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ContinuousWave: Small Boat Electrical
Depth finder readings in shallow water
|Author||Topic: Depth finder readings in shallow water|
posted 08-25-2009 10:53 AM ET (US)
I have a Northstar fish finder. I don't use it to find fish but do rely on it for water depths and temperatures. When I am in very shallow water, (four feet or less) it gives me erratic or no depth readings. No being that experienced with fine tuning this instrument (for beam width, gain, sensitivity thresholds, etc.) I was wondering if anyone had information on settings to optimize reliability for depth readings in shallow water. If that would result in trade offs for detecting fish or scanning bottom structures, etc. that is OK. My main concern is reliable readings in very shallow water.
posted 08-25-2009 02:02 PM ET (US)
I had a similar problem with a very basic Raymarine fish locator. When I'd be coming in toward shore at about 4-6 feet of depth it would freeze up completely or just show 4-6 feet of depth and plot along like I was still at that depth or it would just act very erratically. Sometimes it would still say 4-6 feet deep while on the trailer in the parking lot.
I replaced the unit.
BUT I found out from a number of people who install them and do trouble shooting that it was probably due to running the transducer wires too close to other wires under the floor of the boat. When I did the new install I ran the transducer wire down the other side of the boat. All the boat's wires run down the starboard side including the speed and temp sensor wire for the locator. I ran the transducer wire down the port side all by itself taking care to not get it close to any other wires. No problems.
Now I never was able to see if that would have fixed the previous locator problem but I've had no trouble with the present unit.
The experienced installers say this is a somewhat common problem and that rerouting the wires usually cures it. Cheap fix and a good starting point.
posted 08-25-2009 03:08 PM ET (US)
This is completely normal.
The signal from your transducer is bouncing off the bottom, then your hull, the the bottom again.
posted 08-25-2009 03:08 PM ET (US)
If I'm in less than 4' of water, I'm tilting up the engine, firing up the kicker, and trying to the the HELL out of wherever I am. At that point, who really cares what the sounder says the depth is....you know it's too damn shallow.
posted 08-25-2009 03:52 PM ET (US)
That is NOT normal.
And since when is 4 feet too shallow. I've run in less plenty of times. Heck, some lakes only have a max depth of 5 feet which means most of the time you're running in 3 or less.
My Eagle locator works right up to 0 feet deep. They should all work like that. It is NOT normal for a locator to work as you describe.
posted 08-25-2009 03:59 PM ET (US)
I had the Navman/Northstar 4350 factory unit on my Montauk and it would read depth to 2' (never had it less than 2'). This was in brackish water, very little salt. You may have (accidently) changed some of the factory default settings and this may be the problem. Read the manual, if you still have it, and try to go back to the factory settings (i.e., auto gain, bottom lock, shallow water alarms, etc.). The unit should be able to read depths less than 4' otherwise, there is a problem with the unit.
On a side note, my dealer told me he had seen quite a few problems with Navman/Northstar equipment (within the past year). I recently was shopping for a combo unit and he recommended I scratch this brand off my shopping list. Just my .02 cents worth, keep the change.
posted 08-25-2009 04:50 PM ET (US)
Just replaced my factory fitted 4350, it's been average at best and now completely garbage in shallow water (20ft) with soft sandy bottom.
It was always unreliable at speed and although the screens will scroll no depths are shown, ie no bottom lock.
I tried all the functions and reset it numerous times and checked the wiring, no joy
Someone else reported similar problems about 6 months back
It was not worth fixing so I have replaced it with a Lowrance HDS-5
posted 08-25-2009 05:23 PM ET (US)
One of the cool things about Lowrance and sister brand Eagle is you can download an emulator of the model you want or have and play with it on your computer. Almost like a test drive.
My Eagle Fishmark 320 has been flawless from day one.
posted 08-25-2009 05:29 PM ET (US)
I was recently in a gunkholing situation where my chart said 10-12 ft in the area I was boating but as I progressed, my depth finder was reading 12, 10, 8, 6, 5, 4.5, 4, ---. At --- I stopped, looked for signs of churned up mud behind my engine, tilted up my engine as far as it would go and still have a pee stream, and checked the depth the old fashion way, with my boat hook pole. I was in about 3 feet of water. I had two choices, proceed about another 300 to 400 yards to where my chart showed the beginning of a channel with 16-24 feet or turn around and backtrack a very long distance. I proceeded at minimum idle speed, about 900 rpm, and made it to the channel, did not hit ground, but was nervous as hell the whole time. I later noticed and editorial note on the on-line NOAA chart of that area about reports of shoaling in that area. It was obvious that both my paper chart and chart plotter were not up to date about that particular area. I did not plan on being in that situation but it sure would have been nice to have reliable and accurate depth readings when I was.
posted 08-25-2009 06:38 PM ET (US)
From my L470 manual:
posted 08-25-2009 08:59 PM ET (US)
Actually the unit performs very well except this particular issue. Here is what Northstar support said about this.
"The problem with extremely shallow water is the "cone" of the transmit pulse is only about 6" around... If the bottom is muddy, it may have a tough time obtaining a bottom. I would suggest MANUAL range to about 10'. And adjust your gain accordingly. You may have to INCREASE it to penetrate the bottom, or DECREASE it to eliminate "noise"".
I found this to be somewhat helpful but not completely. I am not sure what "adjust your gain accordingly" means except the following explaination says I may need to increase it or decrease it. I suppose the best thing to do would be to go to a shallow area and experiment with different settings.
posted 08-25-2009 09:42 PM ET (US)
Sapple, A lot of good that'll do if you're running up into shallow water. I feel your pain, man.
posted 08-25-2009 10:26 PM ET (US)
You may be able to dial-out this problem by adjusting the settings of your SONAR.
My L470 will show a depth at nearly any depth, but when I am in shallow water to a point (about 4' as it were), it will not show bottom detail or "fish."
It will still tell me the water is 4' deep though.
You really need to talk to the manufacturer, or their representative.
It has nothing to do with interference or proximity of wires, etc.
posted 08-26-2009 09:20 AM ET (US)
The speed of sound in fresh water is roughly 1440 m/s and 1500 m/s in salt. That means in 4 feet of water (1.2m) that each sonar pulse takes 0.0008s (0.8 milliseconds)or 0.16ms in two way travel time. Given an approximate "cone" of 20 deg. you're covering just over 12" inches of bottom. Given your sonar settings "gain, frequency, sonar sample rate, boat speed, transducer location, etc" can potentially cause erranous depth readings... Bottom conditions also play a part, as the softer the bottom, the more energy is absorbed and not reflected back to the transducer. Shallow water also imparts a lot of "noise" to the transducer in reflections that echo off the boat hull and also reflections from the water bottom, back to the water surface, then back to the water bottom and then finally to the transducer referred to as a multiple. So potentially an older fishfinder with narrow cone, low power rating, and other conditions could provide "false" depth soundings by not being as robust in filtering out the unwanted "signal". Though most modern units are capable of filtering this "noise" in providing accurate depth readings in less than 1 foot of water.
posted 08-27-2009 06:34 PM ET (US)
I had a Northstar depth finder on my 220 Dauntless. It would not read the bottom in the bay where I live. It would just show "__.__". I tryed to contact Northstar and they would not answer my e-mails. The boat dealership monkeyed around with changing the transducer to no avail.
After two years of poor performance by that unit, I threw it away and bought a Furuno FCV620. It works great. When it starts to show depths of about 2.5' or less, I know it's time to start to trim up the engine and get out the push pole. You don't want to "get the hell out of there" as one poster suggested because this is where you will find redfish, specks and flounder. Also, I have found some pretty good oyster piles to fish when I saw spots where the water changed depth rapidly, for instance - from 12' to 4' and then back to 12'.
The Furuno has a navigation mode which is great for cruising and when the Furuno shows fish in the fishing mode, fish are there.
posted 08-27-2009 08:32 PM ET (US)
Glad you junked your Northstar 4350, I have also replaced my one with a Lowrance HDS-5 after a year of not seeing the bottom.
The difference is outstanding even at 3ft.
I just have to learn what all those reflections and arches mean now that I can see them.
posted 08-28-2009 07:06 PM ET (US)
I have an elcheapo Garmin 80 fish/depth finder and it reads all the way down to 1' without any problem, muddy or hard bottom. Never adjusted anything (wouldn't even know how).
posted 08-30-2009 07:39 PM ET (US)
I glad I did not get a chart plotter/fish finder combo unit. I am satisfied with my chart plotter. If and when I decide to replace my fish finder it will not cost too much for a solo unit.
posted 08-31-2009 07:23 PM ET (US)
With the engine down, my Outrage draws at least 3'. One submerged rock, log, crab pot, oil drum, boat motor, abandoned logging gear, piece of rope, cable, or whatever and you're going to have expensive problems. 5' deep lakes around here are for rowboats only. That's not a lake, it's a swamp.
If it's shallow enough to walk in, what do you need the boat for?
posted 09-01-2009 02:52 AM ET (US)
Any Outrage drawing close to three feet must be sitting way too low in the water. Down here in coastal GA some of my favorite short cuts will just float the boat unless the tides high and high muck or ground when low. One of these days will get my depth finder installed as an aide to my navigation.
posted 09-01-2009 09:05 AM ET (US)
I guess you've never been to Florida. Snakes, gators, killer algae, oyster beds, stingrays, sharks, etc.
posted 09-01-2009 09:49 AM ET (US)
Uh, Pacific NW waters can be extremely hazardous - rocks and flotsam, sweepers and logs bigger than you can imagine unless you've seen them in real life. When those coastal mountain rivers blow out, everything in them gets blasted out to the bays and sea. I'm talking rivers that run 1.5 - 2.0ft summer flows blow out to 15 ft in heavy rain in a matter of hours. Sometimes you can hear the boulders groaning - moving underwater from the river banks. Ya'all ain't got none of that in Florida. Oh, 6 - 10 ft tides too.
posted 09-01-2009 10:05 AM ET (US)
We have Black Creek here. In the 2 years I've been here, it has gone up over 15 feet from rain a couple times, over 20 feet in places. Hard to believe water can "pile up" like that. Lots of uprooted trees, no rocks. We wakeboard and waterski there. A little scary zipping past gators. When the waters rise all the weird wildlife in the woods and swamps ends up in the creek.
Oh, boulders groaning. Some people's roof over their boat lifts groan as their boat floats up off the lift in high water and gets crushed under the roof. Bad planning.
Mother Nature will getcha.
posted 09-01-2009 03:46 PM ET (US)
Yeah, well up here in MA, we have syringes, condoms, and brown trout in our rivers after a heavy rain. My flotsam is worse than your flotsam. Nah, Nah, N, Nah, Nah
posted 09-01-2009 04:19 PM ET (US)
In the south end of Puget Sound, the tidal swing is about 15' every day. The farther into the waterway you get the more extreme the tide gets, due to the funnel effect.
There are places up north where the tidal current runs at over 10 knots. That's just from the tides...that means it turns around and goes 10 knots in the opposite direction three hours later.
To me, it's about the the things that could be littering the bottom, natural and unnatural. None of them tend to be good for lower units. And I've actually done quite a bit of boating in the Florida Keys, so I'm entirely familiar with the water depths down there. In my experience, you didn't see very many boats the size of an Outrage in the 4' water. Most of it was about 8-10', and yes, the bottom was littered with junk; barrels, sunken boats, erratics (rocks), and abandoned lobster pots. When you're in water that's only 4' deep, you need someone posted on the bow to keep an eye out for those things, and yes, the shoals do come up without much warning so you really can't go fast in those places. The Keys is the only place I've ever run aground. I was going about 15 knots through a marked channel. I missed a marker and the depth suddenly came up from 10' to 2' in no time, and I was hard aground.
So, I firmly disagree....4' is too shallow to be running in at any speed in a boat with a propeller drive; even in the relatively flat-bottom areas like Florida.
posted 09-01-2009 04:45 PM ET (US)
Totally agree with "at any speed" comment but would add an exception for those with current local knowledge and prior experience in the waters.
posted 09-01-2009 05:34 PM ET (US)
I have a Furno 582L it will read in one foot of water to 1200 feet plus.
posted 09-01-2009 09:36 PM ET (US)
Florida is a big state. Most of the boating in Florida is not like the Keys. You are right, in the Keys it goes from deep to shallow in a big hurry, with lots of coral, etc to look out for. But if you restricted your boating to 4' or greater on the West coast of Florida you would eliminate almost all of your inshore fishing and much of your boating as well. Much of the bottom is soft sand, and the majority of outboards you see in west Florida on inshore boats have the paint rubbed off the skeg from hitting a sandbar at speed. If it's soft sand & not an oyster bar it usually doesn't hurt anything.
I agree that a deep vee outrage wouldn't be ideal for inshore fishing here, but there are plenty of boats built in Florida, up to 30' in length, designed to operate in 2' or less of water. Gause 26', Dorado 23' & 30', & Shaeffer 23' are three of many custom boat brands built in the area for the big, shallow, inshore bays around here, in addition to the more common "bay boats" like Pathfinder's, etc. That's not even considering the tunnel hulls, flats boats, etc., some of which can run on plane in 6" of water. Plenty of smaller Whalers, including my 1971 Outrage 21', are well suited to shallow water running.
Lowrance has a pretty big market share down here, and one reason is shallow water performance of their depth finders...depth changes are usually gradual, but you want to know if you're changing from 3' to 2.5' to 2'.
One thing that is so interesting about this site is hearing about the dramatic differences in boating from one area of the country (or the world) to another. I was in Savannah, GA for the past 5 years, and there we had big tides (not as much as you describe, but up to 9'), strong currents, and murky water that made knowing where you were critical. I'm happy to be back in Florida now where youy can read the water & see underwater obstacles...but a depth finder that will verify that you are in 3' instead of 2' is reassuring.
posted 09-03-2009 12:45 PM ET (US)
Lowrance works best in shallow water. I've owned many boats, most with depth sounders, some with more than one, like my current Tiara. The Tiara has a dinky little Lowrance 3400 digital depth sounder which always works, period. The boat also has a color Furuno bottom machine. No matter how you tune the Furuno, it will not reliably give you an accurate depth in shallow water, particularly when you are on plane. Bob
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