Author Topic: Towfish...Why? Serious Question  (Read 4427 times)

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Offline promapper

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Towfish...Why? Serious Question
« on: April 09, 2013, 07:57:06 PM »
Good day to all.  I am a freshwater fisherman but I also like to dabble in things not many others have tried.

The idea of a towfish interests me but I would appreciate learning the advantage of running a towfish vs a typical surface mount.

I am currently replacing a 997 with a 998 and could use the 997 unit in a towed configuration if there was a distinct advantage to fishing or even perhaps SAR as there are only a few near me that could respond to a water incident.

So, without flames please!!!  Can someone give me a very brief advantage of converting a HB unit to a towed unit?



Offline Bob B

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Re: Towfish...Why? Serious Question
« Reply #1 on: April 09, 2013, 08:25:09 PM »
I don't run a towfish, and therefore should not be considered the last word on the subject, but my understanding is that the towfish would be valuable primarily in deep water.  It will allow you to get the transducer down closer to the bottom structure and therefore get better detail.

It may not be desirable for fishing if you are interested in seeing suspended fish an bait balls.

I am sure some of the guys who run a towfish will check in and give you a more detailed answer.
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Offline RGecy

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Re: Towfish...Why? Serious Question
« Reply #2 on: April 09, 2013, 09:25:33 PM »

Bob is correct.  The lower the transdcuer is to the bottom, the better the detail and coverage for objects on the bottom, especially on smaller objects.  It also is used in deeper water where the depth is too deep for the sonar to reach.  Sometimes, bodies may be suspended in the water column and if the towfish gets too low it could miss it.  Its important to know the characteristics of what you are looking for and this will determine how high you should fly your towfish.

If the Transducer is high above an object and almost directly above it, it will not show a good shadow sometimes depending on the objects shape and size.  The shadow is usually the real key to determining what an object is.  You can determine the height an object sits off the bottom, its width and length, and also the overall shape by looking at the shadow.  Getting the transducer closer will typically produce a better shadow. 

Its also important to understand the beam angles of the transducer.  With 800khz, getting too close to the bottom will cut down on the range to each side.  With 455khz, the beam is pretty much 180deg so you can see greater distances side to side at shallow depths.

Here is a good post with images to show you how 455 vs 800 khz can affect the range.  You will notice the 800khz is clearer, but the range is cut down significantly.

You also have to take into consideration if the bottom terrain is sloping, this may cut down on the range if the bottom is rising up and the sonar cannot see past the slope.

Here is some good reading to understand distances, shadows, etc.

Hope that helps,


« Last Edit: April 09, 2013, 09:28:19 PM by RGecy »
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Offline Jolly Roger

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Re: Towfish...Why? Serious Question
« Reply #3 on: April 24, 2013, 02:44:19 PM »
Another advantage might be that you get better recordings if the water is rough. Your hull mounted transducer will be rocking with the boat, while a towed fish is still stable in the water if you are running it under the waves with enough cable out.


Offline promapper

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Re: Towfish...Why? Serious Question
« Reply #4 on: April 29, 2013, 08:32:38 AM »

Good info in the links you provided,  Thanks!  The biggest advantage as I see it in my shallow water fishing will be in having greater resolution of objects and better and larger shadow detail.  I think a Towfish is in my future with this old 997c unit.

My background is 40 Years of photogrammetry with a bit of LIDAR (LIght Detecting And Ranging) thrown in at the end.  Our 3D technology and interpretation of measuring images relies on perspective angles, shape, color and YES the shadow!

The Laser works much the same way as the SI except it uses a rotating mirror to scan from side to side at a very high frequency and each laser return has an angle associated with it making the ranges, attitude of the sensor and aircraft position an integral part of the elevation calculation.  EACH return (think gazillion) has Accuracies that are in the cm ranges from 4000 feet elevation of the aircraft.

As I see it we (SI users) are only missing the angles/attitude of the sensor to process true 3d data.  At that point, since we have depth (elevation) of each data point we can also make the assumption (or not) that anything not connected to the bottom is a fish and display it appropriately.  The same goes for contour mapping of the bottom in wide sweeps.

It all sounds good but I really think real time display will be a thing in the far future as post processing is the only way to accomplish this at this time due to processing power or lack there of.

Can't wait to try comparisons from surface vs half the depth.


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