Why slopes are bad for anchoring

[bikedaft]

Many of our favourite anchorages have been in locations (usually deserted) where anchoring is difficult for some particular reason. The location may have a poor substrate, a severe downslope, or is deep limiting the available scope that can be deployed etc etc. Invariably, we will have the place such as this to ourselves even in very busy cruising grounds such as the Mediterranean or the Caribbean. Bliss.

Of course when we have been cruising quieter areas such as the west coast of Ireland or the outer Hebrides there is little need to avoid popular locations, but the ability to use more challenging locations that are not "recognised anchorages" opens up many more possibilities, some of which are the most beautiful.

Modern anchoring gear has improved enormously. Not only do we have much better anchors that work well in diverse substrates, but we have powerful and reliable electric anchor winches that enable large anchors to be easily handled by a small crew. These two improvements have opened up many locations that were previously untenable, especially when contemplating anchoring overnight. Strangely, few cruisers have taken advantage of these improvements and will only use "recognised anchorages".

It is a pity they miss out on some fantastic locations.

sssssh!

 

[NormanS]

"Obviously nobody would attempt to anchor" is sort of like "recognized anchorage." I'm sure you've seen some strange anchoring and I would not draw hard lines! I've seen many try to anchor along channel edges, because they had too much draft for the flats. Sometimes it works, sometimes not. Often it's a traffic hazard. Often they ground when the wind changes. Tidal swings tangle ropes around keels.

Maybe there was a smiley emoji missing.

 

[Neeves]

I find it interesting that apart from this thread and one member making the occasional comment - it is not such an issue that anyone has made mention (apart from that one exception) and it has never been a subject of a thread in the last 20 years.

Thin water has started a thread and it has invoked little response, Chesapeake might be an issue (but impinges on few), sea lochs/fjord deltas are exceptions to most, it, sloping seabeds, seems to be an exception that occur in almost unique locations. Pittwater and Cowan water is a flooded river system, ria like - but there is little runoff (over the centuries) no deltas - just vertical to 20m depths - this issue ist not a sloping seabed but how to anchor with any scope and not drift into the vertical cliff faces in a light breeze.

I just get the feel - apart from the few who are posting that it simply is not a big issue - or we would have seen more posts over the decades.

Let's see if I am proven wrong and a rash of locations are mentioned from a cross section of members.

Jonathan

 

[thinwater]

I find it interesting that apart from this thread and one member making the occasional comment - it is not such an issue that anyone has made mention (apart from that one exception) and it has never been a subject of a thread in the last 20 years.

Thin water has started a thread and it has invoked little response, Chesapeake might be an issue (but impinges on few), sea lochs/fjord deltas are exceptions to most, it, sloping seabeds, seems to be an exception that occur in almost unique locations. Pittwater and Cowan water is a flooded river system, ria like - but there is little runoff (over the centuries) no deltas - just vertical to 20m depths - this issue ist not a sloping seabed but how to anchor with any scope and not drift into the vertical cliff faces in a light breeze.

I just get the feel - apart from the few who are posting that it simply is not a big issue - or we would have seen more posts over the decades.

Let's see if I am proven wrong and a rash of locations are mentioned from a cross section of members.

Jonathan

Good point. I can think of dozens of places this is an issue in local cruising grounds, but most just avoid those areas, except for fishermen. Others are in open roadsteads that would be fine places to anchor, except there are so many creeks in the Chesapeake, most habitually go there, even if they are bug-infested and hot. They feel safe.

BTW, Norman and Nolex actually triggered the thought in the Fortress thread.

 

[Neeves]

BTW, Norman and Nolex actually triggered the thought in the Fortress thread.

I missed NormanS' post, but it is a forum , Nolex has raised it a number of times - I have seen no other references, until your thread - and even then I'm underwhelmed by the exposure of others to sloping seabed

People are way over-analysing things. The seabed is varied far beyond worrying about calculating holding power to the nearest 10%.

The comment is being made that some of the best anchorages are not covered by cruising guides - forgetting that most owners are lucky to get a decent weekend sailing. They simply don't have the motivation nor time to explore the isles of Scotland nor the fjords of Norway, they are responsible for the kids and are striving for career success. Their ultimate dream might be to sail the Seven Seas - but by then they will know enough to survive the very occasional sloping seabed (as they will have read this thread ) .

Jonathan

 

[thinwater]

I made an example of the effect of slope on sheer strength with the exaggerated case of angle of repose. Gotta wake people up. But the issues in my area relate not to the slope, but the effect the causes of slope (currents, underlying structures, etc.) have on the soils. Sloped areas are often terrible holding ground because they are too hard (scoured clean) or gravel/cobble material. That was the real point. If there is a slope or interesting tide/current condition, confirm that the bottom is still good.

Really, the soil is always the main factor. As for those that always find good mud and believe we are over thinking things, that is not the rule everywhere and best of luck to you. The more I sail, the more weird stuff I encounter. Not often, maybe, but occasionally and not always when you expect it. I still remember the first time I just could not get and anchor to hold in good sand. I was going for a swim any way, so I investigated the anchor. The sand was only 2-4 inches deep. I took a screw driver and tried to penetrate the clay under the sand. I could barely scratch it. That was perhaps my first anchoring learning experience, about 40 years ago. Not 400 yards away is one of my favorite testing areas, with deep, perfect sand. Nothing in the geography gives it away, other than trying to set an anchor.

 

[NormanS]

I find it interesting that apart from this thread and one member making the occasional comment - it is not such an issue that anyone has made mention (apart from that one exception) and it has never been a subject of a thread in the last 20 years.

Thin water has started a thread and it has invoked little response, Chesapeake might be an issue (but impinges on few), sea lochs/fjord deltas are exceptions to most, it, sloping seabeds, seems to be an exception that occur in almost unique locations. Pittwater and Cowan water is a flooded river system, ria like - but there is little runoff (over the centuries) no deltas - just vertical to 20m depths - this issue ist not a sloping seabed but how to anchor with any scope and not drift into the vertical cliff faces in a light breeze.

I just get the feel - apart from the few who are posting that it simply is not a big issue - or we would have seen more posts over the decades.

Let's see if I am proven wrong and a rash of locations are mentioned from a cross section of members.

Jonathan

It doesn't surprise me in the least that there are few comments about anchoring on sloping seabeds. Apart from the obvious benefit, when the pull on the anchor is guaranteed to be uphill, as when mooring Med style, most people avoid anchoring where the slope is noticeable and significant. Most people have some common sense.

 

[Neeves]

It doesn't surprise me in the least that there are few comments about anchoring on sloping seabeds. Apart from the obvious benefit, when the pull on the anchor is guaranteed to be uphill, as when mooring Med style, most people avoid anchoring where the slope is noticeable and significant. Most people have some common sense.

I confess I totally lack the example of common sense. I wonder how I have survived so long.

What we have always done is

Wind is off the land

Approach an anchorage from offshore. We think the shore will give us shelter. As we near the anchorage the sea shallows and as we close the beach it becomes really shallow - so we stay outside the surf line (making a guess where this might be at low tide). We deploy our anchor, reverse away from the beach, water gets deeper - the seabed nearly always slopes to 'offshore' and then power set.

Wind is off the land (which might be an island), that's why we chose this location.

Common sense apparently suggests we should have reversed this. Deploy anchor in deep water, reverse up, anchor sets more securely into the rising (shallowing) seabed. This is the common sense approach. Setting the anchor in the 'downhill sloping' seabed is suicidal

As far as I experience the seabed always slopes away from the shore, sloping seabeds are commonplace (if not universal). Many people anchor every day in sloping seabeds, they don't give the sloping seabed a second thought - they contradict 'common sense' because wind direction is factorially more important.

People Med Moor because its sensible - its actually not common sense - but standard practice. Common sense would be anchor alongside, every one else raft up to you. Med mooring is more sensible and crams more yachts than you will find than those that raft up. Why Med mooring is not more common - is a bit of a mystery.

Sorry I'll do without the common sense as defined in post 27.



Jonathan

 

[GEM43]

We anchor in an awful lot of places which are not "recognised anchorages", some more suitable than others.
Re sloping seabeds: Many of the sealochs in the West of Scotland were gouged out by glaciers, and are classed as fiords. Many of them are deep. Usually there are rivers coming in from the hills at the head of the lochs. These rivers bring down a collosal amount of material eroded from the hills, and deposit it in the form of deltas, above and just under the littoral zone. Most of the material is rock flour and stones, so it settles very quickly, and doesn't travel far. This means that a steep bank is formed, just out from the LW mark, falling sharply away to the much deeper depth in the body of the loch. Obviously nobody would attempt to anchor on this steep slope, but when approaching from seaward, can give the unwary quite a surprise. 25m suddenly becomes 2m.

Loch Riddon comes to mind…..

 

[NormanS]

I confess I totally lack the example of common sense. I wonder how I have survived so long.

What we have always done is

Wind is off the land

Approach an anchorage from offshore. We think the shore will give us shelter. As we near the anchorage the sea shallows and as we close the beach it becomes really shallow - so we stay outside the surf line (making a guess where this might be at low tide). We deploy our anchor, reverse away from the beach, water gets deeper - the seabed nearly always slopes to 'offshore' and then power set.

Wind is off the land (which might be an island), that's why we chose this location.

Common sense apparently suggests we should have reversed this. Deploy anchor in deep water, reverse up, anchor sets more securely into the rising (shallowing) seabed. This is the common sense approach. Setting the anchor in the 'downhill sloping' seabed is suicidal

As far as I experience the seabed always slopes away from the shore, sloping seabeds are commonplace (if not universal). Many people anchor every day in sloping seabeds, they don't give the sloping seabed a second thought - they contradict 'common sense' because wind direction is factorially more important.

People Med Moor because its sensible - its common sense.

Sorry I'll do without the common sense as defined in post 27.



Jonathan

Jonathan, I suspect that you may have (possibly intentionally) missed the bit where I said "noticeable and significant".
Even I realise that the seabed isn't flat.

 

[Neeves]

Jonathan, I suspect that you may have (possibly intentionally) missed the bit where I said "noticeable and significant".
Even I realise that the seabed isn't flat.

No, not at all,

I took advantage of your use of subjective words (and should apologise)

Loch Riddon comes to mind…..


View attachment 183356

I obviously don't know Loch Ridden but it seems a perfect, well maybe not perfect, let me say possible location to use shore lines (dinghy) and a shallow draft yacht

Jonathan

 

[oldmanofthehills]

Its not just Scottish sea lochs that give an issue (been there with both caution and glee) but also tidal scoured places such as Plymouth sound and parts of the Bristol Channel (eg Steep Holm). Various places have unsuitable shallow shallows with excessively deep water maybe 50m away, and the deeper slopes are can be rocky and hard to get an anchor in. It takes judgment to find the sweet spot and sometimes patience to try anchoring till it sets OK.

None the less I will continue to anchor in the unsuitable anchorage of Barn Pool and row ashore to the Edgecombe and expect boat to be still basically where I left it, on my return and when I wake next morning.

I understand Thinwaters argument, but in practice it mostly come down to knowing the bottom, and so far in 35 years I have never felt the need to add weight (kellet) to the chain, though I do have a suitable one stored in the side locker

 

[vyv_cox]

Even I realise that the seabed isn't flat.

A noticeable feature of Greek cruising is that the islands have clearly been mountains at one time, either sunken or flooded millions of years ago. Sand/mud that has been washed off the islands since flooding has accumulated on the seabed and levelled itself. We often notice that we sail, or more often motor, for long periods, 30 minutes or more, with virtually no change in depth. I noticed a few days ago that over 10 minutes the depth varied between 105 and 106 metres and never went outside these limits.

We are currently anchored in a large enclosed bay, perhaps a couple of square miles. The depth is 4-5 metres everywhere.

 

[Bouba]

Logic suggests that if you anchor in the uphill direction...then if you drag it will increase your scope...keeping you more in touch with the sea bed therefore more likely to reset. If you anchor on the downhill direction, dragging will cause the scope to shorten...eventually to a point where the anchor is dangling

 

[thinwater]

We needed another anchoring thread ....

I like hearing about other peoples regions. It's too easy to believe your own experience is universal. I know mine isn't.

 

[bikedaft]

A noticeable feature of Greek cruising is that the islands have clearly been mountains at one time, either sunken or flooded millions of years ago. Sand/mud that has been washed off the islands since flooding has accumulated on the seabed and levelled itself. We often notice that we sail, or more often motor, for long periods, 30 minutes or more, with virtually no change in depth. I noticed a few days ago that over 10 minutes the depth varied between 105 and 106 metres and never went outside these limits.

We are currently anchored in a large enclosed bay, perhaps a couple of square miles. The depth is 4-5 metres everywhere.

remember the old seafarer echo sounder where you could tell the nature of the bottom with the shape of the return dots, and the gain button...?

 

[NormanS]

remember the old seafarer echo sounder where you could tell the nature of the bottom with the shape of the return dots, and the gain button...?

These old Seafarer sounders were great, - far better than just a number on today's digital display, because they gave an indication of weed etc. My elderly Raymarine plotter has the option of a Fish finder, either as a split screen or the whole screen. I find it invaluable for anchoring, when it can reliably find a nice clear patch among weed or other hazards.

 

[thinwater]

These old Seafarer sounders were great, - far better than just a number on today's digital display, because they gave an indication of weed etc. My elderly Raymarine plotter has the option of a Fish finder, either as a split screen or the whole screen. I find it invaluable for anchoring, when it can reliably find a nice clear patch among weed or other hazards.

In this age of multi-function displays, I wonder that that is not a standard option.

I did UT inspections for years, and a practiced eye can tell a lot from the echo. It's not that hard, after comparing what you see with what is there a few times.