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Just for those people out there that still have a passing interest in the 981 sidescan. I have had another 'fiddle around' with the set-up that I have (detailed on the 80ft transducer extension' thread) and although I haven't done anything remarkably technical, the results aren't bad at all.
Beforehand I had added a single (manufacturer-sourced) extension cable and was towing the transducer in a home-built towfish, which was, as somebody pointed out, a bit on the short side. So, I've increased the size of the fins to increase the longitudinal stability....simple enough modification....no real problems.
I also went out and bought another transducer cable and added that into the system. This has been trialled and the results are alright. I do however get the impression that just a little something has been lost and I might not get away with adding a third extension. However, it still works very well on the lower frequency...plenty of detail....even at the default range, which is 250ft (I think).
The higher frequency is another matter! Noise has reared its head and intermittently becomes a serious problem. Additionally the sonar picture lacks contrast. Maybe you can fiddle with the settings a bit but basically it looks (and I have only tried it the once at the minute) like the higher frequency may only be useful for short-range work.
Finally.....my original shots were alright....as long as the sea was calm. All the shots shown on the other thread were taken inside the breakwater at Plymouth in Devon, UK. Outside the breakwater was another matter, as serious distortion occurred when my boat was bounced around in a 'choppy sea'. Even the shots inside the breakwater showed evidence of boat movement.
So, I had a think and came up with the idea of a sub-surface drogue....basically to put an angle in the tow cable just below the surface of the water. The idea was that if I could get the tow cable to run almost horizontal off the back of the boat, so that it entered the water four/five metres or so behind the transom then the cable could go 'down' from that point and not descend straight off the back of the boat. In effect I'm using the drogue to pick up the cable four metres away and allowing it to drop at a steeper angle from there. This means the boat motion is 'lost'....because the tow cable is near-horizontal off the boat....and all the bouncing around is not transferred to the fish.
Additionally, the drogue (a one metre square of bedsheet formed into a airfield windsock type of arrangement) has no bouyancy....therefore, unlike a float, it is not bounced around by the sea. In my current setup the drogue is straight-through and does not get narrower.....the drag of the linen seems to be enough!!
And.....it works!! Carried out the trials in very similar conditions to before and there isn't even a hint of distortion due to boat motion. When I've done some more work on this and convinced myself it can all work in choppier conditions I will post some results. Cheers!
Beforehand I had added a single (manufacturer-sourced) extension cable and was towing the transducer in a home-built towfish, which was, as somebody pointed out, a bit on the short side. So, I've increased the size of the fins to increase the longitudinal stability....simple enough modification....no real problems.
I also went out and bought another transducer cable and added that into the system. This has been trialled and the results are alright. I do however get the impression that just a little something has been lost and I might not get away with adding a third extension. However, it still works very well on the lower frequency...plenty of detail....even at the default range, which is 250ft (I think).
The higher frequency is another matter! Noise has reared its head and intermittently becomes a serious problem. Additionally the sonar picture lacks contrast. Maybe you can fiddle with the settings a bit but basically it looks (and I have only tried it the once at the minute) like the higher frequency may only be useful for short-range work.
Finally.....my original shots were alright....as long as the sea was calm. All the shots shown on the other thread were taken inside the breakwater at Plymouth in Devon, UK. Outside the breakwater was another matter, as serious distortion occurred when my boat was bounced around in a 'choppy sea'. Even the shots inside the breakwater showed evidence of boat movement.
So, I had a think and came up with the idea of a sub-surface drogue....basically to put an angle in the tow cable just below the surface of the water. The idea was that if I could get the tow cable to run almost horizontal off the back of the boat, so that it entered the water four/five metres or so behind the transom then the cable could go 'down' from that point and not descend straight off the back of the boat. In effect I'm using the drogue to pick up the cable four metres away and allowing it to drop at a steeper angle from there. This means the boat motion is 'lost'....because the tow cable is near-horizontal off the boat....and all the bouncing around is not transferred to the fish.
Additionally, the drogue (a one metre square of bedsheet formed into a airfield windsock type of arrangement) has no bouyancy....therefore, unlike a float, it is not bounced around by the sea. In my current setup the drogue is straight-through and does not get narrower.....the drag of the linen seems to be enough!!
And.....it works!! Carried out the trials in very similar conditions to before and there isn't even a hint of distortion due to boat motion. When I've done some more work on this and convinced myself it can all work in choppier conditions I will post some results. Cheers!
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