Against the tide

[PabloPicasso]

If one were coastal sailing against the tide on a long passage say, would you be more likely to fight faster tide inshore in shallower water than further out in deeper seas.

Assuming no other geological features like headlands etc, are squeezing the flow.

 

[rogerthebodger]

Where I sail we have a north to south current so sailing south you keep offshore but when sailing south to north you keep inshore where there is a small counter current

This is where a speed over ground and a speed through indicator helps alot to find the best over ground speed

 

[KevinV]

If there was no geology you wouldn't be fighting a tidal current, it's a non-question.

Any normal coast it will vary - you may well get a back eddy nearer the coast, but in general terms deeper water will move the same amount of water slower. On the other hand it may well be deeper because a strong current has cut a gully. Isn't this was tidal stream charts are for?

 

[Chiara’s slave]

If one were coastal sailing against the tide on a long passage say, would you be more likely to fight faster tide inshore in shallower water than further out in deeper seas.

Assuming no other geological features like headlands etc, are squeezing the flow.

Shallow water will slow the flow. Back eddies are usually caused by geographic features, but if you’re fighting the tide, go shallow. We race in a very tidal area, this is a golden rule. You can see the truth of it on any buoy, lobster pot or anchored boat you pass. We sail otherwise ludicrous routes because of it, often apparently over standing a mark by hundreds of metres, knowing that as you approach, and the water gets deeper, the mark will suddenly not be overstood at all.

 

[dunedin]

Shallow water will slow the flow. Back eddies are usually caused by geographic features, but if you’re fighting the tide, go shallow. We race in a very tidal area, this is a golden rule. You can see the truth of it on any buoy, lobster pot or anchored boat you pass. We sail otherwise ludicrous routes because of it, often apparently over standing a mark by hundreds of metres, knowing that as you approach, and the water gets deeper, the mark will suddenly not be overstood at all.

As Chiara says, generally head closer to the shore to shallower water if fighting an adverse tide. Racing Enterprises in a very fast flowing river we used to roll tack every 3-4 boats lengths going up the shore against an extreme tide. Could occasionally beat a 505 or even Tornado round the windward mark as they couldn’t tack so quickly and had to go out into more tide.
Then look for opportunities to find a tidal eddy - generally behind headlands built also, in Scotland with a a lot of them around, using small islands for this.
Gets risky in a deep keel boat where the edges have solid rocks, rather than soft mud.

 

[Dellquay13]

Shallows and bays tend to have slower rates or back eddies, but without tidal charts to hand you are just guessing. I have laminated prints of tidal charts in the cockpit even when just pottering in the local area. If I’m making passage I will have checked the tides for each area and plotted my course and timings to make best use of any fair flows and eddies. I’d rather sail a bit roundabout or further and make best headway than take direct routes and make little or no progress in foul tides, but it’s in the planning.

 

[johnalison]

Coast sailing along the South Coast is mostly a matter of passing headlands and bays, while on the East we tend to have mostly rounded headlands past river estuaries. It is true that, say, from Dungeness to Beachy Head the current will be less inshore, though in my experience you have to go quite a long way in to gain much benefit (and risk pot markers). Around the East Coast this is less easy to achieve without risk. Coastal sailing is very much a matter of planning to ensure that you meet the head tides at the least unfavourable places.

 

[Chiara’s slave]

Shallows and bays tend to have slower rates or back eddies, but without tidal charts to hand you are just guessing. I have laminated prints of tidal charts in the cockpit even when just pottering in the local area. If I’m making passage I will have checked the tides for each area and plotted my course and timings to make best use of any fair flows and eddies. I’d rather sail a bit roundabout or further and make best headway than take direct routes and make little or no progress in foul tides, but it’s in the planning.

No guesswork involved. The current is slower in shallow water.

 

[Dellquay13]

No guesswork involved. The current is slower in shallow water.

That’s also where back eddies fair or foul can form, sometimes counterintuitively, but predictable with tidal charts so not worth trying to guess. They are not always present all through the ebb or flow, sometimes they are only present in the first or second half of the tide.
As the exception to the norm, there are a handful of banks and shallows near me where the tides form races considerably faster than the deep water surrounding, like Hats and Barrels, Turbot bank and the Wild Goose race.
There are places around Pembrokeshire where slack water is 2 hours after HW or LW, such as Jack Sound and Ramsay Sound, and that isn’t worth trying to guess.

 

[Chiara’s slave]

That’s also where back eddies fair or foul can form, sometimes counterintuitively, but predictable with tidal charts so not worth trying to guess.
As the exception to the norm, there are a handful of banks and shallows near me where the tides form races considerably faster than the deep water surrounding, like Hats and Barrels, Turbot bank and the Wild Goose race

I’d say that back eddies are more predictable than guesswork, and better than the tidal charts, often they’re too small to be charted. Observation of the water surface, and any fixed features in it is key. We see ruffles, wavelets and lines dividing smooth from rippled water on our patch. Plus the steepness of the waves if the wind is against the tide. When you have other identical boats to compare your progress, it focusses the mind on the reasons they might be doing better or worse than you. I spend about 5 hours a week staring at the water looking for advantage in the wind and tide, in a non regatta week.

 

[Dellquay13]

In my area, the Port Authority have produced detailed tide atlases for both Spring and Neaps, at 30minute intervals. It can be a bit too much detail to search through, and a very old hand drawn atlas circulating among the local sailors has enough information and quick to read most of the time.
Reading the water is very useful locally to your position, I can see that being key if you are strictly following a course to the next turn in a race, but it doesn’t tell you what conditions are like a mile closer inshore or off, or where the better course would be a few miles and an hour ahead. That’s where atlases come into their own.

 

[Roberto]

There is also timing: with alternate currents and uniformly spaced parallel isobaths the flow inverts in different moments if it is shallow or deep. At full flow, the deeper the stronger. As current slows and reaches the inversion moment, the inversion will begin in shallow water before deeper water; then there will be stand in deeper water with inverted current near shore, then eventually stronger current in the deeper water at full flow. Timing your position in/offshore depending on tidal cycle moment can help a lot. Any non-uniform spaced isobaths and the current will be locally stronger where the tightest gradient is.

 

[Chiara’s slave]

If anyone needs a demo of Roberto’s post, sail past Yarmouth about 2 1/2 hours before high water Portsmouth. You’ll be on the magic carpet heading east at 3.5kn more than your usual, yet all the boats moored inshore will be facing east, into the ebbing tide.

 

[fisherman]

Round here, SW cornwall, all headlands I have frequented, N and S coast, have early ebb on the E side and early flood on the W side. So, Bass point at the Lizard, the ebb starts before HW Falmouth. Mullion to The Rill, W of Lizard, almost permanent flood. Between Kennack and Black head, NE from lizard, the flood starts well before LW Falmouth. Porthigga Point East of Godrevy, ebb starts before HW St Ives. Even between Black rock and Pendennis has an awful lot of ebb.
I often wonder of this is a general widespread rule.