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Old 21-08-2019, 06:42   #1
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Probing the Depths of Depth Finders

Our boat is equipped with two depth finders and I still cannot reliably know how deep the water is unless it’s deeper than thirty feet – at which point I really don’t care. I’ve read a number of posts and articles on the issues relative to depth finders but am not sure what corrective actions to take.

One depth finder is an old SR40 unit. The transducer is fixed in the keel in the forward part of the boat. The gauge portion of the unit became so old and degraded that it was no longer possible to read the LCD display unless you put your eyeball right on it. I replaced the display unit over the winter with a Humminbird HDR600, using the existing transducer. This was reported to work well. It does work, but not well. Lying at anchor on a flat bottom in ten feet of water, the depth fluctuates by as much a four feet. This unit only operates at 200Hz. (And, you can't read the LCD display with polarized sunglasses on!) I do have the transducer for Humminbird, it is a shoot through unit. Is this likely to work better than the old SR unit? Would moving it far from the Garmin unit help?

The other unit is our Garmin 540s chartplotter. The Garmin has a shoot through transducer located in the bow, not far from the SR transducer. The transducer has been affixed to the hull with silicon caulking. The Garmin is very accurate but it has a tendency to cut out at any depth below thirty feet. The shallower it gets, the more it cuts out. It is also very confused by mixed salt and fresh water, which we frequently encounter in the river where we moor. (Note that this may have nothing to do with the issue at hand.)

Thinking that the two units may be interfering with each other, I’ve tried an assortment of configurations with one unit or the other turned off. And, I’ve tried setting the Garmin for 50Hz only (though I’m not sure this really turns of the 200Hz frequency) and this has made no difference. I’ve also tried using just the Garmin at 50Hz, 200Hz and auto select, none of which improve the reliability of the Garmin unit.

I would happy if just the Garmin worked reliably. Its display is closer and clearer than the Humminbird’s. What I am not sure about is how to improve the performance of the Garmin. I have read that silicon is not such a good material for shooting through. If I go to epoxy, how much improvement will I get? (I won’t be able to make any further changes once the transducer is epoxied down!) Does the thickness of the hull make a difference? The hull in the bow is quite thick. If I move the transducer more amidships, where the hull is relatively thin, will this help? And, can someone explain why the Garmin is unreliable at shallow depths? I would think that a depth finder would have more difficulties at deeper depths, but this is not the case.

At this point, the only reliable way to know my water depth in shallow water is a lead line – which I do carry. But, it’s not much use under weigh. What would you recommend?
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Old 21-08-2019, 07:19   #2
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Re: Probing the Depths of Depth Finders

The sentence, The Garmin is very accurate but it has a tendency to cut out at any depth below thirty feet. Should read, The Garmin is very accurate but it has a tendency to cut out at any depth less than thirty feet.
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Old 21-08-2019, 07:46   #3
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Re: Probing the Depths of Depth Finders

Is the river water clear? I wonder if suspended silt etc in the water causes your problems?
-Thermoclines and fresh salt layers can be a problem for proper working units.
-I have had shoot through the hull types work well on 47 footers, so I doubt your hull is too thick.
-But the fiberglass must be solid no voids in the layup etc.
-You can easily test the problem of one interfering with the other by turning one off.
-Maybe try dangling the sender (you are not currently using) over the side sometime when that unit is acting up with the in hull sender. If it then reads well, you know the problem is in your mounting of the senders.
-The silicone used must be bubble free and not too thick. I have used both silicone and epoxy and both worked.
-Is your bottom often slimy. not clean?
-Do you ever have a chance to just test them in good clar ocean water?
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Old 21-08-2019, 18:45   #4
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Re: Probing the Depths of Depth Finders

I don't think the river water is the issue. The reliability of the SR/Hummenbird and the intermittent actions of the Garmin are consistent wherever I go. Dropping the transducer overboard is a good idea. I have the transducer for the Hummenbird and it would not be hard to temporarily hook up. Removing the Garmin from its silicon tomb would be a bit more difficult and I'll likely have to cut and splice the wire to re-route the transducer to a place where it could be dropped overboard. If I can get the Garmin unit lose, I may try the bag of water trick first in the area where it is now located. If that works better, and I'll know pretty quickly, I'll see about building a small tank that I can fill with anti freeze or mineral oil.
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Old 21-08-2019, 21:04   #5
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Re: Probing the Depths of Depth Finders

Quote:
Originally Posted by Homer Shannon View Post
Lying at anchor on a flat bottom in ten feet of water, the depth fluctuates by as much a four feet.

That would indicate a problem with the signal to noise ratio.


Quote:
This unit only operates at 200Hz.
Good place to be for what you want.


Quote:

(And, you can't read the LCD display with polarized sunglasses on!)
Buy a $100 sonar, have $100 sonar problems.



Quote:

I do have the transducer for Humminbird, it is a shoot through unit. Is this likely to work better than the old SR unit?
Probably, if it installed correctly.


Try hooking it up and just holding the transducer in the water or clamping it to the transom, and see what sort of performance you get. If you're happy with it, proceed with a permanent install.


Quote:
Would moving it far from the Garmin unit help?
I don't think that's your problem.


Quote:

The other unit is our Garmin 540s chartplotter. The Garmin has a shoot through transducer located in the bow, not far from the SR transducer. The transducer has been affixed to the hull with silicon caulking. The Garmin is very accurate but it has a tendency to cut out at any depth below thirty feet. [....]
You need to know what the hull is made out of at that particular spot. Shoot-through transducers work fine in solid fiberglass, not so well if there is some sort of core (foam, balsa). Angles matter, is the transducer facing straight down? If not, it will perform poorly. How much silicone? If a great deal, it will attenuate the signal. Since the bow area of the hull isn't usually horizontal, this may be your problem. You can cut the transducer loose and put it in a baggie full of water and try different spots and orientations, then epoxy it the way it works best.


If your hull has a core (unlikely, but possible), you'll have to make an area that is solid fiberglass by gouging out the balsa or foam (as the case may be), and laminating layers of fiberglass with epoxy to build it back up.



Quote:
Does the thickness of the hull make a difference? The hull in the bow is quite thick.
Not much.


Quote:
If I move the transducer more amidships, where the hull is relatively thin, will this help?
Maybe, because the hull is more level, not because it's thin. Or perhaps because it is solid glass rather than cored.



Quote:

And, can someone explain why the Garmin is unreliable at shallow depths? I would think that a depth finder would have more difficulties at deeper depths, but this is not the case.
In the bow of a 40' boat, there are lots of things that reflect sound that are less than 30' away, such as the transom, bulkheads, prop, rudder, etc.--any vertical surface. The Garmin has to figure out which of those are the boat and which are the bottom. It usually gets it right.



In deeper water, well, the false echos have died down by the time the pulse comes back from the bottom, so it's easier for the Garmin to pick out the bottom.


Quote:

What would you recommend?
Focus on the Garmin since you like it. Clean up your installation and try relocating the transducer using the baggie modality, and epoxy it in place once you find the sweet spot.
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Old 22-08-2019, 09:40   #6
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Re: Probing the Depths of Depth Finders

If you are running two depth sounders at the same time they will interfere with each other. Especially if they use the same frequency transducer.

My depth sounder has an adjustment for shallow water gain. Bottom conditions affect reliability. IOW soft mud makes for erratic readings. Does your sounder have a gain adjustment?
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Old 22-08-2019, 11:44   #7
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Re: Probing the Depths of Depth Finders

My depth finder has never failed me, it records perfectly, and it doesn't need power: attach a 4 oz. weight to the end of a line, with a colored marker every 10 feet. Cast it out a bit ahead of the bow as you go slowly along. It works like a charm.
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Old 22-08-2019, 12:53   #8
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Re: Probing the Depths of Depth Finders

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Originally Posted by stormalong View Post
If you are running two depth sounders at the same time they will interfere with each other. Especially if they use the same frequency transducer.

That's not something I would have ever figured out on my own and I have that problem.
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Old 22-08-2019, 14:15   #9
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Re: Probing the Depths of Depth Finders

We run two depth sounders /fish finders at the same time with no apparent issues.
Transducers are probably 6 ft apart.
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Old 22-08-2019, 14:26   #10
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Re: Probing the Depths of Depth Finders

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Originally Posted by Simi 60 View Post
We run two depth sounders /fish finders at the same time with no apparent issues.
Transducers are probably 6 ft apart.
Mine are a bit closer together than that, but at least I have something to try now. There is no sense to the faults I'm getting.
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Old 22-08-2019, 14:33   #11
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Re: Probing the Depths of Depth Finders

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Originally Posted by Ecos View Post
That's not something I would have ever figured out on my own and I have that problem.
Quote:
Originally Posted by Simi 60 View Post
We run two depth sounders /fish finders at the same time with no apparent issues.
Transducers are probably 6 ft apart.
Depth sounder 101:

A depth sounder works by transmitting a sound pulse and then listens for it's echo. The time it takes for the transducer to hear the reflected signal is used to calculate the depth. If there are two sound pulse sources the listening phase of finding the depth will not be able to distinguish between the two sound sources. If the two transducers use the same frequency the received echo is indistinguishable. If they use different frequencies the sounder's ability to filter unwanted noise will determine if they interfere with each other.
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Old 22-08-2019, 14:51   #12
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Re: Probing the Depths of Depth Finders

One thing I notice with my Lowrance fish finder display is that if I have the automatic gain turned off and the gain cranked up, I can see two or three "bottoms" on my screen. In other words the echo coming back from the sea floor bounces off the hull and back down, only to echo back up, and so forth. it is possible that your bigger Garmin just stops showing any reading if it is getting confusing returns.

However, with automatic gain this does not occur. If you have the capability of automatic gain, use it.

We have two depth sounders, one operating at 196khz and the other at 200khz (plus a second 50khz transducer) and there is no interference or loss of accuracy that I can tell.

We get depth readings down to 8'. Less than that there is another indicator: our lack of forward motion!

But with all these signals going out from our boat the whales must be quite annoyed.
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Old 22-08-2019, 15:15   #13
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Re: Probing the Depths of Depth Finders

The transducer should be mounted forward in the vessel and angled slightly forward. How much of an angle depends on how fast you intend to travel while checking the depth. The signal is transmitted in a cone shape, usually about 15 degrees or so. This means the bottom will be visible if the transducer is angled about 5 degrees off verticle. The depth sounder is 200kz but in actual fact it is slightly different to 200hz to prevent interference between neighbouring vessels. The variations in depth you are experiencing is probably fish passing under your boat. The only difference between a depth sounder and a fish finder is the spelling! Both use the same transducers.
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Old 22-08-2019, 15:25   #14
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Re: Probing the Depths of Depth Finders

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Originally Posted by stormalong View Post
Depth sounder 101:
.

Reality says something different.
Commercial fishing vessels often have multiple devices running.

Both our sounders works accurately until we touch bottom.
At that stage there would be 3 ft of water under the transducer.
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Old 22-08-2019, 16:15   #15
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Re: Probing the Depths of Depth Finders

I may have to resort to the "potato" method - at least in fog.

I went to the boat and worked on both depth finders. I had hoped to switch the SR transducer on the Humminbird to the through-the-hull Humminbird transducer, but it would have required splicing a shielded cable, something I avoid while the boat is in the water. I left that one alone.

I was able to move the Garmin transducer to a location quite a bit aft, but it's a natural puddle in the hull and usually full of water. Today it was dry, but after moving the transducer, which was quite a hassle as I had to re-run the wire that went from the bow to the wheel pedestal, I put the transducer in the natural low spot and added some water. The depth finder seemed to be working fine but I won't know if it cuts out (just when you need it) until I take the boat out for a spin. Probably Monday. I'll update then.
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