Accuracy of Depth Sounders ??

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kens
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Accuracy of Depth Sounders ??

Postby kens » Tue Nov 21, 2017 7:46 pm

In a similar way that navigation has evolved from compass, to GPS, to Chartplotter;
Such has depth sounding evolved from rock-on-a-rope, to depth sounder, to fish-finder (sonar)
Can you guys tell of your findings as far as accuracy of your depth sounder.??
Please inform as to type of transducer and equipment

If you are entering the lock, can you 'see' the lock doors threshold on your depth finder?

Can tell the difference between:
rock bottom,
sand bottom,
mud.

If you are on a body of water such as TVA lake or Navigable inland river, and you 'see' something on the bottom, can you tell if it is:
an old sunken tree,
an old Volkswagan,
the olde' outhouse?

If you are sounding fish, can you tell if they are:
Bass:
Crappie:
Tuna:
an old tire?

Can you heave your anchor overboard and 'see' it go down?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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JoeM
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Re: Accuracy of Depth Sounders ??

Postby JoeM » Thu Nov 23, 2017 8:58 am

I have some experience with a Garmin 73SV and a dual frequency transducer, believe 77 and 200kHz. I don't have EXTENSIVE experience with other units, new or old.

I don't have freshwater experience so I couldn't tell you about locks. I can say that the resolution is good enough to be able to pick up kelp, able to see many individual strands. Though if it's dense enough it's just a blob.

I can tell the difference between bait and larger fish with somewhat ease. You can tell the difference between fish after experience because it's more than just seeing a mark on the screen. What depth is the fish at, are there more than 1, are they moving slowly, what type of bottom are you around, etc. If you know that White Sea Bass and White fish are around you can tell the difference on the sonar as the White Sea Bass will have an air bladder that makes them look quite different(they show up like long worms on the screen) compared to White Fish. So while I can tell the difference between fish, I need a lot more information than just seeing a mark on the screen.

I can tell the difference between bottoms fairly easily. Depending on how your unit is set up, the more dense an object, the darker color shows up on the screen. Rock bottoms show up like a somewhat thin real dark strip. Sand Bottoms are show up as thick light strips. Mud shows up kinda between the two. Usually I look on my chart and see what bottom is called for at the location I want and I confirm with fish finder.

I haven't seen my anchor as the transducer was a transom mount but I have seen fishing lines/bait/jigs show up as I drop into the zone.

If you're thinking of getting a new unit I would seriously look at one with structureScan like capabilities. Heard good things.
Check out my project and blog! http://fv-escape-hatch.com/project

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kens
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Re: Accuracy of Depth Sounders ??

Postby kens » Thu Nov 23, 2017 11:27 am

If you are cruising, say 20mph, can you still discern fish and bottom structure?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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JoeM
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Re: Accuracy of Depth Sounders ??

Postby JoeM » Thu Nov 23, 2017 12:49 pm

I noticed that anything above 10mph or so we would start to get "dirty" water in and around the transducer which dirtied the signal. Above 15-20mph and many times we'd lose bottom. I found that if we're looking for migratory fish(yellowtail, whitesea bass, etc.) rather than fish that have a territory "rockfish, whitefish, etc.) , we are better off going no faster than 10mph so we can stop on the spot easier. Otherwise, by the time we slow down, turn around and get back on the spot the fish had already moved off. Another thing I found was that the faster you go the less time a fish has to be in the "cone or zone" for your transducer to pick it up. Depending on the beam angle of your transducer and the depth of water you are in will determine the "cone" or "zone" that your finder will display. The blip on the screen could be a fish that is in a 10' diameter area instead of being directly below the transducer. I'll have to dig around my research and see if I can find the formula to determine the size.

The boat was a 24' Skipjack deep-v planing boat with a single SBC engine with 290 dual prop outdrive. The Transducer was a transom mount. It was operated in coastal waters crossing channels that drop to 3kft from 100 ft in short order. We fished mainly in 50-300 ft(partnership fell through before we could make a go at tuna's). The typical swells were 2-3ft at 10+seconds, not flat like a lake can be so there is a little discrepancy as you move along with the swells.

The reason I bring this up is to show that your findings might be different. The attitude of the skipjack changes quite a bit at different speeds and another boat may change little. If I recall, your Double-Eagle maintains a fairly steady attitude through your speed range, which i'm guessing has a lot to do with the fact that it is not a planing hull and is a central inboard engine. That could mean that you won't lose bottom as easily at higher speeds. Depending on where you mount the transducer may also change effectiveness.

Edit: This site doesn't include the formula for the area you are seeing. But has several rules of thumb and a nice graphic that explains this a little better than I did. http://www.louisianasportsman.com/details.php?id=486
Check out my project and blog! http://fv-escape-hatch.com/project

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Bill Edmundson
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Re: Accuracy of Depth Sounders ??

Postby Bill Edmundson » Thu Nov 23, 2017 5:48 pm

On the Bartender, I have a Lowrance Elite 7 Chirp w/ Transom mount. One screen does show the relative size of the fish. This boat has unique transom arrangement. At full plane, I loose signal. I don't fish much. But, on the Alabama Gulf Coast you can get into skinny water quickly.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
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kens
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Re: Accuracy of Depth Sounders ??

Postby kens » Fri Nov 24, 2017 4:01 pm

When I am running at cruise, (20mph) sometimes I get a clean signal, sometimes not. It always get dirty in choppy water though.
If Im running on glassy water, I get a good signal. But how often does that ever happen?

Also, if the water is murky, muddy, sandy, I have trouble seeing much of anything.
If looking for a shipwreck marked on a chart, I can spot it, sometimes make out a mast on the ship or such.
I have noticed that if I see something I suspect as fish, and throw a line, I catch fish.
If I throw a line into a known good GPS position, and not marking fish, I don't catch anything.

I'm not good enough with it to tell species, other than bottom dwellers, or mid column, or top column.
If I throw the anchor in deep water I can watch it go down.
I have not tried to track a lure or fishing weight.

One day I had 2 guys with me, trolling, I was on the helm, 2 guys on the rods, trolling a 5 rod spread.
Watching GPS, trolling over a line of wrecks, I saw fish blips near surface, and called out 'Stand by' , sure enough the rods went off.
We repeated this over and over for 3 hours.
Furuno LS6100 with Airmar B60 (tilted) xducr. mounted forward of engine.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:


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