Tuesday, March 20, 2012

Selecting the right chrip transducer for the GSD 26

For fishermen, the use of fish finders have always required a little black magic to get the most out of them. Tweaking settings to find thermoclines, and playing with knobs with names like gain or TVG. Trying to decide if the big target on the display is really a school of bait fish, or a Warsaw grouper.

As good as sounder systems have become through the years, the approach of doing a single frequency ping (tone burst) has been, and always will be limited by basic physics, and the industry has basically taken the technology about as far as it can go. Now with "Chirping" sounder systems, the game has changed. In 800' of water, you can now tell if that target is a single fish, or a school of bait. At 2000', or possibly more with the right transducer you can still see larger fish targets, and bottom can be found at depths of up to 11,000'.

Talk about fish finding now includes jargon like "number of elements", "Q" values, figures of merit, and DSP processors. Now with about two dozen chirping transducer options, what's best for your boat? This is what we're going to explore, and along the way give you some of the tools to help you make the right decision.


Transducer mounting options
This is the very first step in broadband transducer selection, and this decision is driven by your boat's design, whether you trailer it, use a lift, leave it in the water, and the materials the hull is made of. Some of the broadband transducers range from large, to very large, and their size should be carefully considered. Choosing well here will make all the difference in the systems overall performance.

Transom Mount
At this point in time there is only one available transom mount unit, and it is in the B265 family. Don't let the picture fool you, this is a very substantial unit in size. Care must be taken when installing it to insure that it is far enough outboard on the transom so it will not interfere with water flow to the propellers. This is a good option for many outboard, and I/O powered vessels.

Inhull
These tank transducers can be an excellent choice for a broad range of boats, and there is a wide variety of units available starting at 1000w, and going up to 3000w. The tanks must be cut to match the dead rise angle of the hull, and bonded to the hull with Marine Tex epoxy, or traditional fiberglassing techniques. There is some minor db loss, in the range of around 3-5% based on typical hull thickness of less than 1". This loss primarily affects range, and not target resolution.

The hull must be solid fiberglass, with no delaminations causing air gaps. Wood, steel, aluminum, and cold molded hulls are not suitable for this technology. Cored hulls can be used, as long as the core material has been removed leaving only the exterior fiberglass layer of the hull exposed.

Care must also be taken to keep the fiberglass surface under the transducer free of the materials used to bond the tank to the hull, and use of the cork insulation is important. There are some anecdotal reports indicating some Kevlar hulls may have very high amounts of signal loss when using inhull units, and this should be carefully considered before purchasing an inhull transducer for these boat hulls.

Through Hulll
From the low profile B75 to the 3000W R509/599 beast, the entire line of chirp transducers are available as through hull units. The smaller units are not typically an issue for most boats, but the larger ones can be under some circumstances. 

For boats that are trailered, you need to carefully measure and check your trailer clearances if you want to use the larger transducers. The R509 transducer with its faring block can easily hang down over 6 inches, it's close to 7" wide, and nearly 2' long. For boats on lifts, the larger transducers may require bunk readjustments, and in all cases, you should clearly mark the "No travel lift strap locations."


Pocket Mount
The 265, 111, and 500 families of transducers have pocket mount versions for larger vessels. Installing a pocket mount transducer requires cutting a substantial hole in the hull, and fiber glassing the transducer into the hull with its face flush with the hull. The installation location must be flat at the keel line allowing the transducer to point straight down. This is not a DIY installation project, and should be only be done by skilled personnel.

Q's, and elements
The hunchback of Notre Dame pulls on the cord, and the bell rings, for quite a long time. This bell would have a large Q value because of the long ring duration.

If the bell is rung, and then you immediately put your hand on it dampening it, the Q value now becomes much lower. Lower Q values are important for a transducer, and especially for a chirping transducer.

The piezioceramic elements both create sound waves, and listen to them. When a current is applied to the transducer element, it makes sound waves. When the sound wave echo bounces back to the element, it causes the element to vibrate creating an electrical signal. If the element is still ringing, it can't hear the returning echo well. Lower Q values provide better performance overall.

Since the piezoceramic elements are both the voice, and ears of the transducer the greater the number of elements, in general means the better sensitivity of the transducer. The R111 2000w transducer has a Q value 0f 1.6, and 16 elements. The M265 transducer a Q value of 2.2, and 8 elements. The R111 is twice as sensitive in the high chirp mode, and four times more sensitive in the low chirp mode. But to really put this into perspective the M265's inhull chirp transducer's round trip sensitivity is 50 times better in the high chirp range, and a 1000 times better in the low chirp range than the standard Airmar P79 50/200 kHz inhull non-chirp unit that has a Q value of 21.

Frequency selection
These transducers come in dual chirp, and single chirp configurations. The dual chirp units are either high/low, or low/medium frequency chirps. From the sounder module's view point, a dual chirp transducer really counts as two transducers, and is wired as two separate transducers. Your selection depends on how deep the waters are where you fish.

Here are the basics. The high chirp (80-210kHz up to +-500') has a narrower beam width and provides the highest quality bottom definition. It also works well at higher travel speeds and is good at detecting smaller fish. The medium chirp (80-130kHz up to 1500') has a wider beam width. The low chirp (+-28-60kHz up to 2000')  has the widest beam width, and is used in very deep waters at depths up to 11,000'.

These suggested depth windows can be variable depending on power/water conditions, and are for fish detection. Bottom tracking depths are much deeper. The maximum depth for larger fish target detection, in low chirp, or with very low frequencies is now around +-2000' with the 3000 watt transducers. The suggested frequency depth ranges actually have overlaps. For example the medium chirp may in some circumstances perform better than the high chirp as it nears its depth limits.

The chirp approach provides loosely up to about 10 times resolution improvement over traditional tone burst fish finders, and the usable depths of both the high, medium, and low frequencies are dramatically increased.

The actual beam width of the transducer depends on the frequencies being chirped, or the single frequency being used. For example, the R599LH's low chirp range is from 28kHz to 60kHz, and the beam width varies from 9 to 23 degrees. In this case 28 kHz would have a beam width of 23 degrees, and the beam width of 60 kHz would be 9 degrees. That would make 44kHz in the middle about 16 degrees. When chirping at the low frequency, the beam width starts at 23 degrees, and sweeps in to 9 degrees. The same general rules apply to the high chirp. The lower the frequency, the wider the beam width, and the systems always chirp up in frequency.

Power matters, to a point
The more energy you can put into a target, the better the return echo. The GSD 26 is capable of producing a total of 5000 watts rms (85 watts average) of transducer output allowing it to use the maximum power (rms and average) capacities of Airmar's largest chirp transducers.

Power is also automatically modulated by the GSD 26 based on the depth, and mode allowing the more powerful transducers to be used in shallow waters. This allows the transducer to throttle back to about 30 watts rms in very shallow waters, and in deep waters it's capable of simultaneously delivering up to 2000 watts for a high chirp, and 3000 watts for a low chirp. You can adjust the power down some, but you can't turn it up. There is no sense in spending money on a 3000 watt transducer if you only fish in shallower waters. But if you fish in deep waters bring it on, the more power you have, the more energy you can deliver to the fish targets, the more fish you will see.

What about my existing transducer?
Since the GSD 26 can support two transducers (or one dual chirp), there is a good probability that your existing through hull 50/200kHz Airmar manufactured transducer will work with the GSD 26 using one of the two ports. Although you can do this, and it will work well, you're not utilizing the capabilities the GDS 26 was really designed for. This would leave an open port for a second chirp transducer such as a B75 low, medium, or high chirp transducer. 

The second reason for leaving an existing through hull/transom mount transducer in place, is to access its temperature thermistor, if it has one. This is particularly valuable if your chirp transducer is an inhull installation, and would then have no sea temperature capability. The GSD 26 has two temperature inputs available. If you're unsure about your existing 50/200kHz capabilities, you can typically find the Airmar, or OEM vendors part number tag at either the connector, or transducer end of the cable. You can then look it up in the Airmar database.


Where is all of this technology going?
Everyone is still learning how to use this technology. Never before have such clear images at these depths been available from fish finders, and new things are being discovered almost daily.

In the picture above, there seems to be a thermocline at about 500 feet, and below it another line at a slight angle around 750 feet. The captain watched the lower line fall during the evening, and rise during the morning. He believes this was a layer of bait fish associated with the thermocline. He had been playing with the gains when the two lines appeared. 

The point is there isn't enough enough experience with the technology yet to know what the limits really are yet, and all the things you can potentially see with it. But it is crystalline to me that this technology is orders of magnitude better than what we all have been using to date.

I think, given some time that these chirp based systems will be able to identify specific fish species. New software will be developed to view and interpret the information, and all of this will change very rapidly. If you want to really see the fish, and all of them, chirp technology is the best tool going. Look John, that's Bob the sword fish down there, I caught him last year during the tournament Let's see if we can catch him again.

You can read more about this technology below.
Airmar's brochure for smaller boat chirp transducers.
Airmar's brochure for inhull transducers.
Airmar's brochure for pocket mount transducers.
Airmar presentation "Getting to the bottom of CHIRP technology: by Jen Matsis VP Sales and Marketing, Airmar Technology.

GSD 26 screen shots courtesy of Gordon Daniher.
The transducer range picture is a substantially modified Garmin screen shot. 
Transducer photos/art work elements are from the Airmar website.
The lightning photo is from Wikipedia user Evdaimon.

And many many thanks to all of the technical professionals who provided input to me.

P.S. By the way, if you happen to know what frequencies, or ranges specific fish species are seen at, please feel free to post them in the comment section below. The Airmar website mentions Tuna at 60kHz, and squid at 133kHz

5 comments:

  1. Any idea why garmin doesn't include a users manual with the GSD 26. They include installation instructions but nothing else?

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  2. The manual is included as a subsection of any Garmin chartplotter manual, that came in the gray binder. If you have an older manual you can print out the latest version on line.

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  3. I'm not understanding the pic with the model numbers and depth........anyone explain it?

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  4. These are just graphic representations of the suggested depth ranges each transducer family will operated best in. For example if you look at the third line down (M275LM/B265LM)these are 1000W medium and low CHIRP transducers. If you take where the line starts, and draw a line straight down to the bottom you're at about 500'. The other end of the line is at about 1000'. If the line is below the bottom draw up to the bottom. Another example is the second line showing the range of the 2000W high CHIRP and 3000W low CHIRP transducers. They cover from very shallow to around 2000'. The depths are for fish detection only. As we found out later these larger transducers can track bottom in excess of 17,600'.

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  5. I see. Thanks very much

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