Humminbird 981C 80ft transducer extension

[ChrisPreece]

More stuff.......

Chaps,
My transducer is multi-frequency, just like Rob said.....can't imagine why Humminbird would want to sell a 981 with anything else to be honest.

The transducer is built in standard American fashion (ie very well) and I think (that is I think..but don't absolutely know) that it would be good for most inshore water use. Mine has been down to 10m with no problems and I feel an extra 10 or 20 metres would not be a problem....it really appears to be solidly built.

Rob. You mentioned secondary returns on my screen grabs. I'm not very experienced with all the subtleties of side-scan but seeing as my towfish wasn't that deep, I did wonder whether some of the 'ghostly' bits that have appeared were 'surface returns'. I don't know whether I posted any of these....I'll check. My main problem just seems to be that the cable is too short....therefore I cannot get the towfish far enough behind the boat in order to isolate it from boat movement. Thus....every time there is a choppy sea (that's just about always in UK waters) the boat 'bobs' around and pulls on the tow-fish.....hence less than perfect pictures!!

I'm going to have to just buy the other cable extension and see how I get on.

This screen grab has ghostly bits and secondary returns!!

Chris

[Rickard]

Hi,

I learned some principles from a successful fish builder how they acquire stability. This is what I remember. The fish must be long, one meter or more. The towing line should be attached to a rod, about 30 cm long, and the rod should be firmly attached to the fish with a bolt. The bolt must allow the rod to move up and down but prevent the fish from twisting around the longitudinal axis of the fish. To reduce the effect of waves they hang up some led weights (5 to 10 kg i guess) on the towing line some meters in front of the fish. The result is the weights will pull the towing line almost straight down. The line between the weights and the fish will have a more horizontal attitude. The fish should not be too heavy as this will reduce the effect. With this arrangement most of the boat movements will affect the weights but not the towfish. So he said.

Rickard

[Francisco Xavier]

Hi,
After hearing from you I build up the idea that Humminbird are selling the standart 981c unit with the tranducer you post in the picture above (262/455 KHz).
So, do we have to worry about when ordering one 981c unit ?
But Robert wrote "You do not have to purchase an additional transducer if you already have the side scan sonar version" and it seems to contradict the initial ideia.
(My initial ideia is that Humminbird, - like they did with Matrix 67 unit I acquire - we have to order another-diferent transducer to perform side scan - 455 KHz and I have now two M67 transducers - the standart and the side scan)
Rickard you post good info about assembling a towfish I use a depressor on my homebuild tf and have seen on pro equipment with heavy torpedo like depressors.

Thank you to share,

Francisco

[ChrisPreece]

Towfish design

Hello again chaps!

Seems like my fish is too short! I can't remember the exact size but it's nowhere near a metre long. Also the towing arrangement may need to be changed in line with what Rickard suggested......I have seen something similar on other designs but didn't really understand what was being done.

I also understand what was said about changing the angle of the tow cable, in order to isolate the towfish from boat motion. I was going to go about it in a different manner but I'll bear the weights idea in mind.

Francisco.....I didn't ask for anything special when I bought my 981. I just found somebody on the internet in the US that sold Humminbird equipment and ordered it on-line. Humminbird don't actually suggest anything different on their web-site.....the transducer you get does everything!

A photo of somebodies successful depressor vane would be interesting. I did build one of my own a while ago but it had a life all of its own.....used to keep trying to jump out of the water!!

Chris

[Rickard]

Oh, now I remember another thing. If you use weights in front of the fish you can have a shorter fish. The wobbling tendency is reduced by the inertia of the weights. I wonder if a depressor vane is efficient since it dosn't have inertia like led weights? I guess there is nothing sacred about having a long fish. Rather, it must be the ratio between diameter and length thats critical. One could steel these proportions from photos of fishes. You could keep the loop on your fish and replace the line with some fork-like thing that embraces the loop and thus prevent from twisting of the fish. It seems as there is hope for much improvement without too much effort.

Do you know the horizontal beamwidth of the transducer? I can't find it in the manual. If there is no specification of that one can compute the beamwidth easily from the frequency and the length of the array (ie the length of the transducer.) I'm very curious about the beamwidth!

Regards
Rickard

[ChrisPreece]

Beamwidth

I can't remember what the horizontal beamwidths are for each frequency but I'll look it up when I get back home, tomorrow. I also have a formula somewhere in a radar-based document that relates beamwidth, frequency and transducer dimensions.....I'll see if it works and post that as well.

Just as a matter of interest (but....bearing in mind I've only managed to get my little system into the water twice this year (failure!!)) I've included another screen grab.

This shows the change in contrast when switching between frequencies. The upper half of the screen shows 262kHz and the lower half, 455kHz. No attempt was made to optimise the picture.....I haven't got that far yet although, as the image is a bit map (.bmp), photoshop or something similar could 'do a job'.

I'll keep my fish as it is for the minute but will get some larger fins (easiest to do!) to try and increase longitudinal stability.

Chris

[Unregistered]

Hello!

I think the towfish works fine! I believe the "line" you see between the towfish and the bottom is an echo of the surface with waves.

That you should be able to check easily by pulling up the fish slowly; the bottom ecko should then get farther away and the "surface" should get closer???

[Unregistered]

purchase

been reading this thread with intrest as i am considering buying one of these 981 units .
Anyone know were's the cheapest place to buy one one of these units ,am based in the uk but am happy to order overseas.

[Mats]

price

I dont know what price this units have in UK , but in Sweden you can buy a 987c SI Combo (including a GPS unit) for 17200 Skr (about 1252 £).

A 981 unit costs 15000 Skr (NOT including a GPS unit) , about 1092 £.

\Mats

[Unregistered]

Anyone interested,
Humminbird is coming out with model 997C SI. This is published in the 2007 catalog. They are saying the sideimaging is now high definition and also this system allows you to zoom in on objects.
It looks like this is starting at $1999 US dollars as far as what I have seen.
The transducer is model XHS-9-HDSI180-T.
This unit is supposed to have high definition added to the side imaging frequency of 455Khz.
Any comments?
I would like to set up a side scan tow fish, any more info would be great.
Awesome Web site.
Thanks

[Diver]

New Humminbird

The 997c SI highlights are as follows:

-New sunlight brilliant 8” diagonal LCD

-ZOOM capability

-New High Definition Side Imaging Sonar

-It will have as its low frequency a enhanced 455khz

- The high definition higher frequency will either be 600 or 800 khz. Either of these will give incredible ( even better resolution images- if you can believe it).

200/83 khz dual beam plus downlooking sonar

[rgecy]

Happy New Year Everyone,

I just wanted to update this thread and let you guys know that I have been told by Humminbird, with the upcoming release of the 997 SI they will also update the software of the 987 to have recording capability. I am not sure if any of the other new features, like zoom, will be in the software release but it is a promising step. This will certainly make the Humminbird models the best bang for the buck in my opinion! The 997 will have better resolution, but I also should have asked if the 987 would be upgraded to handle the new 600khz transducer as well.

Just some good info for those of you who already have the 987 and those who are looking to buy soon.

Good Luck,

RGecy

[Unregistered]

UK units

I dont know what price this units have in UK , but in Sweden you can buy a 987c SI Combo (including a GPS unit) for 17200 Skr (about 1252 £).

A 981 unit costs 15000 Skr (NOT including a GPS unit) , about 1092 £.

\Mats

Try Adventure electronics (on web) my 981 unit was about £1200, cheers, Alec Collyer, Devon.

[gainted]

997c SI Combo

Just in the process of purchasing 997c SI Combo £1250 including import duty, VAT, and delivery, to UK FROM USA.
Any suggestions as to the best transducer for using it for towing behind RIB (as stated in the other threads) because the transducer that is packaged with the 997 is the transom type.

[Bryan]

Is your problem stabilizing the transducer or getting it deeper or both?

Bryan