Monday, March 17, 2014

Dialing in Your Humminbird Sonar / Down Imaging / Side Scan Unit

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***thanks to Wayne P from BBC for straightening me out on some stuff***
I had some vacation to kill on Friday before our last club tournament, so I decided to get out and look around. Initially I figured I would get some quality casts in, check some spots that I thought might be good to me for the club tournament, and if it slowed down, I was going to spend some time tweaking my Humminbird 798 which I haven't really used since I installed it, other than for basic GPS operation. But, after spending some time with Basswhacker and seeing how he utilized his when I Learned to Ledge Fish, I thought it wise to get up to speed. Luckily for me (or unluckily) the skies were very high, the wind was whipping and there was a BILLION boats on the water. I mean, it was nuts. There were 5 or 6 boats on every spot. Even on spots I had never seen anyone fish. So, I decided to start playing around with the Humminbird and stored my rods. I did manage to dial it in, which you can read my post about. I was amazed at how far off I was.

Sadly, I admit that I haven't spent any time dialing in that unit, simply because I was unfamiliar with it. I have tried to dial it in, but have been frustrated with it. In fact, I was fairly certain at one point that it simply wasn't working. I would cover miles of water and never really mark a single fish. I would dial the sensitivity up until it was nothing but noise. The sidescan would work, for the most part. I would mark rocks and stumps, but when I motored over them to inspect them with sonar, the bottom would be as flat as could be.  Additionally, I noticed the sidesscan wasn't as sharp as others I had seen. There really aren't a whole lot of knobs to twist, but the instruction guide doesn't do a lot to help you. Instead, I went to Bass Boat Central and did some reading. While every unit and install is different, and each act a little different in varying water conditions, I at least knew where I needed to be looking.

The more I thought about it, the more angry I became. After all, ultrasound is what I do for a living and I should be able to figure it out. So, instead of trying to learn what all the catch phrases and proprietary names of the Humminbird software meant, I instead read what it did. So, what did I change? How did I know it was working?

The first thing I did was find a way to aim at a stationary target. Obviously that's hard to do in a boat on the water. But, my friend John had suggested dropping a bait. So, I did just that. All I did was drop a good sized bait under the boat and dialed the unit in until I could see it on sonar. I would vary the depth and make sure I could track it. What did I change?

Well, first it is important to know what we are dealing with. The 798 comes with the XNT 9 SI 180 T transducer.  The XNT 9 is a compact transducer with  the same 2D crystals.  The SI crystals in the HDSI transducer are longer. The longer SI crystals in the HDSI transducer produce a thinner SI pulse and that produces sharper images. After doing some research, it is apparent that it isn't quite the best transducer that can be bought for the unit and I could enhance my SI capabity. alas, I am stuck with what I bought, since I bought it so cheap. So, we are already dealing with a little bit of a disadvantage, but it is what it is. It certainly isn't useless. And besides, I was most interested in making my 2D work.

The first thing you will notice about your unit is that there are two different frequencies that the transducer can pulse. It can operate at 83kHz and/or 200kHz In ultrasonics, the higher the frequency, the greater the resolution (or detail) but the greater the attenuation and scattering. What does that mean? In dirty water, the signal will be scattered a lot more. Additionally, the ability to penetrate is reduced. So, the ability to see fish in grass or in cover is diminished. On the flip side, the lower the frequency, the less scattering and attenuation and the greater penetration. However, the ability to see fine detail will be lost. The designers made it so that you could easily change between transducers for any type of fishing, whether it was bass or salt water. Depending on what you were after and where, you could dial in the unit. But, sometimes neither of these is perfect. I was looking in relatively cold and dirty water. With the lower frequency, I could see distinguish the general shape of more objects, but there were no fine lines. With the higher 200kHz probe, the whole screen was cluttered with noise and there was no way to distinguish what was noise and what was real. Yet, in neither case could I see the bait under the water. It didn't appear at all with the lower frequency and I couldn't distinguish it among the noise of the higher.

So, I quickly noticed that neither frequency would accomplish achieving what I wanted to see. But, I noticed that I could run both frequencies simultaneously. Additionally, I turned on "switch fire", which alternates between sensitivity levels. It can operate in "clear" which is automatic, or "max". I set it to max. Immediately, the visibility if my target was apparent. Of course, the more power I pumped through the system, the more "dead zone" was apparent on the screen. In ultrasonics, probes are focused on a point. Though they reach a maximum efficiency at that point, the data after that point, called the "far field" is useful data. The "near field", however, is a collection of constructive and destructive sound waves. It produces a useless signal. It is a fact of physics and is based on the probe diameter, frequency, and power applied. Since it is not likely that a fisherman is looking for fish 5 feet and closer to the boat, it is a fact of life that can be lived with.

Another thing I have been told is to match your GPS speed to chart speed for getting the best SI imaging. Otherwise structure on the bottom could be missed or distorted. And with 2D, I set my chart speed to 10 to get the fastest data.
Just like that, I could see great detail on the bottom and I was able to see fish, which I could see before. Check out this series of images!
You can clearly see a series of stumps on the bottom which are holding suspending fish above them.
Another one where you can spot the stumps on the right side of the boat on sidescan and also see the stumps with suspending fish above them.

It isn't perfect, but at least I am seeing structure and fish, where I hadn't been before. It should be noticed that changes in water temps and conditions have great affect on the day to day settings of your unit. Don't think you can set it one time and roll. The units are fairly simple to use, so once you learn what does what, it's easy to play with.