Topic: 998si side scan interferences?

[peterv6i]

After mounting my transducer on my RIB..
I get picture like this:

Left side as you can see has "black" lines?! (interferences?!)
Also you can see a lot of "dots" like snow all arround... why?


My transducer is located at point  "B right"




Thank for any suggestion

[rnvinc]

Lines like in your image are sometimes indicative of the scanning speed being too fast for the specific xducer installation spot...(caused by bubbles from the motor/hull/obstructions)...

(RF interference "usually" shows on both sides of the SI image)...

You can try slowing the scanning speed...adjusting the xducer...or reinstalling the xducer lower on the hull down into cleaner bubble free water...

What speed was the OP image scanned at...??

Rickie

[peterv6i]

speed was about 5-7 mph, 900 rpm on yamaha f80 engine.. (salt water)
from humview I can read that speed was 2 to 2.2 m/sec
Rght side is ok.. left has "black lines"...

[rnvinc]

speed was about 5-7 mph, 900 rpm on yamaha f80 engine.. (salt water)
from humview I can read that speed was 2 to 2.2 m/sec
Rght side is ok.. left has "black lines"...

I would try slowing down to see if the lines to away...maybe even drifting with the motor off..(this will define if the lines are caused by bubbles)...

If bubbles are the culprit ...then slower scanning or repositioning the xducer will be the next decision...

Rickie

[peterv6i]

Ok I will try to:
1. reduce speed
2. trim the outboard
3. switch off the outboard

[rnvinc]

Ok I will try to:
1. reduce speed
2. trim the outboard
3. switch off the outboard

Post back with your results...we all learn from each other...

Rickie

[RGecy]

The only way to completely get rid of it is to buy a second transducer and the Y cable and have one mounted on each side.  I had this same issue, and yes slowing down will help a little, but it will not fix it completely. 

I use my 1198 to search for wrecks and perform geophysical surveying and I want the image to be as good as possible. Its always good to have a second one around anyway!

Good Luck,

Robert

[newkid4si]

Robert

    Off topic some. I imagine that your scanning deeper water than me, but I'd like to know what speed you usually use.
    I'm running 3-4 mph in 5-50 FOW.

              Mike

[peterv6i]

I will scan (for me interesting parts of sea) between 10-24 meters deep.

If I run engine on minimum (yamaha f80) I have "vibrations" because it run on 800-900 rpm and on this rpm my boat is doing about 3-5 mph..
So I prefer to run my engine on 1000-1500 rpm.. the engine runs smoother but the rib is doing about 4-7 mph..

[peterv6i]

What about "dots" on my screen.. If you look at first image you can see a lot of random dots over the screen? what are they?
I must say that sea in my part it's quite murky a lot of sediments are mixed in water... so the visibility on this location is about 1-2 meters..

[Rickard]

About dots,
I also see similar dots when operating the system in my old wooden boat with an old petrol engine. In my case they are definitely interference from the ignition system or the alternator. I can make that conclusion because of two things; they vanish when I stop the engine and the dot pattern is mirrored between left and right sidescan views. Thus, the dots can't indicate objects in the water.
 
Rickard

[peterv6i]

Ok! for the test..
I will switch off the engine and see what will happen..

Another question..
I have a battery 75Ah on my boat.. What is the consuption of 998 fish finder? I notice that quite fast my voltage from 13V comes down to 11V (afet 10 minutes) if the engine is not running.. 

[Rickard]

The power draw is low, less than 1 A. There is a list with most units' power consumption here: http://www.humminbird.com/FAQ.aspx?ContentId=2903
 
Rickard

[peterv6i]

Here I have found two pictures of my transducer...

The transducer is mounted below hull


what do you think? Can I move the transducer a bit left and lower?

[RGecy]

Robert
   Off topic some. I imagine that your scanning deeper water than me, but I'd like to know what speed you usually use.
    I'm running 3-4 mph in 5-50 FOW.

              Mike

Mike,

I am usually in 15-50 fow and scan at about 5 mph.  If I am scanning any deeper, depending on what I am looking for, I will break out the towfish.

I will slow down my scanning speed if looking for smaller targets or when I want to get a sharper image.

Robert

[newkid4si]

Robert

           Thanks.

                 Mike

[peterv6i]

I tryed to switch off the engine and also trim up my engine.. There were no differences...
Sometimes the picture is good (excelent) sometimes there are some "noise"...

I have another question..
My rib is doing 3-4 mph on minimum..
what is recomanded chart speed?

[newkid4si]

peterv6i

      The general recommendation is to match the boat speed and the chart speed.
      3mph= 3 chart speed....4mph= 4 chart speed.....etc. I have found this to be a good start.

      One suggestion is to take a known target and make a scan at different chart speeds while keeping the boat speed the same.
      See what looks good to you. I have made scans at a normal boat speed (3-5) and a high chart speed (7-9) in an effort to
      magnify the target for more detail. It distorts the image but also shows added detail.
      Turning up the chart speed doesn't cost anymore unlike the old days of $4-6.00 paper rolls.

      One other part of chart speed that isn't often discussed is what size target that you are looking for.
      A faster chart speed would show a small group of fish better than a slow speed. A slow speed would be OK for a larger target.

             Mike

[peterv6i]

      One other part of chart speed that isn't often discussed is what size target that you are looking for.
      A faster chart speed would show a small group of fish better than a slow speed. A slow speed would be OK for a larger target.

             Mike

I have in minds this.... If the chart speed is to slow and the object is small than the object size on the sonar screen will be small (low details because of slow drawing speed (chart speed))..
I'am looking for object's which are 3-7 meters in length.. (wood, concrete, iron)..
When I was searching the bottom for objects my boat speed was 3-4mph and chart speed sometimes 6 sometimes 7..

[RGecy]

Probably a little high on chart speed.  I usually keep mine around 4-5 when trolling around.  I usually keep the boat speed around 4-6 mph and the chart speed doesn't need to moved.

You will get better images if the chart speed is close to boat speed, not stretched or compressed.

See if you can find a tire and scan it in say 8-10' of water and get the image of the tire to be round, then you know you are tuned in correctly.

Robert

[newkid4si]

   Robert brings up a good point in the need to establish a baseline ( chart speed/boat speed ) for scanning.
   If the baseline is kept constant it will become easier the understand the images.

   My above post about the increased chart speed was for after a target is found. It distorts the crap out of the image,
   but may allow you to see details not seen in the normal scan.

   Another factor to keep in mind is the orientation of the target to the scan.
   One of the items we are looking for is 30" wide and 25' long.
   Assuming the SI beam is 4-6" wide and I go parallel to the target, I'll most likely see it.
   If I were to pass perpendicular to it with a slow chart speed, all bets are off.

   I like Roberts recommendation as a starting point.