Fishfinder For Anchoring

[noelex 77]

When anchoring, the quality of the substrate is vital. I have learnt when observing the sea bed when diving that the official chart description of the sea bed is often hopelessly wrong. Pilot books/cruising guides are slightly better, but often they simply have a reference to “poor holding” with no other detail. Frequently the poor holding refers to hard sand or moderate weed, which defeated older generation anchors (hence the notation), but modern anchors can set quite well in this type of seabed. In fact, these are often the best anchorages because the comments put off many boats. However, sometimes “poor holding” means rock and even new generation anchors cannot penetrate rock.

When diving it is easy to make an assessment of the seabed, but if conditions prevent visualisation, the fish finder is a possible resource for assessing the substrate.

So I am hoping I can improve my skills using the fishfinder to assess the substrate.

It would be great if someone with more experience could comment on my fishfinder results. So here is a screen capture from yesterday. The equipment is the Airmar 200/50 hz P319 600w transducer coupled to B&G Zeus 3 plotter.

For the screenshot below the settings are on 200 hz and auto, but this can be adjusted if anyone has any suggestions.

Some links to articles describing how to use the fishfinder would be helpful. It would also be great to hear about user experiences, good and bad. Perhaps I am wasting my time trying to get accurate information about the substrate from the fishfinder

What is the structure in the centre. Is it weed or something more substantial?

[a64pilot]

The structure could even be fish, but they usually have a shape that resembles a Christmas tree.
It is most likely weed or other form of growth. Is that Depth feet or meters? If meters then that is some big weed.

The bottom type is actually the bottom line on either side of the weed of course, and not knowing you machine, Id say it’s soft bottom, a hard bottom would be a thicker line. This is where auto mode can fool you, it can auto adjust and make it look different.

This may help. Interpolating and setting up a bottom machine is a lot like Radar, the auto modes can lead to confusion since they auto adjust and make things look different.
http://www.fishtec.co.za/hard1.htm

On edit I believe the higher the frequency, the better. 200 isn’t very high, but its what we are stuck with, much higher would be better.
Low frequencies can shoot DEEP though and I think that is their use, best way to shoot deep without excess power is to lower the frequency, but you give up resolution.

[a64pilot]

You really want to see bottom structure, get a chirp machine, in smooth water they can return an almost photo like picture. However they can’t distinguish Boat movement from waves from an irregular bottom, so in waves they are less useful.
I had a Dragonfly on my fishing boat mostly as a back up Plotter, but it was a phenomenal bottom machine for very little money.
I have bought one for my dinghy, but haven’t installed it yet, need to figure out how to mount and best power supply first.

[noelex 77]

Thanks for the feedback and for the link.

The depth scale is in metres so the “structure” is almost 2m high. I have not seen the bottom so no answers are wrong, but it would be great to get some more feedback, if nothing else so I can learn the thought process behind the interpretation.

[deblen]

To determine the "hardness" of bottom,increase your scale & gain so that you get a second & third bottom echo.
Adjust gain so that you have just a bit of green in third echo on known mud (soft) bottom.
As you travel over bottom,harder bottom will produce a thicker reddish color in second & third echos.
The thicker & redder-the harder the bottom.


Anything above or just touching the top edge of the first bottom echo is not part of the bottom substrate. It willbe fish,weeds,structure-etc.


Sample: http://www.vexilar.com/blog/2014/08/...ng-hard-bottom



Cheers/ Len

[noelex 77]

Thanks for the tips and the link,Len. I have never looked at the second and third echo so that is a good new piece of information to add to the mix. I have just given this a try and it looks very useful, especially as means of assessing bottom hardness.

My interpretation of the bottom from the fishfinder image is that it is soft, but with lots of thick weed, but I sure others can make a better assesment.

[deblen]

Originally Posted by noelex 77

Thanks for the tips and the link,Len. I have never looked at the second and third echo so that is a good new piece of information to add to the mix. I have just given this a try and it looks very useful, especially as means of assessing bottom hardness.

My interpretation of the bottom from the fishfinder image is that it is soft, but with lots of thick weed, but I sure others can make a better assesment.

It is difficult to tell the hardness of bottom in your pic without comparing other places. You need to move around over different bottoms with different hardness,preferably one(s) that you know & at different depths. Then you can compare the thickness of echos.


One thing that I suspect about your pic,due to the "stretched" look of targets,is that your screen advance is set very fast.
You will get better resolution if you run screen advance slower.
The way that screen advance speed works is that at a particular moment,the screen stops,the transmitter fires,the receiver waits for the echo to return,the computer analyses the voltages of the return & the computer prints pixels in a vertical line using colors related to the voltages.Then the screen advances & the process repeats.

If you slow the screen advance down,it gives the transmitter/receiver a chance to send out several pulses to the same spot,compare the received echos,& print a much more accurate return due to averaging.
Note-a fishfinder shoots a cone signal straight down & ,generally speaking,can only show what is in that cone. You will not see any more,or farther ahead,by running the screen advance faster.


You have a lot of pale blue clutter showing. This could be due to air bubbles/disturbed water from current,your prop, etc.
Most fishfinders have adjustments to reduce or eliminate this-especially in the first few feet of water.


Hope this helps. Read your manual & keep experimenting. Fishfinders take practice & experience to fully utilize these extras.


A note on frequency: 50Khz vs 200Khz.
The lower the frequency (50Khz) the deeper it will penetrate water--power,t-ducer & other things being equal.
But the lower frequency comes with a longer(time/depth) pulse.
If you have two fish,one sitting a foot over the top of the other,50Khz may show them as one blob,whereas the shorter pulse of 200Khz may show two distinct targets.


200Khz will give you better detail,but it is useless for showing fish beyond about 20fa (120ft). It is excellent for showing pelagic fish that stay near surface (salmon,herring,mackeral,etc.
50Khz is great for bottom feeders beyond 120ft to 720ft(120fa)
50Khz will pretty well flood your screen at depths of less than 30ft at normal gain settings-basically useless at these depths.
It is helpful for learning purposes to run both frequencies for comparison though.


At the extreme right of your screen is an A-scope display. This display was used before graph paper & video displays were developed. I've never seen the point of it when you have a graph display,but I knew old draggermen that loved them.


Len

[noelex 77]

Thanks again for the comments, Len. The screen was very cluttered in the last photo. The water is quite turbid here with lots of suspended weed especially with a bit of wind stirring things up.

We are still at the same anchorage although we have swung to other side of the swing circle. The wind is very light at the moment (about 15 knots) which I think is helping to reduce the bits of weed floating about midwater.

I have ditched the A screen. I agree this no use. I have also reduced the scroll speed from auto to ½ which seems to have helped the bottom definition. Thanks again for the tip.

This was a screenshot taken a few minutes ago.

[deblen]

Originally Posted by noelex 77

Thanks again for the comments, Len. The screen was very cluttered in the last photo. The water is quite turbid here with lots of suspended weed especially with a bit of wind stirring things up.

We are still at the same anchorage although we have swung to other side of the swing circle. The wind is very light at the moment (about 15 knots) which I think is helping to reduce the bits of weed floating about midwater.

I have ditched the A screen. I agree this no use. I have also reduced the scroll speed from auto to ½ which seems to have helped the bottom definition. Thanks again for the tip.

This was a screenshot taken a few minutes ago.

Looks much cleaner.
Try doubling or tripling your scal depth so we can see the second & third echos. / L

[noelex 77]

Here is the photo of the deeper echos. I have adjusted the manual gain up until the screen becomes very cluttered, but there is only a second not a third echo visible. Even the second echo is reasonably faint, which I presume means the bottom must be quite soft, but correct me if I am wrong.

[deblen]

Originally Posted by noelex 77

Here is the photo of the deeper echos. I have adjusted the manual gain up until the screen becomes very cluttered, but there is only a second not a third echo visible. Even the second echo is reasonably faint, which I presume means the bottom must be quite soft, but correct me if I am wrong.

Nice. I agree it is relatively soft bottom-not rock for sure.


Increasing the gain is required to look at bottom composition & you will get clutter in the water column,especially in that shallow water.Can't be helped. You have to adjust for what you are looking for.


Notice under the zero in 6.0-there is a slight lengthening of "tailings" of first bottom echo & a corresponding show of red in the second echo.
That is a slightly harder spot & is what I've been trying to describe.
No idea what that harder spot is but it is harder(better reflector) than it's surroundings.


Cheers/ Len

[noelex 77]

Thanks Len. This type of detailed analysis of the picture is very helpful in knowing what to look for when examining the fishfinder return.

I don't want to put you or anyone else on the spot, but do you think the “structures” are beds of kelp? There does not seem enough rock at the base to support kelp, but perhaps the kelp itself obscures the rock return. The “structures” to my eyes do not look solid enough to be something like old mooring so I am struggling to think what else they could be.

There is also an almost continuous return extending a meter or so above bottom. Is this general weed or something else?

I don't like giving my position on an open forum, but I am in an area with kelp in some anchorages. This spot is listed as “good holding”, and there is no weed shown on the official charts, but as I have indicated previously I have little faith in any of these descriptions. This is why I am hoping better skills using the fishfinder might fill in some of the gaps, even if it can only give a rough idea.

[deblen]

Originally Posted by noelex 77

Thanks Len. This type of detailed analysis of the picture is very helpful in knowing what to look for when examining the fishfinder return.

I don't want to put you or anyone else on the spot, but do you think the “structures” are beds of kelp? There does not seem enough rock at the base to support kelp, but perhaps the kelp itself obscures the rock return. The “structures” to my eyes do not look solid enough to be something like old mooring so I am struggling to think what else they could be.

There is also an almost continuous return extending a meter or so above bottom. Is this general weed or something else?

I don't like giving my position on an open forum, but I am in an area with kelp in some anchorages. This spot is listed as “good holding”, and there is no weed shown on the official charts, but as I have indicated previously I have little faith in any of these descriptions. This is why I am hoping better skills using the fishfinder might fill in some of the gaps, even if it can only give a rough idea.

This is where the guessing begins.
There is a fine black line on the top edge of your red bottom. I'm assuming that is a feature of your sounder put there by the computer to help delineate the top edge of the bottom echo.
you will note that the black line goes up over the top of "lumps" just below the red stuff in the water column in 2 places.
My guess is that those lumps,being under the black line,are part of bottom. They could be rocks,traps,junk,tree,etc-but they are sunk into bottom.
The stuff in the water column above them is floating,but is attached to those lumps because you can see thru it & under it. It is not very substantial material relatively speaking.

If you know there is kelp in the area,I would guess it to be kelp. Kelp needs a rock or some other heavy object to root to-so the evidence points to kelp growing on something harder. No kelp on the flat bottom suggests a "soft" bottom that kelp & seaweed will not grow on.


Q.E.D. Len


Now dive on it & find out

[deblen]

Eel grass & other grassy type weeds will grow in soft bottom & can make anchoring "slippery".
But there is no sign of anything above the flat bottom,so it should be clean sand,gravel or mud.
Rocky bottom is not usually that smooth.

[JPA Cate]

A comment about kelp: In some areas where it is shown on charts, it has gone away. Whether it is cleared out by storms, or disease, I do not know. But it is something that we have encountered. Some places where it once was thick, now is much less, and a few years ago, was less still. It is as if something cleared it from the areas and it may be re-establishing itself...maybe a natural cycle, maybe due to something else. Since you are in an area where kelp is known to grow, kelp is a likely phenomenon, if the bottom will support it. Usually takes rock, like deblen said. Maybe always takes rock, I am not certain.

Ann