depth finders

[wargofcak]

Hey I am new  to the website.  I have a question about depth finders.  Where does the sensor at the back of the boat read in the water and how come the screen is always moving?  I am a marine in afginistan and the only fishing i can do out here is from a magizine and answering the question that bother me thanks alot.

[Turtle]

All a depthfinder is a sonar unit, it shoots soundwaves through the water and when the waves hit something it bounces back.  Most transducers shoot straight down.  Some transducers are mounted with a bracket on the back of the boat and have the ability to swivel, backwards, straight down, or towards the front of the boat.  There are also transducers that are epoxied into the fiberglass in the hull of the boat, those always shoot sound waves straight down.  the advantage of the transducers epoxied into the hull is that they won't get damaged by underwater hazards.  The reason a depthfinder constantly scrolls across the screen is because it is constantly regenerating an updated reading of what it is seeing.  If you are sitting still in your boat and there is a fish below that is sitting still below it, the screen will show a straight line.  The sound waves are constantly reflecting the same area of the fish, hence giving you the same reading on the screen.  If you are moving your boat and it passes over a stationary fish it will show an arch.  As the sound waves pass over the tail and move toward the head it will give you arch, just like the top profile of a fish, or it could be head to tail.  If you are stationary in your boat and see a line that is moving across the screen but is moving upwards or down there is a good chance that fish is actively swimming through the cone (area covered by your sonar).  The Lowrance website has a great tutorial and can probably answer most of your questions.  Thank you for your service and stay safe, trust me, we have left plenty of fish for you.

[papa1]

'Morning, Marine. Thanks for your service. This old Sailor (radar tech) will tell you what little I know about depth finders. Maybe someone else can fill in the shady areas for both of us.

"Where does the sensor at the back of the boat read in the water" Generally speaking, directly under the sensor. The width of the beam depends on how deep the water is. The deeper the water, the wider the beam where it hits the bottom. Mine is a dual beam, so it shows solid color fish in the main beam; outlined (ghost) fish in the wider beam.

"how come the screen is always moving?" The screen appears to be moving because the depth finder is updating the information it is receiving from the signal it sends out.

Basically, a ping is sent out then recieved in an instant. The computer breaks down the returned signal as to depth and bottom contour as well as any obstacles or irregularities within the beam width. If you stay in the same spot, it refreshes what lies directly below the transponder. If your boat is moving, then each time it refreshes the presentation, it is showing the depth, bottom, and obstacles in the new location the boat is in.

That's the quick and dirty. If you want to read more about depth finders, Humminbird has a great site filled with information. Here's a great tutorial that will answer about any question you might have. Beams, beam width, theory of operation, etc.

http://www.humminbird.com/generic.asp?ID=403

I hope I was able to shed a little light on the mystery of depth finders. I'm going to go back to bed now and try to get some sleep. I will say a prayer for you in the meantime. Stay safe, Marine.

papa1

Looks like I need to modify my profile. Gender's wrong. Just what I needed. ;D

[Glenn]

Hello,

The best way to remember what you're seeing under the boat is to remember the "1/3rd rule".  That is, for every 3 feet of depth, the transducer is painting a 1 foot diameter circle under the transducer.

So if you're in 10' of water, you're seeing a 3' diameter.  If a bass is shown on your screen, he could be anywhere from a foot and a half in front of or behind the transducer (or to the sides).

For much more detailed information, see this article:

http://www.bassresource.com/fishing/fishfinders.html

Enjoy!

[hambone]

Thanks for your service! Be safe and America appreciates you.

Now as far as depth finders go. I understand radar theory, I just have a problem finding where the structure, drop off, fish, etc are in realtionship to what I see on the screen. If I show a fish at 7' does that mean he is under my boat, to the side of the boat (the beam is like an upside down ice cream cone) That is my biggest problem, but I'll use the 1/3rd rule i read earlier. I will fancast into the area I think the fish are showing.

I truly think that is my weakest area....understanding how to read my electronics. I'll keep reading this thread and hopefully we all get more educated.

Semper fi

[Chris]

Where does the sensor at the back of the boat read in the water and how come the screen is always moving?

The sensor sends sound waves that bounce off the bottom. The higher the scroll speed the more frequently the senser sends the sound waves. Your depth finder is always moving because its updating itself between sound waves.

[Glenn]

The higher the scroll speed the more frequently the senser sends the sound waves.

I'm sorry, that's incorrect.  The two are not related.  Please read the articles on this site for further explanation.  

Honestly, I thought I knew everything about them until those articles were submitted.  I then had a Lowrance executive take me out on the water for the day and we did nothing but paint the bottom.  Very informative!

I'd also highly recommend Don Iovino's book and video on sonar.

[Chris]

Well Glenn ya made me put on my jacket and hook up my depth finder. lol I'm not saying i am wrong or right but try it yourself listen to your transducer at hyper scroll and at a low setting and tell me if there is a difference.

P.S. I am glad to see that my batteries still work ;D

[hambone]

yeah, I also put a jacket on and went and looked at my depth finder, the only thing I changed was the fish ID on/off. I read in one of the articles that Glen posted that said that if you turn the ID off you won't get so many "fish" symbols on your screen. That was some great reading, Glen, Thanks for posting that. I'm going out tomorrow and study my electornics, my wife even said she would go! This might turn out to be a good year

[Glenn]

I stand corrected.  The pulse length (the amount of time between "clicks") is related to the resolution, or detail, diplayed on the screen.  Resolution, in this case, is the ability of a sonar to separate targets. Scrolling the chart speed faster requires more detail.  So it stands to reason the pulse length will shorten.

That said, pulse length is also determined by depth.  Too short a pulse length in deep water will result in wrong readings, so it needs to be lengthened regardless of chart speed.  

Lowrance units automatically adjust the pulse length when the depth changes. I believe you can manually override this too, but I'm not sure because I've never tried it.  

Keep in mind this is but one of many factors that determine what is shown on the screen.

[Bud_in_OR]

Hey Wargofcac,  

Think of your screen as the left window of your truck as you drive down the road (safely, we hope). Everything that is visible behind the right side of the window is stuff you have already passed. If you are going real slow, what you see at the right side of the window is just behind your transom. If you are going fast, what you see is a long way back.

Something that is extremely important is understandind 'dead zones'. The machine sends down an inverted cone. It gets wider as it goes deeper. Say the center of the cone is exactly 20', transducer to bottom. On a flat bottom, the  distance at the edges of the cone will be  22', transducer to bottom. The machine will NOT read farther than the closest thing that enters the cone,...the bottom at center.

The edge, from 20' to 22' will NOT be read. It is a dead zone, a 2' thick black line. By the same token, If you are running your boat parrallel to and over a sloping bottom, the machine will read NOTHING below the closest point that the beam touches. So... If the bottom slopes downward, from 20' to 30' as you travel along the slope, you will see a ten foot thick black bottom line from 20' to 30'. This is a dead zone. Everything deeper than the closest cone, contact point  at 20' will be blacked out.

To read the slope you must run UP or DOWN it. As you travel up the slope the front of the beam 'reads' and what you see will be at the front of the cone(perhaps at the center of your boat in 30' of water). Every thing behind the front edge of the cone is in a dead zone. As you travel forward the front and back cone edges will , eventually, both be over the slope edge at equal depths and the bottom band will be very thin (remember the original scenario).

If you travel down the slope, only the back edge of the cone will 'read'. As you progress past the edge of the slope with the back cone edge at 20', all things in the cone angle AHEAD of the back edge is in a dead zone and the back edge(what is being read) is ten or so feet behind your boat. Again, there will be a ,10' thick, black bottom band  that gets progressively narrower as the back and front of the cone get closer to touching the bottom at the same depth, 30'.

If this doesn't make sense, get a chalk board and draw the bottom configuration on it that I've talked about. Also draw the surface. Get a piece of cardboard and cut out a 20 deg. cone and a little, teensy-tinesey boat. Lay the boat/cone on the chalk board and it should become clear.

Remember, nothing, in the cone angle, that is below the closest contact to the transducer will be read. All machines, regardless of make or model,work in this manner

Oh. Oh. By all means order Iovino's book and, FOR SURE, his tape. Any one who buys a sonar should get these as a freebie.

Hope this helps

Bud

[Bud_in_OR]

Hi,

I screwed up in the first paragraph of the post above.

What I should have said is; As you are driving, looking out the left, drivers' window, what you see at the right side of the window is what is happenning NOW, under your transom.

As you drive, what was,'NOW' is passed over and behind your transom. Only things/data seen at the absolute, right edge  is real time. The screen is registering things that you are moving away from. It shows past history (an Iovinoism).

While I'm at it, be aware that useing 'fish symbol' or ASP(Lowrance) de-tunes your machine. You'll likely not see baitfish, unless they are huge, with these operational.

Also, unless fish are stationary as you go over them, you wont see arches or hooks. A fish moving with the boat will be a straight level line on the screen. One moving up or down, trying to get away from the boat,  or moving in and out of bait will show a short diagonal line.  This is good. It shows activity (also an Iovinoism).

If you do see arches, do not worry about length of arch for reading size. Thickness of arch is what shows respective size. If the fish is run over fast, the arch will be high and  narrow. If it is on one side of the boat/cone or the other it will have a tail, one side pointing down farther than the other.

Left or right side of boat? You don't need 'fish symbols' to tell you. Read the 'tail' on the arch. If the 'tail' is on the right side of the arch, the fish is on the left side of the cone. A fish arch in the cone center will have no long side/tail. Tail on the left, fish is on right side of cone.

Look at www.iovino.com and get his video. You wont be sorry.

I'm gone.

Bud