Question About Fish Finder

[Bigmikerc]

My son and I are new to fishing. We bought a boat that has a Lowrance 510 on the front and a x25 in the dash. I noticed today when we were trolling around that fish kept showing up on the screens. So heres my question, When you see fish on the screen, how do you know where they are located based on the screen? It gives me a depth but I am unsure of the general direction or location. I noticed on that the arrow on the trolling motor appears to point in the opposite direction as the large part of the sensor. I'm sure this is a stupid question but all help would be great.

[KevO]

If you look at the screen from right to left... a fingernail width from the right is directly under the transducer you are looking at. If you look at the fish arches you can tell which side of the boat they are on if its only a half an arch in either direction also. I wish I could explain it better without a screenshot sorry. Hopefully WayneP will chime in he knows much more than I do.

[NBR]

Not a stupid question. If the arch is symetrical the fish is right under the boat. If slewed it is off to one side. Always the fish is 6 feet or more behind the transducer. If the fish is a straight line it is moving along with the boat. Same thing if the boat is standing still. I am always a lot more interested in the structure and cover than the fish images.

[Jig Man]

When a fish shows up say 20' on the screen it can be 20' in any direction as long as it is in the strong cone angle. Just because it looks like it is below you doesn't necessarily mean that is so. It may be suspended in any direction.

You would be way better off locating cover or structure and fishing that instead of trying to catch something that shows up as a random arch.

[Wayne P.]

With regular sonar, the shape of the sonar pulse is somewhat an inverted cone that gets larger in diameter as the depth increases. Anything that can reflect the sonar pulse will show up on the display but it's direction is not shown and it is impossible to determine which direction. An incomplete fish arch just means the fish was not directly under the transducer so it could not be displayed at the variable distances it takes to create the arch shape. Refer to the tutorial on the Lowrance web site about interpreting sonar.

Only the very first right side vertical row of screen pixels is current data. Everything to the left of that is history. If a return is not constantly being recorded on the first row, it is not within the coverage of the sonar pulse.

For a common frequency 200 kHz with a 20 degree cone angle, the amount of coverage is about 1/3 the depth but at the spec. strength (usually -10 db). The actual coverage can be a lot more at weaker strengths. I have seen some tests that show a recorded return of a floating object with the 200 kHz frequency.

The distance from the boat position to a displayed return depends on where on the screen the return is recorded, the screen scroll speed, the speed of the fish, and the speed of the boat. It can be any distance. Just remember all recorded data is HISTORY.

There is technology that will indicate which side of the transducer a return is on, and technology that will show actual direction and distance from your present location to a return.

[Bigmikerc]

I think I get this. So whatever is shown in the right first inch is currently whats under the trolling motor transducer. Everything after that is history. So if I see fish, brush pile, or a step edge on my screen then that is actually in the middle to the back of the boat. Does this matter if the front of the transducer is pointed in a different direction?

[Jig Man]

I think I get this. So whatever is shown in the right first inch is currently whats under the trolling motor transducer. Everything after that is history.

Not necessarily. It just means that it is what is currently in the cone. Like Wayne stated the cone angle on a 200 kHz unit is normally 20° which gives you a diameter at the bottom of roughly 1/3 of the depth. You also have the cone from the ducer to the bottom and stuff shows up in it not just on the bottom. Where it shows is how far from the ducer it is not where it is.

We would all like to thing of it like a camera pic but it isn't.

[Wayne P.]

I think I get this. So whatever is shown in the right first inch is currently whats under the trolling motor transducer. Everything after that is history. So if I see fish, brush pile, or a step edge on my screen then that is actually in the middle to the back of the boat. Does this matter if the front of the transducer is pointed in a different direction?

No, not the first inch. The first right side vertical row of screen pixels (they are tiny things). The second row vertically is HISTORY and so are all to the left of that.

The Lowrance 510 screen is 480 pixels wide and 480 pixels high.

A 2D sonar pulse is not directional so the transducer can be "pointed" in any direction. If you pay attention to when a return is recorded and use a visible reference point when that first row if recorded, then you maybe can determine where that return is.in relation to the boat. As I stated before, if something is not repeated on the first row of pixels, it is not within the coverage of the sonar pulse.

[Bigmikerc]

Thanks for the info, I will give it a try again in the morning.