Sonar Distance Behind Boat

[Baxter]

First to Admin: Wonder if there is a need for an "electronics help" type forum somewhere as well?

--------------

Next my question. Is there a formula for figuring out how far behind the boat your sonar reads? See image if I don't make sense

Using the depth, and perhaps knowing the angle from the Transducer I imagine a combination of Pythagorean Theorem and SohCahToa a person can do the math to figure it out, but the problem here I assume comes when a combination of boat speed and ping speed come into play, perhaps with other additional factors. I imagine the answer comes with more than just simple "triangle math".

I have searched to try and find a tool or calculator to help figure this out and have come up blank (so far). I hear some units display this figure, but as far as I know Lowrance does not (or at least I have not found where in the settings I can turn this on).

May be good to know considering I can easily figure out how far behind me my gear is and at what actual depth.

 

[easydoesit]

First to Admin: Wonder if there is a need for an "electronics help" type forum somewhere as well?

--------------

Next my question. Is there a formula for figuring out how far behind the boat your sonar reads? See image if I don't make sense

Using the depth, and perhaps knowing the angle from the Transducer I imagine a combination of Pythagorean Theorem and SohCahToa a person can do the math to figure it out, but the problem here I assume comes when a combination of boat speed and ping speed come into play, perhaps with other additional factors. I imagine the answer comes with more than just simple "triangle math".

I have searched to try and find a tool or calculator to help figure this out and have come up blank (so far). I hear some units display this figure, but as far as I know Lowrance does not (or at least I have not found where in the settings I can turn this on).

May be good to know considering I can easily figure out how far behind me my gear is and at what actual depth.

Until my retirement a few years back I worked in the sonar industry. I am not a technical expert, but I think I can help you out.
The good news is that several manufacturers provide charts to help.

First some terminology. Most often fishing sonar transducers are single beams, oriented vertically, pointing down. Nearly always the case with simple depth finders. The transducers emit pulses of sound downward and outward to some extent. The beams of sound are focussed electronically. For practical purposes using simpler fish finders, it is often referred to as a “cone” and the beam angle is defined by degrees. This relates directly to what you “see” under your boat. Multi beams and side scan sonar operate on the same physics on a related but different basis.

To use one of my transducers as an example, it has a beam angle or cone of 45 degrees. Airmar has a chart on their website that you can use to estimate the size of your cone at different depths. According to Airmar this transducer would produce a beam diameter of 80 feet at 100 feet depth. I expect Lowrance and other manufacturers can provide this info.


Lets say you have your gear at 100 feet depth and are slowly drifting or stationary. Your gear will be visible within that 80 foot diameter sonar cone, no problem. Now you start moving and your gear will start to “blow back” and extend further behind your boat due to drag caused by your cannon balls, Downrgger lines, flashers and so on. The deeper your gear is and the faster you troll the more blowback there will be, and it is not uncommon in salmon trolling to lose sight of your gear.

Now there is another calculation required, how to predict roughly the extent of blowback. The fine people at Cannon downriggers have calculated this and produced charts. (I think this may have been printed in their manuals under Blowback & Troubleshooting). Variables here are downrigger weights, speed, and cable length deployed.

Other factors: Ping rate doesn’t factor into beam coverage. Frequency choice (kHz) affects range and resolution. Lower frequencies have less attenuation (signal loss) than higher frequencies. Lower frequencies have greater detection range of larger targets, but provide less detail. What you would normally be seeing with a single beam fish finder display is a time series of snapshots, one after the other, scrolling across the screen not a full screen display in real time. Multi beams and sidesscan sonar are different.

Determining the cone angle of your transducer will help you determine (in theory) under what conditions of depth and speed you can expect to track your gear. Blowback will be less of a factor.

pm me if I can be of any further assistance.

 

[Rain City]

First to Admin: Wonder if there is a need for an "electronics help" type forum somewhere as well?

--------------

Next my question. Is there a formula for figuring out how far behind the boat your sonar reads? See image if I don't make sense

Using the depth, and perhaps knowing the angle from the Transducer I imagine a combination of Pythagorean Theorem and SohCahToa a person can do the math to figure it out, but the problem here I assume comes when a combination of boat speed and ping speed come into play, perhaps with other additional factors. I imagine the answer comes with more than just simple "triangle math".

I have searched to try and find a tool or calculator to help figure this out and have come up blank (so far). I hear some units display this figure, but as far as I know Lowrance does not (or at least I have not found where in the settings I can turn this on).

May be good to know considering I can easily figure out how far behind me my gear is and at what actual depth.

The short answer is what you're seeing on the screen does not reflect what's behind you. It's a scrolling log of what you've passed over. It takes one slice and then moves it over. You're looking at slices at whatever angle your transducer is set to.

 

[Baxter]

The short answer is what you're seeing on the screen does not reflect what's behind you. It's a scrolling log of what you've passed over. It takes one slice and then moves it over. You're looking at slices at whatever angle your transducer is set to.

Yes, I do realize this. What you see on the far left of the screen is what you were over previously, but essentially it is what is behind you as you have since moved on past it. (at least with structure, not so much fish, as they move). I realize it is not a real time view of precisely what is behind you.

 

[Rain City]

Yes, I do realize this. What you see on the far left of the screen is what you were over previously, but essentially it is what is behind you as you have since moved on past it. (at least with structure, not so much fish, as they move). I realize it is not a real time view of precisely what is behind you.

Your drawing does not reflect your understanding of such.

 

[Baxter]

Until my retirement a few years back I worked in the sonar industry. I am not a technical expert, but I think I can help you out.
The good news is that several manufacturers provide charts to help.

First some terminology. Most often fishing sonar transducers are single beams, oriented vertically, pointing down. Nearly always the case with simple depth finders. The transducers emit pulses of sound downward and outward to some extent. The beams of sound are focussed electronically. For practical purposes using simpler fish finders, it is often referred to as a “cone” and the beam angle is defined by degrees. This relates directly to what you “see” under your boat. Multi beams and side scan sonar operate on the same physics on a related but different basis.

To use one of my transducers as an example, it has a beam angle or cone of 45 degrees. Airmar has a chart on their website that you can use to estimate the size of your cone at different depths. According to Airmar this transducer would produce a beam diameter of 80 feet at 100 feet depth. I expect Lowrance and other manufacturers can provide this info.


Lets say you have your gear at 100 feet depth and are slowly drifting or stationary. Your gear will be visible within that 80 foot diameter sonar cone, no problem. Now you start moving and your gear will start to “blow back” and extend further behind your boat due to drag caused by your cannon balls, Downrgger lines, flashers and so on. The deeper your gear is and the faster you troll the more blowback there will be, and it is not uncommon in salmon trolling to lose sight of your gear.

Now there is another calculation required, how to predict roughly the extent of blowback. The fine people at Cannon downriggers have calculated this and produced charts. (I think this may have been printed in their manuals under Blowback & Troubleshooting). Variables here are downrigger weights, speed, and cable length deployed.

Other factors: Ping rate doesn’t factor into beam coverage. Frequency choice (kHz) affects range and resolution. Lower frequencies have less attenuation (signal loss) than higher frequencies. Lower frequencies have greater detection range of larger targets, but provide less detail. What you would normally be seeing with a single beam fish finder display is a time series of snapshots, one after the other, scrolling @croos the screen not a full screen display in real time. Multi beams and sides sonar are different.

Determining the cone angle of your transducer will help you determine (in theory) under what conditions of depth and speed you can expect to track your gear. Blowback will be less of a factor.

pm me if I can be of any further assistance.

Lots of good info, thanks. I will see if Lowrance has any charts for estimates etc.

As for blowback, this is interesting. Of course I am aware of it, but I have always looked at it from a triangle problem. Calculate the angle from the rigger, along with the line out, and you can figure out how far back/deep things are - but I guess that is not necessarily the case. Never occurred to me until now that the hypotenuse of that right-angle is not necessarily straight. In fact, especially with currents, is most certainly not straight and rather curved. Probably the cannonball is always shallower than what simple math would state...

 

[Baxter]

Your drawing does not reflect your understanding of such.

Okay, perhaps if I worded it as (how far have I traveled since that first slice on the far left of the screen...

 

[Rain City]

Okay, perhaps if I worded it as (how far have I traveled since that first slice on the far left of the screen...

It still doesn't matter. That all depends on your speed. If you're going really slow it still "tracks" across the screen at the same speed. There's no relation.

 

[CBsqrd]

Lots of good info, thanks. I will see if Lowrance has any charts for estimates etc.

As for blowback, this is interesting. Of course I am aware of it, but I have always looked at it from a triangle problem. Calculate the angle from the rigger, along with the line out, and you can figure out how far back/deep things are - but I guess that is not necessarily the case. Never occurred to me until now that the hypotenuse of that right-angle is not necessarily straight. In fact, especially with currents, is most certainly not straight and rather curved. Probably the cannonball is always shallower than what simple math would state...


View attachment 64252

Your downrigger is actually deeper than what your simple math would give - the curve actually goes the other way where the greatest angle from vertical is near the boat, with the angle decreasing at depth. i.e. instead of convex it forms a concave line.

To your other question on how far you have travelled since you first saw the return on the screen, that depends on how fast your speed is and how fast the picture advance speed is set on your display. Also, any returns you see are from the entire cone, not what is just directly under your transducer at the time of the return; in @easydoesit 's example, at 100' you are seeing everything in an 80' circle - so from 40' in front of your boat/transducer to 40' behind. So you could have something scrolling off your screen that is actually still in front of you transducer.

[side rant] I don't know why people get hung up on wanting to see their gear on their sonar. Due to the cone from the transducer, the sonar isn't telling you how deep a fish is, but rather how far from the boat it is. Using the example above again, at return at 100' that is on the edge of your cone is going to be only ~71' deep. Likewise, if you gear is on the edge of the cone (i.e. at 71' deep, but showing 100' on sonar / downrigger counter - assuming straight line blow-back), it is going to be shallower than a fish that swims right under your boat that shows up at 100'. Use the sonar to get the approximate depth and set your gear in the general area, a couple feet isn't going to make a difference and any fish that is going to bite that sees your gear will come and get it. [Rant over]

 

[SpringFever552]

I think we need to ask the janitor

 

[BCTony]

I don't know why people get hung up on wanting to see their gear on their sonar. Due to the cone from the transducer, the sonar isn't telling you how deep a fish is, but rather how far from the boat it is. Using the example above again, at return at 100' that is on the edge of your cone is going to be only ~71' deep. Likewise, if you gear is on the edge of the cone (i.e. at 71' deep, but showing 100' on sonar / downrigger counter - assuming straight line blow-back), it is going to be shallower than a fish that swims right under your boat that shows up at 100'. Use the sonar to get the approximate depth and set your gear in the general area, a couple feet isn't going to make a difference and any fish that is going to bite that sees your gear will come and get it.

This is where knowing your transducer cone angles and coverage comes into play. If you have two transducers operating, one with narrower cone angle/coverage you can begin to determine more precisely the position of targets both geographically and in depth. For practical purposes and for most of us, to just to know that there are targets in the general vicinity is enough.