My depth sounder LIES!

[trooper dan]

So I have a uniden depth sounder that is relatively new but purchased with the boat. I have run aground three times already and every single time the depth sounder lies about how much water I have under me. Today I ran aground and while I was stuck the sounder read 6'2" and my boat draws 3'8" so I should be ok right? Each time some kind power boaters towed me off. I am here on the Magothy river Chesapeake and on the northern end. There are very shallow water in the creeks and rivers here.

I have new charts and gps, and follow them closely. So anyways I used a lead line a little bit ago here at my slip and supposedly my boat is in 5'1" of water and clearly not aground. Indeed the lead line indicated 5' of water, So why would my sounder read 6'2" when firmly aground? It stayed at 6'2" the entire time i was aground until I got towed off. I cannot adjust this model of sounder to account for say "0" to be the bottom of my keel or I would. So even in these skinny waters here, what can I do?

I fell really silly getting grounded all of the time, what gives?

TD

[Teknav]

Check the transducer for any obstructions/cracks. Try to temporarily cover it, then check your depth. You should be able to determine where the issue is. (I realize that this will require some work, as it is below the water line and not easy to access.) Check the wiring, while at it! Mauritz

[trooper dan]

Oh yeah the thing is I never ask for help. I would be just as comfortable waiting for the tide to come back in. People are really nice and helpful around here! People just motor over and ask how I am doing... A catamaran looks really sweet right now! What surprised me the most was how firmly I was aground at such a slow speed. This guy today with a 125 HP outboard had a difficult time getting me un-stuck. Thanks goodness for the kindness of others.

[trooper dan]

Originally Posted by Teknav

Check the transducer for any obstructions/cracks. Try to temporarily cover it, then check your depth. You should be able to determine where the issue is. (I realize that this will require some work, as it is below the water line and not easy to access.) Check the wiring, while at it! Mauritz

Well that is the thing, It works accurately.. sometimes, as indicated at my slip. I double checked the usual stuff with it and it appears to work most of the time. I don't even have any kind of idea of what error it has. But even then, here in some places even the entrance to the marina the depth is 9' or so. I am consistently operating in shallow waters with this problem, scary.

TD

[Ziggy]

Also, send an email to uniden: Contact Us - B2C Static Page at Uniden

[DavidLGCrawford]

This may sound dumb, but make sure you aren't in demo mode. If I press and hold the power button on my depth sounder it goes into demo mode and gives completely fictitious readings without any indication it is in demo mode. (You'd think there would be a big flashing DEMO across the screen...)
Cost me a propellor to find that out. Lucky really that only the prop hit and we didn't rip open the hull.

[trooper dan]

Originally Posted by DavidLGCrawford

This may sound dumb, but make sure you aren't in demo mode. If I press and hold the power button on my depth sounder it goes into demo mode and gives completely fictitious readings without any indication it is in demo mode. (You'd think there would be a big flashing DEMO across the screen...)
Cost me a propellor to find that out. Lucky really that only the prop hit and we didn't rip open the hull.

I don't thing I have ever saw demo. I have the manual for it and all its functions consist of a depth alarm. So, yes I have trouble-shooted the usual. It is like black magic is on my sounder. Its EVIL I tell you!

[sailcruiser]

Is it calibrated? I calibrate mine while on the hard.

SC

[trooper dan]

Originally Posted by sailcruiser

Is it calibrated? I calibrate mine while on the hard.

SC

The manual says noting about calibration other than "sensitivity". It is accurate when at the dock, the problem is when moving its not accurate at all! That is whats frustrating sometimes its completely right other times its totally wrong. I don't know what to believe, even the charts are very vague with depth compared to where I have been etc.

TD

[rubyjean]

I used to have a Uniden until I hit the bottom twice in one night reading 35' of water..
I now own a Garmin...... Michael..

[Paul Elliott]

Originally Posted by sailcruiser

Is it calibrated? I calibrate mine while on the hard.

SC

How do you do that, given the huge difference in sound propagation speed of air vs water? Is there some technique? I suppose you could calibrate using a 0.231 scale factor...

[David M]

You can't calibrate it in air. Sound travels more efficiently through water meaning the pulse would be louder and the return louder in water. The transducer may not "hear" the return pulse in air. Also, sound travels much slower through air meaning the Distance = Time x 0.5 / Velocity calculation would be innacurate.

I would call Uniden with your problem and if they do not have a solution then insist on a refund or replacement. It's a liability for them if they have an inaccurate sounder which also cannot be calibrated.

[Reefmagnet]

Have you allowed for the angle the sounder shoots through the hull as well as the angle of heel? Any angle will make the depth appear greater to the sounder. Are you shooting through the hull (i.e. transom transducer stuck in polyurethane or in an oil bath)? Air being introduced between transducer and hull as the boat rolls could cause erratic readings.

Some sounders allow a keel offset setting to be entered, but this is as much "calibration" as I'm aware off.

[daddle]

Because the typical sounder has such a wide angle of sensing the heel angle and mounting angle don't make much difference.

The signal processing to convert the return sound to a numerical depth is non-trivial. A brand-x manufacturer could easily screw it up.

[trooper dan]

I was not under sail at the time, nor under heel. My depth sounder is malfunctioning, I think.