Depth Finder Lies

Roger · Sep 8, 2002

Sailing in the ever -shoaling Cape Cod waters, I keep a weather eye on the depth finder. Now I have read here that when it goes from 35 feet to 2 feet to 35 feet on the display, and you have not gone aground with your four foot fin keel, you have probably pinged some bluefish or stripers. But my depthfinder has taken to telling me that there are holes in the bottom of various 100-foot plus depths. Especially, but not exclusively, in my home channel. I doubt it. Is this a sign of depthfinder dementia? Is it telling me it is going to become less and less dependable? Is there anything I can do, short of replacing my trusted (and costly) friend ? I stand in need of the list's deep experience and expertise.

Randy · Sep 8, 2002

Depthfinder issues

Roger-- Depthfinders can give false readings for a variety of reasons: you mentioned fish--that's one. Air bubbles beneath the surface, such as from a passing boat's prop wash is another. Temperature breaks beneath the surface (thermoclines) can cause inaccurate readings. If the depthfinder's gain or sensitivity is too hig or too low, that can cause problems too. Sometimes a depth signal can bounce twice between the boat and the bottom, giving a reading double the true depth. If a weak signal is not returned, your readout may go blank for a while. Here's what I would do: check that there is no bottom growth in the area of the transducer. (Grass or weed fluttering under the transducer will wreak havoc with readings.) Second, look for a gain or sensitivity adjustment, and use it to make minor adjustments when you are in waters with a known depth to achieve consistent readins. Lastly, check to see if your unit has different operating scales--some have a few different ranges such as 1-10 feet (for lakes), 1-50 feet (for coastal areas) and 1-1000 feet (for further offshore). Use the range that applies to your area. I don't think your variable readings indicate an impending failure of the unit. In my experience, they either work, or they don't, and it sounds like yours is generally ok.Randy