Unexpected peculiarity of Fortress anchor

[sarabande]

A recent event on the Tamar may provide the OP with some help.


About 6 weeks ago, just after the September full moon (springs) I was on the Tamar talking with a moorings contractor of very high reputation. He was busy solving a problem he had not experienced in more than 30 years work.

A partly sunken tree, probably about 3 tons mass, had tangled itself round the riser from large concrete sinker on a fore and aft mooring. The pressure of the tidal ebb (about 4 kts) had moved the sinker, then broken the riser, leaving a 20 ton sailing boat moored by the stern.


Now knowing that the OP is on a tropical river, I could make a case for a similar event with a sunken object lifting the chain and the anchor, and then rolling away, dumping the anchor in the main stream, where it then reset itself.

Are there any signs of damage to the anchor ? That might give a clue to what has happened. It would need a large force to lift the anchor vertically, but it happened to a mooring here in Plymouth recently.

With two decent anchors on the OP's boat, it is hard to see how an anchor can move, after being dug in, except by an external force. And no amount of experience or forethought can counter that.

 

[RupertW]

A Bahamian Moor obviously does not allow the anchor to experience a 360° change in direction of pull. Where are you getting "building up speed" and "snatch loads". Sorry, but I think maybe you need to do some research about that type of anchoring. It is totally different from anchoring stern to a wall.

It was your suggestion of putting slack into the system that didn't make sense to me and I do think there is a close parallel with a fore and aft moor - with slack the boat will then be able to build up speed and snatch as wind gusts and lulls and I don't think that helps an anchor.

The other part is that a setup with slack allow the boat to move a distance downwind in a cross wind so you are unnecessarily causing the anchor to pivot a few degrees. I admit the 360 degrees has no place in this discussion but I think a pivot of 10 degrees due to little slack is much better than 30 degrees with a cross wind and more slack. Obviously in a tidal region then slack is a necessary evil apart from the top of the tide.

 

[NormanS]

It was your suggestion of putting slack into the system that didn't make sense to me and I do think there is a close parallel with a fore and aft moor - with slack the boat will then be able to build up speed and snatch as wind gusts and lulls and I don't think that helps an anchor.

The other part is that a setup with slack allow the boat to move a distance downwind in a cross wind so you are unnecessarily causing the anchor to pivot a few degrees. I admit the 360 degrees has no place in this discussion but I think a pivot of 10 degrees due to little slack is much better than 30 degrees with a cross wind and more slack. Obviously in a tidal region then slack is a necessary evil apart from the top of the tide.

Simple application of mechanics will show that if both rodes are tight in a straight line, when the wind blows from the side, the forces are vastly increased. Think of slinging loads with a crane. Like most things to do with boats, it's a compromise. A Bahamian Moor reduces the possible variation of angle of pull, and reduces the swinging circle, but also can actually increase the load on the anchors.

 

[jdc]

A recent event on the Tamar may provide the OP with some help....
Are there any signs of damage to the anchor ? That might give a clue to what has happened. It would need a large force to lift the anchor vertically, but it happened to a mooring here in Plymouth recently....

No signs of damage to anything except my face a bit red! The peculiar thing was that next afternoon I stayed aboard and it did it again, the Fortress ending up well down tide of where the boat's bows had ever been. The water is completely opaque and cafe au lait coloured and cotains any amount of baulks of timber so in all probablity it was the combination of one of these and the lightness of the anchor: as was sugested above I too bet that had I had the two anchors reversed neither would have moved.

I posted the tale really as it was such an unexpected thing! Shows that even after bedding in, and using enough chain, and the boat being fine for a few days, and, I hope, not being that incompetant, sometimes events conspire to make a fool of one! next time I'm going to try something different:

1. Use two Deltas (although tbh, it's a pretty rotten anchor in soft mud), the 25kg and the 20kg I have as spare

2. Join under water with all-chain of about 30m (I have such a length handy), and

3. Shackle to the mid point of this 30m chain a nylon-multiplait rode. This last one solves, hopefully, the issue of scratching the hull.

So far I can see how to lay it, but if both anchors bed in well am a bit worried about recovery. Pick-up buoys are a pest as one always over-rides them. Maybe a lightish line from one of the crowns to the shackle at mid point.

 

[Neeves]

A couple of issues

Most anchors behave very poorly in mud. Their fluke angles are set for 30 degrees or thereabout which is ideal for firm sand but simply ineffective in mud. A Fortress can be altered to allow its fluke angles to be set to 45 degrees and then it is unbeatable - nothing compares.

But any anchor in mud will take very long distances, 20m would not be unusual - depends on the mud, to achieve maximum, or anywhere near maximum, hold and unless you run the engines for a prolonged periods you will not achieve this hold - and the hold development will occur under wind or tide power. But if you have not developed full power then the rodes, will slacken, in a bahamian moor, as the anchors further set and move toward each other.

As has been mentioned with all that chain and only 4 knots of tide then a Fortress will not fly like a kite - forget the idea unless you know of kites that work trailing chain!

Mud palms have been available for decades, either you have a very old anchor or you lost them.

There is a tendency for people to think their anchor is all things to all men, or all seabeds. This is a complete fallacy. Some anchors are good in weed, some are good in mud. Some anchors need to be set with a shortish scope, others do not. Most anchors work well in sand. Consequently each anchor and each seabed needs to be treated differently.

In sand you can throw the, any, anchor out and it will usually set. No-one would do this in weed - one would look for the sand patches or areas of sparse weed. You would not try to use a Fortress in stones and cobble (and I would not use a roll bar anchor either). Only a Fortress set at 45 degrees offers real reliability in mud etc etc. Its horses for courses and setting techniques to suit both anchor and seabed. The idea that one bower anchor can suit every seabed is simply invalid - it can work much of the time, but not all. and if you are adventurous, sailing NW Scotland or the Amazon you need different anchors and different techniques.

If the fear is retrieving the Fortress in mud - and it could be a real issue then the only answer is some form of tripping line (rigged to suit what you prefer). But given the limited tensions potential you describe, lightish winds low tide flow, I think maybe you over reacted.

I actually hear of more Fortress that need to be abandoned because they have been so deeply set in a Storm - so they did what they were meant to do - than Fortress that trip. Many of the stories I hear of tripping Fortress are simply the same story repeated, often by someone who 'heard' the story second or third hand - and I suspect the anchor was not deep set in the first place (maybe because it was too big or the owner simply did not bother, or know).

In anticipation of the anchor being difficult to retrieve then a tripping line is sensible (and in a Bahamian moor not an issue as long as the lines are well marked - unless traffic is fierce).

I keep thinking with over 500,000 Fortress sold the stories of tripping anchors are simply not reflected in either sales success nor the numbers of Fortress I see on bow rollers of American yachts. We often use our Fortress in tidal rivers, by itself, but the rivers tend to slow flowing with minimal tidal effects - we have never had an issue (but then every situation is unique).

We would not leave home without our Fortress, nor the Spade and Excel, and whenever we anchor we will always have our second anchor (whichever is appropriate) and second rode (neatly coiled in a milk crate) on the bow ready to deploy. Its so much easier if its there and ready at 2am in the rain!

Jonathan

 

[jdc]

Look, I really like the Fortress, and I do have the mud palms as it's neither a very old anchor nor are they 'thrown away'. I wouldn't travel without it and it's been my kedge for 12 years.

But I thought it interesting that the anchor moved something like 70m while the boat moved a fraction of this distance, so it can't be simple dragging, and wondered whether anyone else had experienced anything similar.

I'm not at all sure I buy the 'it can't be flying like a kite with all that chain' statement as I've noticed that the one previous failing being that the anchor never seems to get to the bottom if the boat's moving at 2 or 3 knots.

 

[Neeves]

There must be quite exceptional circumstances to try to anchor when travelling at 3 knots. We would be trying to be moving slowly backwards at a speed not measurable by our GPS.

Why not attach the mud palms - they are there to help the anchor set - you will find it helps a lot in firmer sand.

Jonathan

 

[GHA]

There must be quite exceptional circumstances to try to anchor when travelling at 3 knots.

Not exceptional at all but common in many UK rivers or estuaries (actually many anchorages) , the tide could be easily running at 2 or 3 knots (as jdc said, not 3) so to get a close to zero sog speed through the water could easily be 2 or 3kts.

 

[Neeves]

Always willing to learn. I stand corrected

jdc, no suggestion you were in any way being critical - your experience is intriguing.

Fortunately it appears not a common occurrence.

What is overlooked in the thread is where you are - not many get to sail a yacht up, or on, the Amazon. Beats having an anchor problem in Sydney Harbour!

Edit I had to check as you appear to be on the Amazon - not a common location for someone from Falmouth!.. Not many of us sail on, or in, the Amazon. Beats anchor stories on Sydney harbour! - certainly much more exotic! And must have many unique issues. close edit

Jonathan

 

[NormanS]

There must be quite exceptional circumstances to try to anchor when travelling at 3 knots. We would be trying to be moving slowly backwards at a speed not measurable by our GPS.

Why not attach the mud palms - they are there to help the anchor set - you will find it helps a lot in firmer sand.

Jonathan

We occasionally anchor when going at 2-3 knots. Picture running into an anchorage under Genoa, having furled the main into the mast on the way in. It takes judgement to let the anchor go at just the right time, then let the chain rattle out, before just at the last moment, putting the helm hard over, so that when the boat is brought up short, the chain doesn't foul the hull. You know, when the boat stops dead, that the anchor has a grip.

 

[Neeves]

I'm not sure I'm so keen on the idea - I prefer to come into an anchorage and set as we move 'backwards', with us that means head to wind and then being blown off broadside to the wind (one of the pleasures of owning a cat!). The anchor would be let go as we move down wind and the anchor is then set in the direction we would want. But if we were moving forward at 3 knots the anchor would be set back to front . It would be most unusual for us to need worry about the chain fouling a hull.

The idea of in mast furling - at the time it was one of those new fangled ideas, like dyneema and GPS (in the 80s) and new gen anchors - which would never catch on!

Jonathan