reading a fishfinder

[handball]

This winter I installed a furuno 6100 fishfinder and have since launched the boat and tried out the new fishfinder.I found it very interesting to run the boat slow and study the contours of the bottom . I could see various shaped sizes of depresions in the bottom, and assume they might be good places to try fishing when the time comes.As you travel the pockets in the bottom start to show up on the right side of the screen and slowly travel accross to the left side.One question that I have is when is the boat directly over the hole? is it when the hole is in the middle of the screen? And also because the beam is cylindrical in its shape is what you are seeing a cross section through a round depression?I would appreciate it if someone could answer these questions in a way a novice could understand. <br />Bill

 

[JB]

Re: reading a fishfinder

What is directly under the boat is what is at the far right of the screeen...being "drawn".<br /><br />The beam is not cylindrical, it is cone shaped, with the point of the cone at the transducer.<br /><br />Go to the Lowrance website and watch the demo. That should give you a better idea of what you are seeing.<br /><br />Good luck.

 

[imported_JD__]

Re: reading a fishfinder

handball,<br />Your transducer beam is more like the shape of a cone with a 360 degree view of the bottom. Think of it like a flashlight beam. If you aim the flashlight on a distant object your light has more coverage area. Up close the opposite is true. The actual cone angle of the 'ducer and the depth of the water both come into play. Since sound travels through water @ around 4,800 fps and your transducer pings several times per second, what you're seeing on screen is definitely real time, almost instantaneous. The leading edge of your transducers coverage area would be represented by what first appears on the right side of the screen. Directly beneath the center of the transducer is the actual depth or distance between the face of the transducer and the sea/lake bottom. As you get away from the center of the beam your sonar unit is displaying range from the bottom in a 360 degree area. Your task is to imagine a 3-D world from a 2-D display. Hope this helps.<br />JD

 

[ThomWV]

Re: reading a fishfinder

Handball,<br /><br />Just to clairify something for you, the cone that is being refered to, which you can think of as looking something like an inverted ice cream cone, has sides which are at about 45 degrees when you are shooing at 50 kHz and about 12 degrees when you are shooting at 200 kHz. While you'll see several different cone angles advertised (depends on where you look) for your transducer it was in fact made by Airmar and usest the standard 44mm element - the same one is used in all three transducers that might have shipped with your depth finder. Use the high frequency in shallow water (up to about 200 feet) and the 50 kHz in deeper water. By the way, congratulations on having bought the very best small boat, non-color, fish finder made. You could not have done any better.<br /><br />Thom

 

[handball]

Re: reading a fishfinder

Thanks for the help fellows,<br />If I understand correctly I should think of what is on the right side of the display as what is foward of the boat, and when it is centered on my screen it is below the boat. I will check out the lowrance site to try to understand better what is appearing on the screen.I have used radar and have no trouble understanding what is on the screen, it is easy for me to visualize my boat in the center of the circle and the land outlines on the top of the screen, or the boat blips around me, and my relation to them.<br /><br />With this fishfinder I can also see what looks like the composition of the bottom. It looks to me like I am seeing the layers of mud sand and rocks by their densities.<br /><br />While sitting in the slip in say 15 ft. of water I can see odd shapes at different water depths passing from right to left on the screen. I guess these may be sea weed or grass passing under the boat with the current.What would these shapes look like if they were large scools of bait fish that we get later on in the season in the harbor?Would you see individual blips or a large darker area? I am presently using the 200 khz option.Even my wife Sue has found this unit interesting to watch and hopes it helps our fishing.Thanks so much thomwv for your extensive help in mounting the transducer during the winter, you gave me some really good advice in the mounting.<br /><br />Bill

 

[18rabbit]

Re: reading a fishfinder

There are two ways to look at the sonar return; what was there and what is there.<br /><br />What was there is shown as the transducer’s return in two dimensions, Y (vertical) and T (time). Time is represented on the largest portion of the display screen from oldest time (left side) to most current time (right side). There is no reference to X (horizontal) or Z (width) to locate anything inside the cone.<br /><br />If you look at any given, single column of pixels, you will see a slice of time for what was in the cone. That single column has little value on it’s own. Start adding columns of pixels to the left and right (time before and after) and you start to get a better idea of what was there and when, but not what is there.<br /><br />You can see what is there by looking at just the sonar return without the reference of time. It’s called the A-scope. That is the bar graph to the far right of the whole display screen. It is a representation of what is immediately in the sonar cone based on depth and strength of the return only. Because the A-scope is a representation of what is there right now, it has no reference to time.<br /><br />See section 1-10, page 8 of your owner’s manual for the A-scope.<br /><br />Note: there is nothing on a single channel fishfinder that will show you where an item is inside the cone, other than depth of course.<br /><br />Other note: there are other fishfinders that will display 360 deg around the boat as well at depth. They display on a polar graph, similar to a radar return. They are use by commercial fishing vessels that need to track schools of fish in motion. Furuno does not make anything like these units. They are multi channel and/or rotating sonar.

 

[ThomWV]

Re: reading a fishfinder

18Rabbit,<br /><br />Actually Furuno was the company that first made the sort of unit you mention, the ones that can show what is actually where in real time. They currently make 15 models that can do it that operate on 12 volts and three more that operate on 24 volts. They are extremely expensive though and are, as you so correctly state, only found on commercial vessels.<br /><br />Handball, <br /><br />I hate to say this but you are misunderstanding what that screen is showing you. 18Rabbit explained it quite nicely and absolutly correctly but I think you'd have to have a pretty good understanding of how they worked to understand what he was telling you. Let me give it a try in simpler terms.<br /><br />When the fish finder is truned on it sends power pulses to the transducer, which then pings, and then it waits for a very short period of time for a set of return echos to come back from that ping. One ping can, and often will, result in more than one return echo as more than one thing may exist within the cone of outgoing sound that is capable of reflecting some of the sound back.<br /><br />When return echos come back they are converted to electrical energy by the transducer and that is sent back up to the display unit. The display unit will paint dot on the extreme right side of the screen that represents the depth at which the echo causing thing exists. In the mean time the transducer keeps on pinging every fraction of a second so more return are comming in. Now I'm going to tell it in a way that's not exactly true, but that's OK. Every time a new ping is shot out and its echos come back a new vertical line will be drawn on the extreme right side of the screen and the line that had occupied that space will be moved one line to the left. The process continues and what appears to be happening is that something that is under the boat for a brief time seems to move to the left as if it were swimming off the screen. That is not what is happening though. Also, and this is very counterintuitive, when you are moving you can get the impression that looking at the fish finder screen is like watching a television in that you are moving and so is the image on the screen, so if you see something passing to the left on the screen it appears that it corresponds to your movement. That is not what is happening at all.<br /><br />Think about this, and maybe it will become clear to you. Let's say that you are at anchor in still water. You've got the fish finder turned on and a fish swims under the boat. Let's say that the fish swims up from behind the boat and passes directly under it and keeps on going until it is well away from the boat in front of it. What will appear on the screen is a blip that will appear to move from the right side of the screen to the left and the width of the blip will actually represent how long the fish was under the boat and within the cone of sound. Now, immagine that another fish passes under the boat, swimming at the same speed but this time he comes in from directly to the left of the boat and swims so that he exits the cone directly to the right of the boat. The exact same picture will show up on the screen, with the blip moving from right to left on the screen. Now, immagine a third fish approaching the boat from its front, passing directly under the boat, and exiting directly behind the boat, exactly the opposite of what the first fish did. Know what will show up on the screen? Exactly what you saw when the first fish passed by going in the other direction, a blip that marches across the screen moving from right to left. Now, immagine this, and you can actually do this with your fish finder if you like. Immagine a fish that swims under the boat and just stops directly under it. If you like take a weight of an ounce or two and drop it on a line down below the boat by 10 feet or so. What do you think will show up on the screen? Here's what. A straight line will appear on the right side of the screen and it will march across the screen to the left, appearing to move across the screen even though the object or fish is completly still. If you were to jig the weight up and down what you'd see is a wavy line forming on the screen. Make sense?<br /><br />Thom

 

[18rabbit]

Re: reading a fishfinder

Thom - Didn’t know Furuno made ‘em. I checked out the CH product line and there they are. Anything available in 24v risks being installed on my boat. <br /><br />Take a look at the Wesmar. They have a couple low-end sports versions. Don’t know the cost.<br /><br /> http://www.wesmar.com/sportfishing.html

 

[Boatist]

Re: reading a fishfinder

Handball<br />What is under your boat is the right most row of pixels from top to bottom. Every thing to the left of that row is history, or what was under your transducer.<br /><br />Also what you see in that right most row is right under the transducer. With a 20 degree transducer in 30 feet of water what you see is in a 10 foot circle under the transducer. It is not out to the sides or up front but right under your transducer. If you are trolling what you see in the right most row of pixels is under the transducer. Any thing to the left of the right most row is what you have already pass over, the farther to the left it is then the farther behind the boat it is.<br /><br />If you are anchored you will see what is under the boat in the right most row. It will scroll to the left as time passes. If a fish swims up and stops under the transducer of your boat and in the shade of your boat and he does not move what you will see is a solid line starting on the right and scrolling to the left. This line will be at the depth of the fish until he moves, then the line will clear from right to left.<br /><br /> http://www.lowrance.com/Tutorials/Sonar/sonar_tutorial_01.asp

 

[handball]

Re: reading a fishfinder

boatist,<br />Let me see if I understand you correctly. If I see the start of a depression or hole in the bottom on the right side of the screen, when the hole is shown moving to the left and starts to show a rise on the right side, the deepest part of the hole is now under the transducer.Another question I have is if while sitting in the slip tied up in 16 ft. of water and I see a dark irregular outline moving from right to left and occasionally one of these objects have white lines thru them , what might I be seeing? My guess is they are some type of sea weed or grass.Is there any way to tell the difference between sea weed , grass or a fish? Also what would a school of bait fish look like on the screen?<br />Bill

 

[imported_JD__]

Re: reading a fishfinder

Handball,<br />I have an excellent tutorial written by Lowrance's international technical coordinator. Different brand but the principles are the same. Fully illustrated and in color, in MS Word format. Give me your email address and I'll send it to you. Just send a blank email with tutorial in the subject box to jabshire@cablenet-va.com

 

[Boatist]

Re: reading a fishfinder

HandBall<br />No when you start to see the rise on the right side of your screen the transducer has already passed over the deepest water. The rise means water under the transducer has got shallower.<br /><br />Sitting in your slip you could be seeing many things, a lot depends on how high you have your sensitive set to. It can be grass, air bubbles from the bottom, some silt or plankton in the water.<br /><br />Sea weed, grass, a log will all look pretty much the same. You may get stronger return from a log or rock on the bottom. A school of bait fish can look a lot different with different frequency tranducers. In the ocean useing a 50 khz transducer most bait schools just look like a big round blob. With a 200 khz transducer will sometimes see lots of small marks. In lakes a under water bush or tree is hard to tell the difference between it at a school of bait fish. a big tree lim up off the bottom will look just like a nice fish.

 

[handball]

Re: reading a fishfinder

Boatist,<br />Right now I have the unit on auto setting for sensitivity , and it seems to show a lot of returns between the surface and the bottom. Would reducing the sensitivity remove all these weed, grass and other returns and give me a better chance of seeing fish?<br />I have downloaded the tutorial but am getting an error message when I try to read it.I am using windows xp dont know if that makes a difference.<br />Bill

 

[ThomWV]

Re: reading a fishfinder

Bill,<br /><br />When you are in the auto mode most of your controls will be deactivated. When Furuno says auto them mean auto and they cut you out of the picture so to speak.<br /><br />One of the very nice things about color machines is that the strength of the return echo is deliniated by the color of the display, with soft color referencing weak return echos and harsh colors used to denote strong echo returns. Gray-scale machines to that too to some extent but its harder to tell the subtle differences. At any rate with a color machine its pretty easy to tell weeds from bait fish using the color-defined strength of the return.<br /><br />Its good that you are digging into this and if you've looked at the Lowrance tutorial that was bound to help. Although I think Lowrance equipment is generally grossly overrated I also think they have a great website and the best tutorial out there. One thing that you need to keep in the back of your mind is that the fish finder doesn't really measure depth, it measures the distance to an object. That might sound like double talk but it does have a lot of meaning particularly if you have a transducer that is shooting a relatively wide cone angle. It is the difference in distance, not the depth of the target, for instance that causes the traditional "fish arches' to show up on the screen. They are formed on the screen because as a fish moves into and then out of the cone within which usable echos can be expected to come back the target will become closer to the transducer as it comes closer to directly under it and then will become farther away as it moves away from directly under the transducer, even though the depth at which the fish is swimming may never have changed. Its why the top of the 'fish arch' is indicated higher (shallower) than the wings of the arch..<br /><br />There is another thing that you need to keep in mind. This will sound like blasphemy but here goes, the cone angle does not define the area in which you can detect targets, with things outside of the cone beyond the reach of the machine. In truth there can be things inside the cone that can not be seen and there can be things outside of the cone that can be seen. The cone angle is simply a measure of the transducer's ability to focus outgoing sound waves, no more and no less. For almost all manufacturers the cone angle is measured at what is called the half power point. What that means is that an immaginary line is drawn that corresponds to each horizontal point at which the energy of the remaining sound is half of that directly below the transducer. Power is greatest along a line directly below the transducer and as you move away from that line the power decreases. In truth the cone does not look like an inverted ice cream cone, it looks a lot more like an upside down mushroom with ears (called side-lobes). Radar signals do the same thing. Those side lobes are just as cabable as the main beam of sound at producing reflected signals and that is why you can see things outside of the cone. This is particularly true in shallow water.<br /><br />Any way read the tutorials and keep on paying close attention to the fish finder. Just keep in mind that although it looks something like one its really not at all like watching a TV screen.<br /><br />Thom

 

[18rabbit]

Re: reading a fishfinder

 

[Boatist]

Re: reading a fishfinder

HandBall<br />Most units will allow you to leave the depthfinder in auto mode but still change the sensitivy and gray line setting. In auto the sensitivity will still change on it own as you get in deeper water. I always adjust the sensitivity up until I get lots of false returns then back down until the screen mostly clears up.<br /><br />To see fish the sensitivity need to be pretty high. Most fish are kinda like a stelth fighter airplane. They are wedge shapped and will not return to strong a signal. A big flat rock on the bottom will give you a very strong return but a soft wedge shaped fish will not be seen if you set the sensitivity at a point where the rock and bottom do not return a very strong signal. That is why I set unit to point where I get some false returns in the depth of water I am fishing. Set this way anything that goes under the transducer you will see. Most units this also means that the top of the screen will be blacked out due to surface clutter. If you set to where you do not see grass and weeds then you most likely will not see any fish either. With time you will learn to tell what is likely a fish. Many fisherman do not even look for fish but instead look for structure where fish likely hiding. In link below you will see a example of a bunch of fish that can not be seen on a depth finder because they are in a area called the dead zone. In this example if the boat moves you will be able to see then at some point but they will not make a full arch if you are running from deep to shallow or shallow to deep. Even if your pick them up running along the edge of this bank they will not make a full arch as they would be in the outer edge of the cone.<br /><br />A fish in or near a under water bush or tree very hard to detect as the limbs on the tree will return a signal that looks like a fish. To aid in fishing I think need to know a lot about the fish you are fishing for. For example fishing for Black Bass in the winter with cold water out here you will find the bass 30 to 60 feet down they will still be around structure. In the spring and summer mornings will find bass shallow like 5 to 15 feet and in the hot part of summer they may be shallow in the am but in the heat of the day will move down 30 feet or more. So you look for structure where you think you will find fish and start fishing. When you catch a fish you note the depth now you fish up slope or down slope until you catch another fish. Soon you will know the depth to find fish. You will likely see some fish also but easy to be fooled. Once you know the depth you can move around the lake and find fish right away. If you go up a river inlet with snow melt water coming into the lake you will have to find fish again.<br /><br />Fishing for Sturgeon in San Pablo Bay the bottom is mostly flat and hard to find any rapid change in depth. Here you can actually see fish on the bottom and be very certain they are fish bucause the bottom is so flat and really no structure at all.<br /><br /> http://www.vexilar.com/help/tips/tip006.html

 

[handball]

Re: reading a fishfinder

Hi fellows,<br />I am starting to get the message it seems that practice with the finder combined with written info is the way to understand what I am seeing.<br />As far as I know the furuno does not show fish arches , I guess they feel the arches are missleading or something.<br />I am looking foward to the bluefish run here on long island sound. Sometimes these schools of blues are quite large and surround the boat.Under these conditions it should be easy to see the fish under the boat. If I were fishing in say 25 feet of water should I be using 50 or the 200 ktz., or is it better to use both with the split screen?<br />Bill

 

[ThomWV]

Re: reading a fishfinder

Handball,<br /><br />The Furuno will show arches just like any other fish finder,its just that some of the makers do software manipulation to enhance arches. Furuno does not do that.<br /><br />Understand what causes a fish arch and you will quickly see why any fish finder will show them. When a fish first comes into the cone it will be farther away from the transducer than at any other time during which it can be seen. Being farthest away it's displayed echo will be drawn at some lowe point on the screen. From there, as the fish moves within the cone, it will only get closer to the transducer - the only other option is that it will fall off the screen, if it stays in the cone it has to be getting closer. The display unit will display closer as higher, because that's all it can do. Higher in this case will cause the displayed image to begin to peak into an arch. At some time later, and this could just be a split second really, the fish will begin to move out of the cone, and in so doing it's displayed depth will once again increase, because its not really depth this is being displayed, it the increased distance from the fish finder as the fish move out of the cone, so what you get to see is the second, and down moving, side of the 'arch'. So in the end it looks like this / as the fish move into and closer to center of the cone, and like this \ as the fish moves from its closest to the transducer to out of the cone. Put the two together /\ and you get a fish arch. That is how it works and it works just as well (better actually) when a Furuno is doing it as when any other fish finder is doing it.<br /><br />Now, about frequencys. Use the high frequency for shallow water (up to a couple of hundred feet) and the low frequency for deep water. The reason is that higher frequency sound waves will be attenuated by the water and simply will not work in deeper water. I'd guess that your fish finder won't pick up bottom at all in water greater than 200 feet in the 200 kHz mode but will shoot to somewhere around 600 feet in the 50 kHz mode. Anyway think of it as just assbackwards, bigger the water smaller the frequency and smaller the water bigger the frequency, pretty easy to remember that way.<br /><br />There is also the thing about a wider beam being shot by the 50 kHz side than the 200 kHz side. The 50 kHz side is roughly 3 times as wide as the 200 and so it will cover a larger area in 50, but with corresponding less resolution. The nice thing about using split screen is that it is the only way you can get a sense of how far from the boat the fish, or anything else, might be. If you see it first on the 50 side but not on the 200 you know its a good ways off, but when its on both sides of the screen whatever it is has moved a lot closer to directly under you.<br /><br />Thom

 

[Boatist]

Re: reading a fishfinder

Good Job ThomWV. I agree with everything Thom said. All LCD fish finders will show arches if you set to raw returns. Some fish finders have a feature called fish ID where it draws a picture of a fish or displays a fish symbol. If the FURUNO 6100 has this feature turn it off so you can see what is really there. All the fish ID does is display any return off the bottom as a fish symbol. It could be a leaf, a plastic bag a bush or even a air bubble and it would be displayed as a fish. <br /><br />The 200 khz is better for shallow water and in fresh water good to about 300 feet. Salt water good to about 200 feet. My 200 Khz 20 degree transducer unit in salt water will see bottom to 600 feet or more but will not see any fish over 200 feet. My 200Khz 8 degree transducer will see fish down to about 350 feet of salt water and bottom to at least 1000 feet.<br /><br />The 50 Khz unit work to see fish much deeper than the 200khz unit and will be 37 to 45 degree cone angle. The wider cone also means fish will make better arches, since the fish is seen farther away and be displayed as deeper when first pick the fish up and when about to loose the fish after passing over. The 50khz unit also much better at picking up down rigger balls when useing a downrigger.

 

[handball]

Re: reading a fishfinder

Thanks again thom and boatist for helping me to understand fishfinders better. I would like to know how both of you became so knowledgeable on the subject, would you mind telling me how it came about?Also what is the reason you would use the zoom feature for? and do you use it often?<br /><br />I hope that there are many more fisherman out there that this forum might help as much as it has helped me.<br /><br />Bill