ip485
Well-Known Member
So you have set the anchor, but know the tide will be rushing the other way in a few hours - do you do anything different.
Rotating on anchor in tide
- Thread starter ip485
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GrahamM376
Well-Known Member
Just set the alarm as usual
vyv_cox
Well-Known Member
I have photographic evidence that when that happens my Rocna barely moves, simply turns around. In a particularly good example it can be seen that the chain dragged right over the anchor, pulling down the small float and line attached to the crown of the anchor and trapping it under the chain in the new direction. The marks in the sand show that the anchor remained in exactly the same place. This was a change in wind direction, not tide.
GHA
Well-Known Member
jordanbasset
Well-Known Member
Before we became liveaboards was always a bit anxious about what would happen to the anchor when the tide turned. But after the experience of being at anchor over several months in many different anchorages soon learned to trust the anchor to sort itself out. It always did (a Kobra 2) . When we go back to the liveaboard life we will probably start in the med this time, but if we did buy in a tidal area I would not worry provided I was confident with the anchor.
jimbaerselman
Well-Known Member
Yes. This particularly applies in the estuaries of North Brittany (Lezardrieux, Treguier, Morlaix, L'Aberwrac'h) where. if going west, you'll be arriving on the ebb, and you may wish to hang around for a day time flood before exploring the shallower reaches. It also applies if you have wind over tide during one stream, and the scope needed may allow you to wander towards one or other shore.
The solution is a running moor.
1. Drop bower, let vessel drop down tide until you have twice the scope needed for high water, test holding with astern.
2. Drop second anchor/kedge/hook, preferably with rope rode above 10m (easier to untwist if you've moored for 3 or more tides)
3. Haul in bower rode, paying out second hook rode until you lie half way between both anchors.
4. Dig in and test second hook holding by heaving it in against the bower.
5. Lash rodes together, then lower the lashing so that it lies deeply enough to clear keel/rudder when boat swings with tide.
6. Take strain with chain rode.
You'll now be secure to leave the boat for a week or two if needed, with no risk that a change in pull direction will occur at either anchor. This change in direction is particularly risky in narrow tidal estuaries, where bottom slope may give good holding in one direction, but not the other.
This style of anchoring is also called the Bahamian moor, where it's needed in places to deal with reversing current streams, sometimes with a fair bit of wind over stream.
Sybarite
Well-Known Member
N
Or, you could simply use a Spade anchor which has amply demonstrated that it continues to hold with changes of direction.
Tests have shown that the CQR and the Delta can have trouble in the same circumstances (based on tests in France).
Or, you could simply use a Spade anchor which has amply demonstrated that it continues to hold with changes of direction.
Tests have shown that the CQR and the Delta can have trouble in the same circumstances (based on tests in France).
jimbaerselman
Well-Known Member
I think you're assuming here that there's no significant bottom slope, up or down stream. Fair enough, but these estuaries vary in depth longitudinally by a good 10m over some stretches, so it takes some hunting to find those flat patches. They're usually marked by an anchor on the chart, and often annoyingly occupied with a mooring or two. Outside those flat areas, you may find you're pulling on a 4 degree downslope instead of an upslope on tide change. Suddenly you've lost a lot of holding power, whatever anchor.
Spring tidal range in these estuaries is up to 9m, and anchoring in anything from 3m to 7m LWS. Add a metre for bow height, and high water scope (assuming 5:1) will be from 70 to 90m. Width of channels, between 120m to 240m. Wind over tide you're going to bump the channel mud or rock one side or another.
With a running moor, you can set the anchors with mid-tide or lower scope, and just swing to a 10m riser. The joint of the lines to the anchors can't rise much due to the tension between them, and swings only to a small circle, some 10m or so if tension was good.
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ip485
Well-Known Member
That is an interesting technique.
I am "over anchored" as it happens (long story) but the 55Kg Rocna is a few sizes larger than required for 23 tons of vessel but it does so far seem to hold in anything, even as it happens without much chain! It is a substantial beast. It has yet to have a really good work out in a tidal estuary so I am interested in your experiences.
I am "over anchored" as it happens (long story) but the 55Kg Rocna is a few sizes larger than required for 23 tons of vessel but it does so far seem to hold in anything, even as it happens without much chain! It is a substantial beast. It has yet to have a really good work out in a tidal estuary so I am interested in your experiences.
rivonia
Well-Known Member
Coming down the coast of Portugal we often achored in the RIA's Our huge 25 lbs ROCNA held at all tide turns and we slept peacfully.
noelex
Well-Known Member
As others have said, a good anchor when presented with a new direction of force should rotate on the spot leaving its flukes buried.
The better anchors will do this very consistently if they are set and the substrate is reasonable.
Don't assume your anchor will always rotate because the boat has moved and stretched the chain the opposite way. It takes a reasonable wind force to rotate an anchor. Somewhere around 20 knots is typical.
I don't often get to dive on anchors affected by current. It is very difficult to swim against even a mild current, but people consistently overestimate the force on an anchor from current. The force will depend on the waterline length. For a boat with a waterline length of 10m (33 feet) is has been estimated that a 6 knot current is equivalent to same force as 20 knots of breeze. I think this comparison is overestimating the wind forces, but whatever way you look at it, it is likely to take a hefty current before the anchor starts rotating.
See here for a table showing the force produced by current:
http://alain.fraysse.free.fr/sail/rode/forces/forces.htm
The better anchors will do this very consistently if they are set and the substrate is reasonable.
Don't assume your anchor will always rotate because the boat has moved and stretched the chain the opposite way. It takes a reasonable wind force to rotate an anchor. Somewhere around 20 knots is typical.
I don't often get to dive on anchors affected by current. It is very difficult to swim against even a mild current, but people consistently overestimate the force on an anchor from current. The force will depend on the waterline length. For a boat with a waterline length of 10m (33 feet) is has been estimated that a 6 knot current is equivalent to same force as 20 knots of breeze. I think this comparison is overestimating the wind forces, but whatever way you look at it, it is likely to take a hefty current before the anchor starts rotating.
See here for a table showing the force produced by current:
http://alain.fraysse.free.fr/sail/rode/forces/forces.htm
vyv_cox
Well-Known Member
Having anchored for years in the Menai Strait, where tides reverse direction quite strongly, four or five knots in each direction being common, I have never used a Bahamian moor and do not know anyone who has. I do recall several boats dragging in spectacular fashion, one in particular going over a mile from Abermenai over Caernarfon Bar, but these were all without exception on CQR anchors.
GrahamM376
Well-Known Member
+1
Although we no longer sail that area, we were glad to change to a Manson which (so far) hasn't dragged, unlike the old cqr lookalike which could never be relied on.