Humminbird 981C 80ft transducer extension

[seastrike2]

Hey Guys Cool site!!

I have been running a 981c for a little over a year now and love the unit. Any suggestions on building a makeshift "towfish" for the unit. I was thinking I would buy an extra transducer, 3 or 4 extensions, and try and run it of a downrigger ball?? Will I have any loss in power. Anyone done this yet?? Could I buy the 80ft cable in one piece. The goal is to find VERY small pieces of structure in 100-120ft just like I can now can in 20-40ft with the transom mount

Thanks

[Francisco Xavier]

Hi sea,

I had the same problem when assembling a homebrew modifyed transducer for Matrix 37 (Humminbird) and have to use three humminbird cable extensions. Then I glued the connectors using pieces of plastic tube.
Other way I considered is to use four conductor wiring.
Lenght until 200 feet is oK and would not lose signal strenght.
The fish stabality is the most important !!!

[T-Bone]

Project manager / salt water fisherman

I have been looking for someone who owns a 987c to get their opinion. I need a unit that can look (with detail) in between the legs of offshore oil platforms (50-100' of salt water) for red snapper and also have the ability to (side scan) see out away from the boat in 10 - 30' of salt water for speckled trout. Please tell me how you use your unit and give me the pros and cons of the 987?

[GLB]

Hi T-bone,
My advice to you would be to get the Hummingbird transducer off the boat and in a towfish when in an ocean environment. Because the image is built up over time improper boat movement such as rocking etc will ruin the image. A towfish for the ducer with a long cable will help a great deal to isolate boat rocking and small tracking errors. See "Opinions of side scan sonars" and you will see where GaryF has used a different system. He has one image posted where you can see fish on a shipwreck. Different system, same idea. Can't get that with a hull mounted ducer in rough water.

[ChrisPreece]

981 plus towfish!

Have had a go with the 981 transducer in a homebuilt towfish. There is one extension cable attached (a thirty foot version) but Humminbird were not keen on additional extensions.....they felt that cable capacitance would be excessive and this would adversely affect unit performance. If anybody has made their own 'long' extension or joined together a series of Humminbird ones.....and has had decent results.....I would be quite keen to hear. Cheers.

[Diver]

Results?

ChrisPreece , what kind of results did you get with the 981 fishfinder and the 30´ extension cable ?
And what type of transducer do you use ?
Do you have any pictures ?

[ChrisPreece]

981-based towfish

Chaps,
Think I've attached a photo of the 'gadget' and two screen grabs from the system.

On one you can make out the underwater 'habitat' just inside the breakwater at Plymouth in the UK. Also recognisable is the used tyre artificial reef....the area is used for commercial diver training and consequently there is plenty of debris on the sea-floor. This was taken in calm conditions and boat motion was slight....consequently the definition is good and distortion minimal.

The second one shows operation in a slight 'chop'...therefore efforts have to be made to minimise boat motion (whatever you feel in necessary eg. tow with the wave motion). The 'inside' of the right and left traces show distortion, which gets worse the more the boat moves around and the wreck of the ELK is also distorted. In the lower left corner is 'noise' that might be associated with a high-speed engine (speedboat).

The main problem with the 'set-up' as it stands is that you have to think hard how to minimise boat motion, in order to get the best images, image distortion being a prime issue. The tow cable is simply not long enough to allow you to a). get the fish far enough behind the boat to effectively 'de-couple' boat movement abd b). get it deep enough to throw a good sonar shadow.

Anybody with experience of adding a second extension.....please shout!!

Chris

[ChrisPreece]

981-based towfish



Think these are the images I discussed above......but failed to upload!!

CP

[rgecy]

Is the tow fish balanced? It looks like the tow ring is more in the middle and will cause the fish to be slightly off balance depending on depth and more suceptable to movement, possibly causing it to wobble significantly.

Robert

[rgecy]

I also mention the balance issue because it looks like you are getting a double return. The transducer is not parallel to the bottom and a false bottom is showing up (the lighter line between the center and hard bottom return). It is prevalent in both images.

My suggestion would be to check the fish in a pool or area that you can view it under water and see how it pulls. I would bet the nose is slightly down and this is whats causing your problems.

Good Luck and let us know how it turns out!

Robert

[ChrisPreece]

Tow-fish balance

Rob,

Unfortunately I have no idea how the tow-fish actually 'flies'. It seems to work well alonside the boat though! I arranged the towpoint to be slightly ahead of the centre of gravity, so when the fish is suspended by it's rope it actually hangs slightly tail down. The thinking behind this (even if incorrect) was that, when the fish was towed, water passing over the fins would lift the tail into position....I imagined the balancing act between the tail being slightly heavy and the lifting force of the fins might confer some stability. It does seem to work quite well.

My main problem (I think) is just that the tow cable is too short. You need to have the towfish both vertically and horizontally separated from the boat in order to clear turbulence and isolate the fish from boat motion.

I'll work on this. In the mean time if anybody has already extended the cable a second time time.....please say something!!!!

Chris

[hertsog]

Rob,

I'll work on this. In the mean time if anybody has already extended the cable a second time time.....please say something!!!!

Chris

I asked from Humminbird support about max cable length and they answered:

' The transducer cable can not be extended past 50 feet. It looses strength past that point.'


I planned to use 100-120 feet cable, maybe somehow impossible to use a signal amplifyer...?

[Francisco Xavier]

Hi Chris,
It is the first time I see the 455 KHz transducer from 981c unit. By it lenght it seems really that it have built inside a piezo array.
Here in Portugal they sell the 981c unit with he standart ducer, that I think it is diferent from the transducer you post in the picture above.
Another interesting thing is it that Humminbird in is site about the 981c unit just report three frequencies (specifications): Operating Frequency:
200kHz, 50kHz & 262kHz

Sonar Coverage:
50kHz: 74° @ -10db
200kHz: 20° @ -10db
262kHz: (2) 84° @ -10db

and in the 981c pdf manual report:

The side beams can be operated at one of two frequencies: 262 kHz or 455 kHz.
Selecting 455 kHz produces the sharpest image while selecting 262 kHz provides greater bottom coverage area.
I will negotiate to aquire the 981c unit just with the 455 KHz unit ducer.

[rgecy]

Francisco Xavier,

The transducer is multi frequency and is user selectable via the internal 981c/987c settings. You do not have to purchase an additional transducer if you already have the side scan sonar version. It can be changed from 262khz to 455khz in the Sonar settings menu.

Robert

[Rickard]

What about the ability to resist water pressure at greater depths? I guess transducers in general are not designed for that. Does anyone have experiances?

Regards
Rickard