Erratic Depth Readings at Anchor

[John T1594]

OK, so I have a set of original, factory installed instruments on my 2000 H340. They include Raymarine ST30 Depth and Speed displays and an ST60 wind instrument. The Speedo transducer went bad last year and I was able to replace it. My ST60 wind mast head unit blew away in hurricane Sandy so I had that replaced. This year I am getting some erratic depth readings at anchor. The depth readings seem normal when I am underway with the engine running. We usually anchor in a local cove with a sandy bottom in about 6 feet of water. When I turn the engine off, I get changing depth readings of from 7 to 20 feet with the numbers running up and down. When we leave the anchorage under power the reading stabilize at the correct depth. I have not noticed any problems under sail but then again we don’t sail into such thin water or stand dead in the water. The surface of the transducer is clean of growth and free of bottom paint except for a light coat of transducer paint applied about one month ago. I know my 15 year old transducer may be aging but wonder if anyone else has seen this issue and has any advice on how to troubleshoot.

 

[Ralph Johnstone]

When I turn the engine off, I get changing depth readings of from 7 to 20 feet with the numbers running up and down.

We still have the original ST 30 depth and so far no problems after 16 years.

The fact that the readings run up and down at anchor is interesting.

Couple of questions:

Do the numbers run up and down (at anchor) WITH the engine running ?

Can you get a voltage reading at anchor without the engine running.

Any chance of checking this out at your marina while tied in place ? (engine running and not running). No chance of error on the bottom.

 

[Gordon Taylor]

We had a problem with erratic depth readings from our ST60 system a couple of years ago. Displays and on the chart plotter depth would periodically change to show very shallow water. It's most disconcerting to be in deep water and suddenly see 7' on the depth sounder! I was sure that the transducer must have been going bad but tech responses I got were that usually when that happens the display will show last depth or nothing... Not a shallower one.
We had a 6 month old puppy and he was being crate trained and we had placed his folding wire crate directly on the v berth wood panel with the cushions on either side of the crate to wedge it in place. Our transducer is right behind the v berth. The wire crate and transducer were only 3' apart. With the engine running I had heard the crate door humming on occasion. Anyway we moved the crate and the problem went away. Has not returned in 2 years. Is there something metallic near the transducer that could be vibrating and returning and echo to the sounder though the boat itself?

 

[Sailm8]

Had a similar problem. Cleaned all of the connections and problem was fixed.