so 10 minutes before HW Osea, the tide is dropping at Mersea, but at Osea, the tide is still rising...

 

[LittleSister]

There is just a gentle breeze, perhaps 5 knots, we're in around 4 metres of water with 25 metres of chain out and draw 1m. The boat should surely be being pushed back by the current which is streaming past the bows as if we're sailing forwards at 2 or 3 knots but instead she's straining forwards on the snubber into the stream.

What makes you think the stream was 2 or 3 knots, other than the bow wave? Is that likely in the locality (I don't know it)? Did you look at your log?

 

[RichardS]

What makes you think the stream was 2 or 3 knots, other than the bow wave? Is that likely in the locality (I don't know it)? Did you look at your log?

I don't have a paddlewheel unfortunately but I know from the pilot that the usual flow around mid-tide goes up to 3 knots so it would be in that range. It also looked like that looking at the bows, slightly more if anything.

Richard

 

[nemodreams]

Fisrt time I ever anchored overnight was off of Blue anchor - Bristol Channel - and I was a little worried by a bar tight rode going straight back to the stern ! They didn't tell me about that in them books I read !
The concern was that it was going to get between the keels. I frequently have the Raymarine log there showing 2.5 kts.

 

[jwilson]

Fisrt time I ever anchored overnight was off of Blue anchor - Bristol Channel - and I was a little worried by a bar tight rode going straight back to the stern ! They didn't tell me about that in them books I read !
The concern was that it was going to get between the keels. I frequently have the Raymarine log there showing 2.5 kts.

Exactly what I regularly used to get anchoring on long warps waiting out tides in the Bristol Channel. Really strange the first time..... Not seen it elsewhere, but nowhere else have I had to anchor in deepish water with such strong tides.

 

[RichardS]

Fisrt time I ever anchored overnight was off of Blue anchor - Bristol Channel - and I was a little worried by a bar tight rode going straight back to the stern ! They didn't tell me about that in them books I read !
The concern was that it was going to get between the keels. I frequently have the Raymarine log there showing 2.5 kts.

Exactly what I regularly used to get anchoring on long warps waiting out tides in the Bristol Channel. Really strange the first time..... Not seen it elsewhere, but nowhere else have I had to anchor in deepish water with such strong tides.

That's very interesting ... but what mechanism in causing this? Why doesn't the boat flip around so the anchor rode/snubbing line is streaming out in front.

I'm sure that it's not caused by wind and surely it can't be due to different currents at different levels as I only draw 1m.

I wish I could have seen the chain on the bottom as that might have shed some light on the matter but the water was too cloudy. All I could see was the snubber line disappearing under the water behind the boat. I've no idea whether the chain and the anchor were even further behind the boat or whether they were ahead of the boat and it was only the much lighter snubber that was running aft. The snubber also had a fair amount of weed wrapped around it that was coming in on the tide but I can't see why that would make any difference.

How can a boat lie in a tidal stream with the bows head steady into the stream for hours and the snubbing line being taut towards the stern? I just can't get my head around it!

Richard

 

[Bosun Higgs]

Am I going completely doolally?

Richard

Cant really answer that one, though I too have wondered.

Anyway the phenonemon you describe is very common and the reason is simple. The resistance of your hulls to the water is less than that of your topsides to the wind. At a guess you are in a big french built cat with huge topsides and long slim hulls designed to be easily driven. I used to have a Prout and despite it having more sensible topsides, I used to have the same issue which was a pain in crowded moorings. I still get the problem a bit with a modern mono and the next door boat being a heavy long keeled old wooden banger.

I would add that this issue of topsides is one reason why you dont go to windward that well

 

[JohnGC]

That's very interesting ... but what mechanism in causing this? Why doesn't the boat flip around so the anchor rode/snubbing line is streaming out in front.

I'm sure that it's not caused by wind and surely it can't be due to different currents at different levels as I only draw 1m.

I wish I could have seen the chain on the bottom as that might have shed some light on the matter but the water was too cloudy. All I could see was the snubber line disappearing under the water behind the boat. I've no idea whether the chain and the anchor were even further behind the boat or whether they were ahead of the boat and it was only the much lighter snubber that was running aft. The snubber also had a fair amount of weed wrapped around it that was coming in on the tide but I can't see why that would make any difference.

How can a boat lie in a tidal stream with the bows head steady into the stream for hours and the snubbing line being taut towards the stern? I just can't get my head around it!

Richard

Great thread Richard.

I think the problem needs to be considered as a set of dynamic forces which are balanced. Assuming it's your bow(s) that get blown off in a side wind, then it may be something like this;

With no wind, the boat will face the current.
Add wind from the port quarter and the boat will turn to starboard to some extent (blows bows off).
The current is now flowing over the hydrofoil sections of the hull(s) and keel(s).
Lift is generated (forwards and to port).
The boat sails but is restrained by the anchor warp.

The boat settles at an angle such that the forces turning the boat to port (current) and starboard (wind) balance.

If the boat begins to turn further to starboard, then the angle of the current over the keel will increase. That will result in the lift force increasing and the boat will turn back to port.

Likewise; if the boat begins to turn to port the angle of the current over the keel will decrease. Resulting in the lift force decreasing and the board will turn to starboard.

 

[jwilson]

Great thread Richard.

I think the problem needs to be considered as a set of dynamic forces which are balanced. Assuming it's your bow(s) that get blown off in a side wind, then it may be something like this;

With no wind, the boat will face the current.
Add wind from the port quarter and the boat will turn to starboard to some extent (blows bows off).
The current is now flowing over the hydrofoil sections of the hull(s) and keel(s).
Lift is generated (forwards and to port).
The boat sails but is restrained by the anchor warp.

The boat settles at an angle such that the forces turning the boat to port (current) and starboard (wind) balance.

If the boat begins to turn further to starboard, then the angle of the current over the keel will increase. That will result in the lift force increasing and the boat will turn back to port.

Likewise; if the boat begins to turn to port the angle of the current over the keel will decrease. Resulting in the lift force decreasing and the board will turn to starboard.

I am fairly sure it is nothing to do with wind: have had this often whilst anchored in total glassy calm waiting out a tide. As I said earlier, I've always believed it to be because the drag of the tide/current on the anchor warp/chain to be greater than the drag of a hull bow-on to the same current. If you anchored using a wire with minimal water resistance it wouldn't happen.

 

[JohnGC]

I am fairly sure it is nothing to do with wind: have had this often whilst anchored in total glassy calm waiting out a tide. As I said earlier, I've always believed it to be because the drag of the tide/current on the anchor warp/chain to be greater than the drag of a hull bow-on to the same current. If you anchored using a wire with minimal water resistance it wouldn't happen.

Interesting. This can only be the case if there are two forces in opposite (or nearly) directions. So with no wind there would have to be a surface current opposing a lower level current. In which case your explanation is simpler than mine. But there may be more than a single cause of this behaviour.

 

[RichardS]

Great thread Richard.

I think the problem needs to be considered as a set of dynamic forces which are balanced. Assuming it's your bow(s) that get blown off in a side wind, then it may be something like this;

With no wind, the boat will face the current.
Add wind from the port quarter and the boat will turn to starboard to some extent (blows bows off).
The current is now flowing over the hydrofoil sections of the hull(s) and keel(s).
Lift is generated (forwards and to port).
The boat sails but is restrained by the anchor warp.

The boat settles at an angle such that the forces turning the boat to port (current) and starboard (wind) balance.

If the boat begins to turn further to starboard, then the angle of the current over the keel will increase. That will result in the lift force increasing and the boat will turn back to port.

Likewise; if the boat begins to turn to port the angle of the current over the keel will decrease. Resulting in the lift force decreasing and the board will turn to starboard.

I like that explanation as it suits my "feeling" that something quite complex is happening ..... but, if the hull and keels generate lift (and my keels indeed are a pronounced tear drop section like aeroplane wings with the convex profile on both sides) then wouldn't the boat move forward into the current if there was no wind at all?

Actually, the more I think about it the more I realise that that would be impossible as surely you can't generate more forward drive than the current going the other way otherwise it sounds a bit too much like perpetual motion.

So you basically think that the wind + the hydrodynamic drive from the keels balances the current? It's a very attractive proposition but I'm really surprised that I've never read about this before. Surely there must be something about it on the web but , if there is, I can't find it.

Richard

Richar