Dragging of anchors

[noelex]

We find that if the water is clear(ish) and its not overcast and dark (nighttime) then simply looking offers the most accurate way of determining weed - on a sand seabed. The sand is white, or at least pale, and the weed black. The person on the bow can have the anchor hanging ready and drop into the sand patches - you really want the sand patch to be a decent size.

Yes, I agree. Visually is the best way and it is worth practicing dropping the anchor in a precise spot. With some practice, good communication between the helm and bow, and an anchor design that sets in a short distance you can anchor in quite small patches where it weed free.

But this does not work if you cannot see the bottom.

Most, even very cheap, or basic, fish finder/depth sounds offer an ability to identify weed - its not difficult. The instructions usually include detail on how to enhance the images of bottom type.

I am not sure getting the most out of the sonar information is as easy as you are implying.

Depth sounders potentially have a lot of information about the substrate. Is the bottom hard or soft? Is there a very hard layer or even rock under a thin layer of sand? Is there weed and if so how thick? Is the weed growing on rock or is the substrate below the weed soft?

It would be great to able to interpret the depth sounder image and answer the above questions accurately.

Despite the enormous length of discussion on threads about anchor design the substrate is still the most important variable and deserves more attention.

 

[AntarcticPilot]

I am not sure getting the most out of the sonar information is as easy as you are implying.

Depth sounders potentially have a lot of information about the substrate. Is the bottom hard or soft? Is there a very hard layer or even rock under a thin layer of sand? Is there weed and if so how thick? Is the weed growing on rock or is the substrate below the weed soft?

It would be great to able to interpret the depth sounder image and answer the above questions accurately.

I used to work on various types of radar used to investigate ice sheets. This included both satellite radar altimeters and ice penetratingradars - very different beasts! But one useful way of looking at the returns was as an A-scope - that is, a plot of power returned versus delay. This allows the precise shape of the returned echo to be seen, and allows a great deal of interpretation of the nature of the interfaces being sensed. An A-scope type display alongside the conventional Z-Scope display (the usual fish-finder display) would make interpreting the returns much easier. Things like the width of the return echo, the slope of the rising edge, the presence of secondary echoes etc would all provide a lot of useful information that is to some extent suppressed (or at least, difficult to see) in a fish-finder display.

 

[noelex]

I took this photo a few minutes ago showing the fishfinder trace at our anchorage.

The controls are set to auto and 200kHz but can be adjusted if anyone has any suggestions. I have also turned on the A scan profile as well.

Can anyone provide an interpretation.

 

[Neeves]

Not me, but then I don't recognise your instrumentation nor had time to play with it.

When we installed new I was lucky and had the manufacturer on deck for 2 whole days, primarily as I was able to demonstrate their Broadband was not as sensitive as they were claiming (though more sensitive for commercial vessels at longer distance) - but still perfectly acceptable The advise for fish finder (in addition to the manual) was - spend time checking your actual seabed, what you know is down there, for the response you actually achieve - and correlate the two (and remember). As a diver - you will find this easy.

 

[AntarcticPilot]

I took this photo a few minutes ago showing the fishfinder trace at our anchorage.

The controls are set to auto and 200kHz but can be adjusted if anyone has any suggestions. I have also turned on the A scan profile as well.

Can anyone provide an interpretation.

This site (http://www.radartutorial.eu/12.scopes/sc05.en.html) shows what I mean. Your "A-scan" isn't really very helpful as it doesn't show the shape of the return clearly. Of course, the "Z-scope" display in the rest of the screen is helpful, showing changes in the nature of the bottom as well as features in the midwater clearly.

I always find the degree of crossover between satellite radar altimeters, ice sounding radars and echo sounders very interesting, especially as I have a lot of experience of the former two!

 

[Neeves]

I find it interesting how flat is the seabed. I don't know if that is a feature of your fish finder, or the seabed - but our seabeds are never quite that flat. As you have shown the image and you are a keen diver - it would be interesting to know what is down there. You certainly do not seem to have chosen a very useful spot for fishing Our fish finder seems to offer more clarity, we have a couple on wrecks on the seabed near our mooring and can clearly pick out features on the wrecks, stanchions and the like, at 10m depth.

Kelp is not an issue with a fish finder, as you can see it simply by looking - and kelp grows on rock - so its return is not of interest as for a variety of reason we would not anchor there (and usually lots of alternatives). Kelp is usually well documented (as you may find if you venture into NW Scotland). You should be able to pick out and differentiate between each of mud, sand and weed. Our fishfinder, and I might guess most, cannot differentiate with certainty or determine the thickness of the top layer (unless it is very thin) - so if its thin sand over rocks or rock - you will only find out either by diving, trying your anchor - or reading the Admiralty Pilot, cruising guide or look at the chart (as someone mentioned earlier). It merits mention that errors are repeated, as the Pilot books are based on the same seabed surveys as the charts and the cruising guides abstract from the same sources - so if its sand over thin rock - the lead line will have shown sand - unless the surveyors actually anchored there themselves.

 

[Neeves]

Yes, I agree. Visually is the best way and it is worth practicing dropping the anchor in a precise spot. With some practice, good communication between the helm and bow, and an anchor design that sets in a short distance you can anchor in quite small patches where it weed free.

But this does not work if you cannot see the bottom.



I am not sure getting the most out of the sonar information is as easy as you are implying.

Depth sounders potentially have a lot of information about the substrate. Is the bottom hard or soft? Is there a very hard layer or even rock under a thin layer of sand? Is there weed and if so how thick? Is the weed growing on rock or is the substrate below the weed soft?

It would be great to able to interpret the depth sounder image and answer the above questions accurately.

Despite the enormous length of discussion on threads about anchor design the substrate is still the most important variable and deserves more attention.

 

[Neeves]

Yes, I agree. Visually is the best way and it is worth practicing dropping the anchor in a precise spot. With some practice, good communication between the helm and bow, and an anchor design that sets in a short distance you can anchor in quite small patches where it weed free.

But this does not work if you cannot see the bottom.



I am not sure getting the most out of the sonar information is as easy as you are implying.

Depth sounders potentially have a lot of information about the substrate. Is the bottom hard or soft? Is there a very hard layer or even rock under a thin layer of sand? Is there weed and if so how thick? Is the weed growing on rock or is the substrate below the weed soft?

It would be great to able to interpret the depth sounder image and answer the above questions accurately.

Despite the enormous length of discussion on threads about anchor design the substrate is still the most important variable and deserves more attention.

I

I think you are finding that with a bit of effort and some patience you can use a fish finder to good effect to determine bottom type. Not to be considered - 'as I have not tried it - it therefore cannot work'

It is surprisingly easy - if you try it. But you need to be on your yacht, with your on instruments and the manual to get the best out of it. Many of us do check on bottom type for our chosen anchorage - including using our fish finders.

http://www.cruisersforum.com/forums/f118/fishfinder-for-anchoring-203622.html

It would be even better if, as you have been advocating, you had a dive to see what the actual seabed is like and correlate that with what you see on the screen. I suspect you are finding that for a variety of reasons people do not dive on their anchors - because it is not very pleasant, cannot see anything, cold etc.

I'm surprised you did not do this on your previous yacht over the 7-9 years in Greek waters - maybe you did not have a fish finder.

Jonathan

 

[thinwater]

I used to work on various types of radar used to investigate ice sheets. This included both satellite radar altimeters and ice penetratingradars - very different beasts! But one useful way of looking at the returns was as an A-scope - that is, a plot of power returned versus delay. This allows the precise shape of the returned echo to be seen, and allows a great deal of interpretation of the nature of the interfaces being sensed. An A-scope type display alongside the conventional Z-Scope display (the usual fish-finder display) would make interpreting the returns much easier. Things like the width of the return echo, the slope of the rising edge, the presence of secondary echoes etc would all provide a lot of useful information that is to some extent suppressed (or at least, difficult to see) in a fish-finder display.

^^ This. I used to do a lot of UT work, and the A-scan is far more useful for this sort of detail. That said, the details he mentioned (width of the return echo, the slope of the rising edge, the presence of secondary echoes) are often more subtle and complex to interpret than seems obvious. I also agree that the a-scan is not a very useful format. Too much information is obscured.

Google a-scan and do a lot of diving.

 

[simonfraser]

fussing over bottoms is all very interesting, but why not just put the engine astern and check the anchor actually holds ?

 

[noelex]

The techniques used for a skillful interpretation of the fishfinder image are something that is worthy of more attention, but it is a complex subject. Most of the information available on the internet is related to catching fish and while some of this deals with the bottom quality, it is not written from the perspective of trying to find the best place to anchor.

There have been a few suggestions that I found helpful. The first is to look the density of the second and third bottom return. This means manually selecting a depth setting that will incorporate these returns. The second is to look at the “tailings” on the first bottom echo.

It was also suggested I adjust the scroll speed of my display. This is one setting I have never adjusted and it does seem to have helped the quality so it is a parameter that may be worth fine tuning on your equipment.

I think the above is only scratching the surface of a complex subject, but it is a start.

 

[Neeves]

fussing over bottoms is all very interesting, but why not just put the engine astern and check the anchor actually holds ?

+1

Too easy, works for many.

 

[noelex]

fussing over bottoms is all very interesting, but why not just put the engine astern and check the anchor actually holds ?

I agree Simon, this is an important step when anchoring.

The main limitation is most yacht engines can only generate the equivalent force of about 25-30 knots of wind. The holding of the substrate cannot be tested much above this level in a sailboat although you can use some momentum as well as engine power if you are really keen. The other problem is you are only testing in one direction of pull. Some substrates such as rock can have high levels of grip when pulled in one direction, but the holding can be very poor if the wind shifts and the direction changes, even slightly.

Some knowledge of the substrate is I think very helpful, but reliable information, especially without the benefit of local knowledge, is difficult to find. After many years of diving in anchorages I do not have much faith in the substrate notations on charts or even pilot books/cruising guides. A fishfinder has limitations and needs some experience, but it provides another means of trying to assess the substrate when the bottom cannot be visualised, so if you have a fishfinder think it worth cultivating these skills. However, any information gleemed about the substrate should not replace setting/testing the anchor.

 

[GHA]

fussing over bottoms is all very interesting, but why not just put the engine astern and check the anchor actually holds ?

You're allowed to do both, and maybe learn something along the way


Noelex & SV Panope have uploaded Gigabytes of video and images of what really goes on for the rest of us to look at free, great that some people have some curiosity and are generous enough to share

 

[Neeves]

You're allowed to do both, and maybe learn something along the way


Noelex & SV Panope have uploaded Gigabytes of video and images of what really goes on for the rest of us to look at free, great that some people have some curiosity and are generous enough to share

I'm amongst the first to agree - SV Panope's videos are excellent as are Noelex' underwater images. This does not mean those images represent the real world - visibility being one simple example - is not like that in many places.

Unfortunately for a whole host of reasons in the real world many are unable to dive and have a look at their anchors nor are they able to take videos. Some are simply not fascinated in the whole topic. In the same way they do not analyse the performance of their sails to the nth degree - they simply want their anchor to work. A trusted method is to rely on charted info, 'pilot' info and other sources of literature - as many frequent well know locations, this could then be augmented by information from a fishfinder (hardly new to many of us) - and some use the trusted technique of setting the anchor under power in reverse. These techniques, or a limited combination of same, work for thousands without incident.

The idea you need a University degree in Fishfinder operation is new to me - and for one demands you have a fish finder - which many do not have - and I, personally, think implying that this is obligatory (suddenly for 7 years of describing how anchors set without one) is simply unrealistic. First we needed to dive on our anchors, now we need to have a fish finder.

Anything helps - but one should be able to anchor with some confidence - as generations did before us.

It was ludicrous previously its getting plain daft.

If you cannot set your anchor with confidence with what you have - stick to marinas.

Jonathan

 

[NormanS]

I don't think anyone is suggesting that a fish finder should be compulsory. I certainly am not. I do at times anchor in places where there is absolutely no charted or pilot information re type of bottom, and the fishfinder is ONE of the tools in the box.

If some are slightly OTT enough to dive down with a spade, and excavate a sample of the bottom, good luck to them. I wouldn't criticise them, if that's what they need to have confidence in their anchoring. We all have different ways of doing things, and surely that's good.

 

[noelex]

Jonathan, given the often very esoteric and peripheral subjects that are discussed at length on anchor threads, I am surprised you single out techniques for assessing the substrate using the fishfinder as a topic that is so unworthy that it should not merit some analysis and discussion.

If the subject does not spark your interest that is fine. It is often better to just to ignore that topic.

 

[Neeves]

At the last count this was a thread on Dragging of Anchors - I have no difficulty in discussing use of a fishfinder - for any application (though confess it leaves me a bit cold, as discussions on anchors leaves others cold ) We try not to become a slave to technology for technologies sake. However as I started the thread it would seem counterproductive to ignore its content.

I might suggest, even recommend, that a dedicated thread on Fishfinders might solicit more focussed replies from people who are specifically interested in use of fishfinders and happy to share their thoughts.

Jonathan

 

[thinwater]

fussing over bottoms is all very interesting, but why not just put the engine astern and check the anchor actually holds ?

I suspect many areas are more variable than you are accustomed to, with large patches of rocks and weeds.

* If hooked on rock, it may come off if the wind changes correction.
* If hooked on grass, it may only be on a tuft. If it really blows, the tuft will rip loose.
* If sand over hard pan it will seem to set, but drag if it blows hard.
* If really soft it's probably just a waste of time, or at least drop-and-pull is not going to work. You'll need to wait and then pull gently.

There is potential for useful information. I suspect details will be tough to discern and that the result will be one of three types:
1. Smooth, sharp return. Sand. Could be clay or hardpan, though.
2. Smooth, blurred return. Mud of varying firmness.
3. Bumpy, meaning many possibilities, none of them good. Rocks or weeds.

 

[Kukri]

(engages Ancient Mariner mode):

Lead line? Works for me, and is also a good way to tell if you are dragging