Sonar / Depth-Finder  Usage




The word "sonar" is an abbreviation for "SOund, NAvigation and Ranging."  It was developed as a means of tracking enemy submarines during World War II.   Sonar units consist of a transmitter, transducer, receiver and display.

In the simplest terms, an electrical impulse from a transmitter is converted into a sound wave by the transducer and sent into the water.  When this wave strikes an object, it rebounds.  This echo strikes the transducer, which converts it back into an electric signal, which is amplified by the receiver and sent to the display screen.   Since the speed of sound in water is constant (approximately 4800 feet per second), the time lapse between the transmitted signal and the received echo can be measured and the distance to the object determined.  This process repeats itself many times per second.  CLICK HERE for a link to Lowrance's informational and tutorial link.

As modern times dictate, it would be putting ourselves at a disadvantage to NOT use a Sonar / Depth Finder / Fish Finder in our fishing efforts.   There are many brands out there with prices varying from economical to outlandish.  Under normal conditions, it is suggested that you purchase the best unit that you can afford.   The exception would possibly be if you were only going to use it on a river Jetsled where you basically only wanted to use it for a depth-finder.  In this case there would be no real need to pay $600 for something that a $120 unit would function just as good on.

Brands available are numerous, but not limited to Bottom Line, Lowrance, Eagle, Garmin, Furuno, Hummingbird, Interphase or Rathion, and I am sure others.

Finding Fish ;   It will not do you much good if you are trying to fish in a barren location as far as finding fish.    Also if you are fishing in shallow water, 10' or less, it may be beneficial to turn your power and or sensitivity down, as you do not need the same power to see 10' as you would for 300'.   The electronic signal may be powerful enough in this shallow water that the fish can detect, then move away from your boat.

You will have to look at the charts, try to locate underwater structure that may attract minnows, scrimp, crabs, etc. as this is the attractant for larger fish.   If you are in a lake, depending on the season, look for weed beds, deeper (cooler) holes, a stream running into it, anything that may be an attractant for the fish to be comfortable as to temperature and yet be near a food source.

A river will need to be observed and again make a decision as to what you think would be the most likely locations for fish to stay.  They will not be in the strong current, but on the seams between the strong and the sides where it is easier to swim and where bait may become trapped at least temporarily.   You will need to be able to read the water as to what you see on top to what obstructions may be below.

If you are a saltwater fisherperson, frequent bays or inlets where the tidal movement occurs, you will have to look at the location as if it was 2 different rivers.   One as a river on the outgoing tide and the other as a incoming river.   The bait will not be in the middle where all the current is moving, but will seek shelter, (less water movement) behind a point or island.  This location will change from say the north side on an outgoing tide to the south side on an incoming tide.

If you are an open ocean fisherperson, then you will have to frequent known locations "looking" for bait on the sonar screen or fish themselves on the screen.  At times, you may not "See" bait or fish if they are scattered enough to not be concentrated.  If they are close to the top, you may not even see them as your boat may spook them and since your fishfinder's signal radiates in a cone, you would have to be right on top of them and then may only see a fraction of the numbers that may be there.

Just because you have a sonar/fishfinder does not mean that you are now an expert and the fish will just jump into the boat.   You are just beginning to learn.  Some sonars are very good while others are lacking in detail.  There can be a really big difference in a depthfinder and a fishfinder, while at the same time it being the same unit.  It may well depend on what you want it to do and how you have it set up.  Plus you normally have to get used to how to use it, plus this may take some time on the water just reading the manual, then making changes or adjustments on your unit. 

What you are seeing in the picture below of a color chart are MANY small baitfish being forced to the surface by black rockfish.  This was during a WDFW rockfish tagging trip off Point Grenville.  The split screen represents a dual transducer cone designed to be able to read the structure under the boat better by one being a broad cone while the other being a narrower beam as mention later in this article.  The bottom was at 131' but the fish were caught at about 1/2 that.   Water temperature was 50.5 degrees.

A once in a lifetime picture

If all else fails pay your dues and hire a guide for a day.   Watch exactly what he is doing, as it is for a reason, and that is for you to catch a fish.

Installation ;   I will not dwell on this much as all the units come with very good instructions on how to install their units.   The instructions usually say to rout the cables away from other power wiring to help prevent interference.  This may not be possible in some instances on a small boat.  It could also be a legal liability situation where the company can claim that you failed to follow instructions if you are unhappy with the operation of your unit.  

If you "SEE" fish all over your screen, you might just turn off any of your other electronics, one at a time, like radio, defroster, electric trolling motor, etc.  If you see a change in the sonar screen, like maybe way less fish, then in reality you are probably getting electrical interference off that particular unit and since your sonar is sending and receiving electrical impulses the other electrical interference is being intercepted by the transducer where it thinks that this is indeed fish instead.

Noise suppression really means that you see less garbage on the screen.   Since it is an electric signal that you are trying to recover on the transducer, any electrical static may give erratic readings, or at least clutter.   This could be also caused by the spark plugs on your motor not being a resistor type.  Some units have what may be called an ASP (Advanced Signal Processing), which is designed to help eliminate this type of problem.  However if you turn the ASP on, you very well may loose some of the sensitivity of the unit.   The best thing to do if you have interference, is to eliminate the problem at the source.

Another thing to consider, is where you have your sensitivity set.  One day when we were salmon fishing (unsuccessfully I might add) my partner tried to adjust my sonar to the settings he had on his.  Boy, did we see a lot of fishy looking marks under the boat.  He had set my sensitivity at 85% from my previous75%.  I later played with it on the water the next day and found for that area, 65% was best.

One thing that Lowrance/Eagle says that if you plan on using one of their units near saltwater conditions, that they recommend wiring the unit thru a separate ON/OFF switch.   This is because the normal power switch is in the unit, therefore power is supplied clear into the unit's connectors and with it being near salt air, the plug in connections may have some electrical interaction and become corroded.   However the newer units have a twist locking cable connector that seems to be a lot tighter and more secure attachment that previous units.   But one salesman recommended to not use the regular off/on button to turn the unit off with as this is the button that gets used the most and will be the one that fails first, so the separate on/off inline switch works great in this instance to turn it off with.

Mounting the sonar sender unit (transducer) should be installed according to the company's instructions.  If you mount it flat, depending on your boat hull design and speed, you may loose bottom as you get up to speed.  If your boat is a flat bottomed boat and you get it up on a plane where there is a chance of the transom breaking contact with the water if you hit small waves, you could also loose seeing bottom on the screen because of you being airborn at times creating air bubbles under the sender unit.  Changing the sender angle so it points slightly forward can also help if in deeper water in that the signal has to be sent down, then bounce back to be received by the same sender, which in turn measures the elapsed time giving your depth reading.   If you happen to be going fast enough and in deep enough water if the sender is not adjusted right, the signal may be bouncing back behind your boat. 

Also at a recent seminar put on by Lowrance representatives, we were told that many times to lower the transducer another inch below what the instructions say.  You may get a water rooster-tail, but your transducer will be under the water more giving better readings at plaining speeds.

The also informed us that the transducer itself may loose power over time.  They replace the transducer every 2-3 years.  With this in mind if you are on the water enough that IF your sonar dies, it may well be because of a transducer either dying or being broken off if you ran over debris.  Some fishermen even go to the length of installing a 2nd, standby transducer.

General Operations ;   Initially my recommendations are DO NOT get a combination unit that is a built in sonar, GPS, plotter, etc, etc. unless you have NO room in your boat to mount multiple units. These combination types are sometimes called integrated units.  The problem comes in that you have spent a lot of money to get this fancy unit, and if something goes wrong, and IF it is still new enough to be serviced by the manufacturer, the time that you will be without it for repairs could be a a minimum of a three weeks, (been there-done that).   However my last purchase was a combination unit, as my newer boat has changed to lesser room I have and technology has improved, however I still vote for 2 separate depth-finders, or a t least a 2nd one hooked up an doperational. 

If a GPS unit dies it is usually receiver head or antenna that is the culprit.   On my latest unit a Lowrance HDS5 which has a built in antenna, however I still have the separate GPS receiver head mounted from my earlier 337c as a backup.  This separate antenna can give better skyward reception for the receiver.   You will probably not know there is anything wrong until after your vacation has started, & now you are without any electronics.   

Modern technology has gotten to the point that maybe I am just old fashioned, and or living in the past, and possibly electronics are better than they were 10 years ago, BUT?    Then with electronics being updated constantly, these units become obsolete sooner than you would believe.  I have spent lots of dollars for what was top of the line when I bought it, but it was made obsolete and replaced with a improved model within 2 or 3 years.   Another thing, if a company is selling units for a discount, it is for a reason, and that reason is that usually it is or will soon be discontinued.   Any discontinued item may be impossible to have repaired sooner than you think.

My suggestion is that you do a lot of research, talk to MANY COMPETENT salespersons, see if you can try out their demo, or have them put you in touch with someone who owns the one you are looking at.  Then purchase what will work for you.   Another thought is to the one you do pick, find out whether a cheaper version of the same manufacturer will plug into the existing transducer cables.   If this is so, then if your main unit dies, you can purchase a economy version as a backup while the original is being repaired.   Also some companies want you to send your transducer AND cables in at the time of a repair so they can be sure everything is functioning.   For some boats, this may involve a lot of work pulling the cables out of the side panels etc, depending where you had to rout them initially.  They do not understand that if they simply put a waterproof disconnect coupler 6' from the transducer you would not have to spend the time pulling cables out & back in.

You will also find that the companies who make these units may not repair them, but simply replace an inoperative unit with internal sub-panels.   It seems to be cheaper to do that than pay a technician to troubleshoot a serious problem.   

In use, these units read what's under the water surface and relay it to a mini computer screen.   The screen can be set at different scroll speeds.   It will only scroll from right to left.  The RH side being what is under the boat, with what is scrolling to the left will be history.  How far and how fast depends on how you set it up.  Your boat can be anchored and the screen will still scroll, the screen is not viewed as if the boat were moving.  It could be, but think about it for a second, if the screen did not scroll and all the fish symbols stayed say at the RH edge eventually they would all be stacked on top of each other where you would not be able to distinguish what was there 3 hours ago from what is there at this instant, making the unit totally unusable.  Or if they only showed at the instant you were over them, then someone would have to be watching the screen every instant thereby not having any time for fishing.

The old models that you just turned on with a toggle switch and that was it, have mostly disappeared.   Now there are push buttons and toggle switches that do a multitude of operations.   I defy anyone to get the best out of their unit if they do not have the owners manual nearby and then study it.  But it seems that the printed manuals are made for computer geeks who understand all the newfangled language.  These manuals are not written for the average person.   We also may not understand the terminology in the manual or even realize what it means in relationship to what we may need.  Most of us fish way less than we would like and if your memory is anywhere near like mine, I can not remember which buttons I pushed or in what sequence to achieve the desired effects from the last time.   Sure the unit has memory and probably went back to what settings I had it on last time, BUT if I don't understand and push the wrong button, then OH $hit?

I suggest that when you purchase a new unit, then get it installed, that you spend an afternoon on the lake trying it out before you charge off on vacation and have to do a crash course with little time to understand what is needed.   Most of us never use anywhere near all the features available on these units anyway.   With most of the newer units the average fisherman may only use a fraction of the capabilities of the unit.

There is a difference in a depth-finder and a fish-finder.   They can and are most times the same unit, but just used for different purposes.  To me, a depth-finder is just that, and you do not care what else you see.   Where the fish-finder really gives you meaningful information if you know how to use and understand it. 

Cone angle is important in that depending on the depth of water that you normally fish.  The sonar signal is sent from the transducer usually mounted on the stern of the boat.   This signal starts as about the size of a half dollar and flares out as it gets farther away from your transducer, or the surface.  This signal bounces back to the same sending unit where the depth is measured in the fractions of a milli-second that it takes from the start to get back to the receiver.   Your unit is constantly sending and receiving signals.  Cone angles may be from 8 degrees, to 14 or even 35 degrees.

If you are fishing a shallow lake or bay, then you need a wider angle of a sender unit, as compared to fishing in deeper ocean water 200' or more.   The reason here if you have a narrow angle cone, the spot you will be seeing at say 20' may only be 2' in dia.  This may be good for a depth-finder, but not very good as a fish-finder.   Needless to say you will not be seeing very many fish, as compared to at least double or even triple that size on a larger angle cone.

Some transducers are designed to be mounted inside the fiberglass hull and actually shoot thru the glass.   In mounting it this way, you have to have no air bubbles between the transducer and the hull.  The normal method of installation, is to build a dam of modeling clay around the transducer, then pour fiberglass resin to bond things together.   The problem here is that if you ever need to repair it, getting the transducer out may be a problem.    So many boaters simply use silicon caulking as a bonding agent, being sure there are no bubbles between the unit and the hull.    The problem here is that the transducer has to shoot thru the hull only, so there has to have no insulation, boards etc. between the two.   I have also found that the effectiveness of the unit is diminished somewhat also.

Viewing screen size and how clear the picture is will be determined by the number of pixels available.  Examples may be 240 x 320 or 640 x 480.

Power of the units will be spelled out in wattage, like 100 to 500watts.  Power also designates how deep your unit will read the bottom at.   Freshwater will allow the unit to read deeper than if it is in saltwater.  Of course more power also equates to more cost of the unit. 

Prices can vary from $150 to $2600 depending on all the bells and whistles attached.

Alarms can be preset for shallow, say at 5', or a deep alarm if you do not want to go beyond say 120'.   This shallow water alarm is something that is VERY VALUABLE and may help keep you from going aground.   The deep water alarm could be set if you find the fish are holding in a water depth and you are fishing near the bottom, so you do not want to slide out into deeper water.

Most of the newer units, you can split the screen to allow you to see the whole picture on one side and then possibly just a section on the other side.  Here you could zoom in to only see the bottom 40', while still seeing the rest of the water column in the other part of the screen.  You can also set the brightness of the screen, depending on your lighting situation.   They are improving things all the time and I now see where it is advertised on some brands that the screen does not fade out in bright sunlight.   You can also turn on the backlight for evening fishing.

If you get a unit that also reads speed and temperature, these will usually have an extra rear mounted module that have a built in paddle wheel and thermometer to also show boat speed plus water temperature.   These require an extra cable routed from the stern to the power head.  

Some units have what they call "Fish ID" which when set in this mode will allow you to see a fish symbol with a depth number near it.  One factory representative, suggests that you turn OFF this feature and only use the fish arches display.   He says that the "ID" is not that good in that it may show about anything in the water as a fish, and that it is really only for demonstration purposes to sell the units.

You will have to get used to what your unit shows, and each may be somewhat different.  Some of the older ones only saw the air in the air bladders of fish, this is what signaled the unit that there was a fish down there.   However these units thought air bubbles from rotting vegetation on the bottom were also fish.  

Some units you will just see blips on the screen indicating fish.   Most units ideally will show "Fish Arches".   What this means is that the symbol that you see as a fish is an arch on the screen.   When you see a complete arch, you are right over the fish, if you see only part of the arch, the fish may be on the edge of the sonar cone, and you do not get a clear view of it.  These arches will vary in size depending on the size of the fish.  It takes a while to understand and actually determine just what these arches mean.

Some makes have come up with a  method of separating a fish image from the bottom.   This would be important to a sturgeon fisherman where he could see a "Fish" on the bottom, instead of what may look otherwise like a submerged log.   On my Lowrance LMS 337CDF I have seen sturgeon just off the bottom and could even see the arched tail on one large fish.   Some experienced people say that they can tell the type of fish they see on the screen by the image and or color.   This may however take some experimenting to be able to tell a dogfish shark from a salmon for most of us.

I have seen it on occasion where your sonar will not read below 6' even if you are in the ocean and have the bottom at 300'.  Apparently this is caused by algae in the water.   It is so fine that you can not see it, but your sonar signal can not get thru it.

In somewhat reverse to the above, if you are running at a plane in shallow water, then loose the bottom numbers on the screen, you will probably need to increase the ping speed as you may be going faster than the sonar is bouncing back up to the receiver section in the transducer.

One last thing, most units will have a "DEMO" mode.  This is basically so the salespersons can have a unit running while it is on display to give you some indication as to what to expect.   It will show a bottom, many fish, etc. as an example of what you may expect to see.   Be sure to turn this feature off, otherwise you may run aground in what appears to be 40' of water and you could even name the fish as they repeatedly swim by.

If you are having problems understanding what the nomenclature is or what the manual is talking about, there is one book available by Frank Amato Publications, "Depthfinders - A Guide to Finding & catching More Fish", or through this website which is run by Steve Chamberlin, who does seminars.

Monochrome screen with fish ID symbols

Color screen showing hump in bottom & fish arches

Garmin 441s combo sonar / GPS



Different Types of Units;  
The older type units (mid 1960s) are what is called a "Flasher".  This is a rotating ring with a small neon light attached that flashes at a number printed on the outer sides of the unit when it the bottom relates to the number.  Most fisherpersons have never seen one of these units, so may shy away from them nowadays.   But they still have their place in fishing.  Using these the flash of the light when indicating the bottom will vary from wide to narrow depending on the type of bottom.  If the bottom is hard, the flash will be a narrow flash as the echo bouncing back will be quick.  If the bottom is muddy, then the echo signal will be delayed because of the spongy bottom and the flash will be wider.

Most of these units were made in 120' or 240' models, with the flashes going around the second time for the maximum depth.   This can get confusing if you just turn it on and are not aware whether you are at 50' or 150'.

Fish will show as small flashes at whatever depth they are at.   And you can see the bottom, plush large fish suspended with baitfish being represented by many small flashes possibly above the large fish.   Nothing is saved as like the newer units where the screen is scrolled, what you see is what you get.   So you would have to be constantly watching if you wanted to know what was happening.  The one good thing about this type of unit, they are more reasonably priced however may now be hard to find in that the normal chart types are so common.  They work great as a depth-finder if you happen to be running at night and need to know immediately what the depth is.   Say your unit uses a 60' screen, then if the flash is at 3 O'clock position, you would be at 15'.   You really do not need to read the 15', as you can quickly relate to the flash's position on the screen.

      "Normal Mono Screen"
This will be the type of unit that most of you may be familiar with.  These will have a mono, (black and white) screen, with all the shades of gray.  Most units will come on to an AUTOMATIC mode.   This can be fine under many conditions. but if you need to get the most out of a unit, you will need to set your preferences.

Most  units now you can go into the "Menu" and have the depth show as a large number on the screen along with viewing the graph, which also shows a calibrated side scale.   You can turn on the temperature and speed numbers also.   Temperature is very handy to have when trying to locate fish that are known to seek a known temperature.  You can usually adjust for a "Gray Line", which allows you to read the bottom better, separating fish from structure near the bottom.  You can also set the scroll speed, which gives you a history of what you saw that was under you.   When looking at this type of screen, look only at the RH edge, as that is what is happening NOW.   Anything beyond that on the screen is history and you have passed beyond it. 

You can usually  zoom  in or out to show different sections of the water column.   Also you can adjust the sensitivity to improve the screen readability.  If you are not seeing any fish arches on the screen, but you are sure that there should be some there, maybe you have adjusted it too low and are the screen is washed out.

Many units are so good that you can see the downrigger ball on the screen.   If not the ball, then the air bubble trail or disturbance behind it.   This is a benefit at times to be able to place the ball at the right level that you are seeing fish arches.

      "Color Screen"
Basically the same as the normal chart unit but it has color.  This can be beneficial in that for some people the color helps define better what is down there.

     "Dual Frequency Color"
Dual frequency units will usually be  50kHz / 200kHz.  The 50kHz  uses a 35 degree cone angle, while the 200kHz may use a 12 degree.  Now here is something that can really help if you are fishing deeper ocean type water.  I like to use the dual split screen with the dual frequency unit.   With the normal 50kHz, you may see "General" conditions, but the 200kHz shows way more detail but less depth.   Using the split screen, I can still see what appears to possibly be fish & other debris.   Bottom is usually brown, with the main water column blue.  Anything in the water will show as green/yellow, orange, or red depending on the concentration.  On the 200kHz screen right beside it I can see RED spots that are fish.  If there is bait in the area, depending on the concentration, it will defined in in a yellow/orange or red, depending on the concentration as well.

The 50kHz  side will usually be more cluttered (lots of green lines) because it is scanning a larger area.  This is generally more when you are running.  You can change the sensitivity to remove most of this, but it will also decrease what you need to see.  Part of the reason for the excess of clutter is that since the cone angle is 3 times as wide, you are picking up a lot more than off the 12 degree cone.  Most fishermen just live with it, and they learn to read what they see when they are trolling, (which a lot less than  running).

Newer transducers now seem to be made in 83kHz / 200kHz.

     "Going Digital"
These new units may also be known as High Definition.  When you replace your old units, it will worth your while to consider going to a HD unit.  These new units provide a lot more viewing than you have ever seen before.

These are transducers that are mounted on the sides of the transom, just under water and they are designed to took sideways.   They give a reading of an object on that section of the screen, out the same as if you were observing the regular screen.   This can be good if fishing an area where you may be casting from the boat & using this technology be able to "see" to the side.   Some units have provisions for only one, but most that are set up to handle them are for two sidefinders, out to 200'.

"Recording Chart"
These units are the older more expensive versions using a paper roll that records your travels.   The newer type units allow you to plug it into a computer and download your travels onto a GPS chart.  This can be beneficial if you want to find out where you have been, or where you made a circle to come back to re-fish an area where you just caught the big one.

  Observances ;  One well known fisherman runs his lures back about 60' from the release, then when watching the sonar screen, if he sees a fish either above or below this cannon ball downrigger weight, he immediately will either raise or lower the ball to match just above the fish's depth.   By running the lure that far back, it allows for the time involved to move the lure into the fish's depth zone by the time the lure gets there.  He says he would rather target one known fish than to troll for an hour hoping to drag the lure close enough to hopefully get a bite from an unknown fish.

Some salespersons may tell you that you may not be able to run more than one sonar off the same boat at the same time.  My response is they are listening to the manufacturers electrical engineers as to that there may be a potential for this to happen, so they are priming you, kind of like "I TOLD YOU SO".   I have personally run 3 separate sonars and of 3 separate brands on my ocean boat where one of them had dual sidefinders, and I have not had any interference.

Also, I am amazed at at how many fishermen who when running turn off their sonar and use only a GPS plotter.  Most of these units have the ability to split the screen so they can have BOTH the plotter AND the sonar on the screen at the same time.  OK if the unit is small, (say a 5" screen) maybe they feel they need a larger plotter screen, but most of these units, the viewing window for the plotter can be widened more than the existing 50%.  The sonar screen will be smaller of course, but it really can be narrower as what is important is right under your boat NOW, not what you have already passed over.  If they forgo the sonar, they may well run over fish while trying to follow a radio fish report.   No sense of running 5 miles when there are fish arches a lot closer.

Or they may turn off one or the other when fishing at anchor.  If they are afraid of running their battery down, they need to invest in something better or install a 2nd battery.  To me, a fisherman on the water in these modern times without a sonar functioning is like a house without windows.

OK my rants are over, (for a while).

Life Expectancy of Electronics ;  I hate to say this, but most electronics are only warranted for 2 years.  And after another 3 or 4 years, the factory may well not be able to repair them because of the fast changing technology, the parts for the older units are non-existent.  After that the factory may offer you a upgrade for about 1/2 price of a new unit.  If you wait too long, then this upgrade falls by the wayside.


Copyright 2004 - 2016 LeeRoy Wisner  All Rights Reserved

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Originally started 02-15-2004 Last updated 02-16-2016