Determining Direction Using Straight Sonar

[Chris Bray]

Saw this topic from 2017, but thought I'd revisit it. I'm new to fish-finding electronics. I understand the theory of how the "blips" appear. I operated radar in the navy, so I get it. But with radar, the signal is being sent out from a rotating antenna. So instead of the aircraft passing through the radar beam, the beam is passing through the aircraft - which is how we know the direction of the blip. It's much more complicated than that, but I get it. The transducer doesn't rotate, of course. It's just sending out an omnidirectional signal and painting anything in its path.

 

Another analogy would be radio signals. You can hear the radio signal, but you don't know in which direction the transmitter is located. But at least in this situation, if you move away from the transmitting antenna, the signal on your radio gets weaker. So at least you can turn around and "chase" the transmitter with your receiver. You've now made a reasonable "guesstimate" on which direction the radio tower is, 

 

So I'm trying to apply that to the transducer mounted on my pontoon boat.

 

Let's assume, for the sake of discussion, that fish are (mostly) stationary. As a diver, I watch fish. They're not travelling from reef to reef, they're hanging around an area that they've decided in their little fish brains that they like. At it's slowest, my pontoon boat travels at about 2.5mph. I see blips. Let's also assume, also for the sake of discussion, that these blips are fish. Big fish, little fish, bait fish, whatever. So these fish that appear on my screen are the ones that I'm travelling over as my transducer shoots its signal towards the lake floor. I'm imagining my transducer is like a flashlight, shining down. 

 

What I'm getting at is this: 

 

As I travel around looking at my screen, isn't it reasonable to assume that these blips are either below me or behind me? Seems to me that it would be a waste of time to cast off the bow. 

 

While I'm at it, here's something else I've observed:

 

When moving, I get the "arches". Big arches, little arches, bright arches and faint arches. One after another, at all depths. As soon as I cut the engine and slow to "adrift", they all disappear. I can sit there and stare at that screen for an hour and never see an arch. Happens every time. If I'm using a really big bait, I can see it travelling through the water. When I throw my anchor, I see it travelling down through the water. I even get paints of the anchor rope. But no arches. 

 

I've also read that if the boat is stationary, and there is a stationary fish under me, there should be a straight line being painted. Makes sense. No one is moving, but the object is still there, reflecting the signal of the transducer. 

 

Can somebody please explain why that is? 

 

Hoping @Glenn will read this and weigh in...

 

Chris

 

 

[MN Fisher]

10 minutes ago, Chris Bray said:

As I travel around looking at my screen, isn't it reasonable to assume that these blips are either below me or behind me? Seems to me that it would be a waste of time to cast off the bow. 

The signal is sent in a cone - normally 30-degrees. So the fish that appear on 2D screen (Not DI or SI) can be anywhere in that cone - in front, to the sides or behind.

[Team9nine]

In short:

 

Arches are created by the boat moving over a fish, or the fish swimming under the boat, or a combination. The length of the signals path at the far edge of the transducer “cone” is longer than the ones directly under the transducer, so you end up with an arch.

 

When the boat and fish are stopped, you get a solid horizontal line and no arch.

 

As MN Fisher stated, you can’t tell direction in 2D. However, weaker signals will be to the outside edge of the cone, and stronger signals more directly under you. This is usually represented by the thickness and/or color of the mark, as well as the shape.

 

Obviously other variables involved, but this is a starter.

[Chris Bray]

14 hours ago, MN Fisher said:

The signal is sent in a cone - normally 30-degrees. So the fish that appear on 2D screen (Not DI or SI) can be anywhere in that cone - in front, to the sides or behind.

Sure. I understand that. 

But to your point, if the fish is stationary, and you are moving along (at whatever speed), the fish cannot appear behind you. It can only appear to the forward side or in front of you.

 

And to @Laggard's point, I think you can make some assumptions as to what direction to cast. 

Assume the depth is 15ft. 
Assume that cone covers an area around the boat that measures 30ft. 

Assume that the fish is stationary. 

Assume the fish is on the bottom, and is directly in front of you (ahead, in nautical terms). 

Assume the boat is travelling at 2mph, which is appx 3ft per second. 

Using those assumptions, by the time the blip (arch) is completed, the fish would be on your nose (ahead) at 29 feet. In 10 seconds, if the fish remains stationary and you do not change course, the fish will be 30 feet behind the transducer (astern). You'd also need to take into consideration where your transducer is mounted. If on the bow, and you're in a 15ft boat, then in 10 seconds, the fish is now 15ft astern of (behind) the boat. Conversely, if the transducer is mounted on your transom, the fish appeared on your screen when it 15ft in front of your boat. And in 5 seconds, it is directly beneath you. 

 

But wait - there's another consideration and I want you to know that I'm taking that into consideration: The shape of the blip. If the fish is stationary, the blip is going to be long and shallow, because the apex of the blip will not occur until the fish is beneath the transducer, where the signal (and the return) are the shortest. A blip that is tall/deep and narrow would indicate a fish that is on the move. Just sayin' I realize this. 

And of course you throw in the variable of "where did the fish enter the cone?" Directly in front of you, or off your port or starboard bow (45 degrees left or right)? Port or starboard bow (90 degrees left or right)? If the fish is stationary, it could not have appeared on your port or starboard quarter, because the cone shrinks at that point. Only a moving fish can enter the cone off your quarters or stern.  

If the fish entered the cone off the starboard bow, then your times shrink. But by shrinking, the fact is that your target is already behind you in seconds - so it makes no sense to cast in front of the boat. You should be casting off to the side and fanning back towards the back. 

Right? 
 

[Team9nine]

2 hours ago, Chris Bray said:

 

And to @Laggard's point, I think you can make some assumptions as to what direction to cast. 

Assume the depth is 15ft. 
Assume that cone covers an area around the boat that measures 30ft. 

Assume that the fish is stationary. 

Assume the fish is on the bottom, and is directly in front of you (ahead, in nautical terms). 

Assume the boat is travelling at 2mph, which is appx 3ft per second.

Right? 
 

 

That’s a lot of assumptions that have to be right in order for your guesstimate to pan out ? 

 

...but it all could go that way occasionally 

[redmexican5081]

IIRC, cone diameter is ~1/2 the depth for most 2D sonar units.  In shallow water, if you are moving, by the time you see the arch on the screen that fish is behind you, left right or anywhere in between It passed through the cone to register on the screen but only the pixels being filled at the very right hand side of a scrolling display will show you real time information. 

[InfantryMP]

18 hours ago, Chris Bray said:

As I travel around looking at my screen, isn't it reasonable to assume that these blips are either below me or behind me? Seems to me that it would be a waste of time to cast off the bow. 

Yes.

 

On 2D sonar, the far right of the screen is your boat. Everything to the left on the screen is what the sonar has painted. Meaning your on it, or moving over it. If you have side scan, you will be able to gauge better the sides of the boat the fish are on, and distance from the boat. 

Also it depends on where your transducer is mounted as well for your 2D sonar. 

 

I have mine mounted on trolling motor, but if it were in the back of the boat, then I am physically already passed what has been painted already. My Side imaging and Down imaging transducer is mounted on the back of the boat. When I look at those, and see something I need to understand I have already passed it. I will typically mark it on the graph, and then turn around if I want to fish it. I would need to fan cast that side where I saw it, and the depth range it showed up at.