Setting an anchor when tides do a 180 degree change

[noelex 77]

Originally Posted by kerrydeare

If you find yourself compelled to anchor in a significant tidal stream with no access to a permanent safe mooring, you are probably best served by two anchors (mentioned earlier under "Bahaminan" or such). We have had very good success with a pair of the older style Danforth (or "patent") anchors of the High Tensile variety. I am not even sure these are still available for purchase as new items, but that's another issue................

Best bet: avoid this situation in the first place, and if you cannot, pick a strong permanent mooring.

I think people are overestimating the effects of current. Current does not produce high anchoring forces. There is a theoretical discussion here on anchoring forces:

Forces

This graph (for a 10m boat) of force (daN is close to kgf although then engineers will kill me for saying that ) versus current in knots from the website to makes things clearer.




You can see it takes a lot of current to produce the same force as a moderate wind. The theoretical calculations presented show even a 6 knot current only has the same effect as a 20 knot wind.

I am not sure these calculations are quite correct in the real world, but the message is clear. We should not be concerned about high forces due to even reasonably strong currents. The only caution I would express is that the current does effect the substrate and some areas that are subject to high current can have poor holding.

I think the Bahamian system of two anchors was adopted because some older anchor designs were very unreliable in rotation, particularly the Danforth style of anchors. With modern anchors it is unnecessary.

Unfortunately, if another boat is using this system you need to match their setup, but the use of two anchors seems to be diminishing, as the combination of reliable modern anchors and electric anchor winches means more cruisers are using a larger and more modern single anchor to replace two smaller older design models.

There is still a place for the Bahamian "moor" when the swinging area needs to be reduced to avoid obstructions. It is worth remembering in this situation boat safety is dependent on both anchors holding so the reliability diminishes often significantly if the second anchor is an older and/or smaller model.

[kerrydeare]

Originally Posted by noelex 77

... it takes a lot of current to produce the same force as a moderate wind. The theoretical calculations presented show even a 6 knot current only has the same effect as a 20 knot wind ... I think the Bahamian system of two anchors was adopted because some older anchor designs were very unreliable in rotation, particularly the Danforth style of anchors ... Unfortunately, if another boat is using this system you need to match their setup ... There is still a place for the Bahamian "moor" when the swinging area needs to be reduced to avoid obstructions ... boat safety is dependent on both anchors holding so the reliability diminishes often significantly if the second anchor is an older and/or smaller model.

All points well taken. I did not mean to suggest that current trumps wind, but rather that current may dislodge certain gear in certain circumstances.

I mentioned using patent anchors in pairs, and perhaps should have emphasized "identical pairs." Also I have been the "victim" more than once of another fellow arriving late and setting two anchors in a tidal stream in such a fashion that I, lying on one, would later swing into him. I simply moved rather than push the situation.

Once again, anchoring is a proper combination of gear and experience.

[Cheechako]

Originally Posted by noelex 77

I think people are overestimating the effects of current. Current does not produce high anchoring forces. There is a theoretical discussion here on anchoring forces:

Forces

This graph (for a 10m boat) of force (daN is close to kgf although then engineers will kill me for saying that ) versus current in knots from the website to makes things clearer.




You can see it takes a lot of current to produce the same force as a moderate wind. The theoretical calculations presented show even a 6 knot current only has the same effect as a 20 knot wind.

I am not sure these calculations are quite correct in the real world, but the message is clear. We should not be concerned about high forces due to even reasonably strong currents. The only caution I would express is that the current does effect the substrate and some areas that are subject to high current can have poor holding.

I think the Bahamian system of two anchors was adopted because some older anchor designs were very unreliable in rotation, particularly the Danforth style of anchors. With modern anchors it is unnecessary.

Unfortunately, if another boat is using this system you need to match their setup, but the use of two anchors seems to be diminishing, as the combination of reliable modern anchors and electric anchor winches means more cruisers are using a larger and more modern single anchor to replace two smaller older design models.

There is still a place for the Bahamian "moor" when the swinging area needs to be reduced to avoid obstructions. It is worth remembering in this situation boat safety is dependent on both anchors holding so the reliability diminishes often significantly if the second anchor is an older and/or smaller model.

Not so much the current that is a concern, it's the change of direction.

[noelex 77]

Originally Posted by Cheechako

Not so much the current that is a concern, it's the change of direction.

Even anchors that rotate well lose some grip when they realign themselves to a new force direction, so I understand your point. But I don't think anyone would be concerned with a reasonable anchor in a reasonable holding ground in only a 20 knot wind even if the wind changed direction frequently.

If the theoretical calculations are remotely correct, the force from even a very strong current (for an anchorage) will be lower than in a 20 knot wind, so we should be similarly unconcerned.

If we look at the graph it shows for the more normal sort of currents the forces are so low that the anchor is unlikely to need to rotate. The boat will more to the opposite side of the swing circle, but the anchor will remain unchanged.

The other take home message from this sort of graph is that if you ever have to anchor in an extremely strong current (for an anchorage) that is approaching hull speed, the forces can become very high. So if you get caught in an area like this perhaps in an emergency situation you will need a very good anchor and even then anchoring may be impossible.

[kerrydeare]

Originally Posted by noelex 77

... If the theoretical calculations are remotely correct ...

Speaking of "take home" points, most of us in this discussion know that theory is one thing, and practice another. Not to hammer this home, but we all learn along the way that only results count.

In fact I have now forgotten what my original point was to be, other than just that we all pay attention, try our best, etc. (smile).

[JPA Cate]

Hey, guys, please remember, the OP is not on all chain rode. His is a 25 foot boat, with a similar length of chain, backed by rope rode, generally at 5 or 6 to 1 scope. He is mostly anchoring in mangrove creeks [which give good wind protection] in an area with 4-6 m. tides. He has concerns about where the boat will wind up when the tide changes.

What I'm getting at here, is that it is a particular kind of problem for the OP to solve, and he is really looking for an anchor that will "shuffle" round without disengaging, in creeks, estuaries, and rivers.

Ann