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Full Version: [TG11] Odd readings from an ST800 on an ST70
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Hi, thanks in advance for any help in tracking down a pretty strange issue we're having...

I have a 35 foot sailboat that is new to me as of two years ago. Last year the old analog depth transducer which had fed an old Tridata ST50 display and an ST70, failed.

A new DT800 was installed beside the deprecated analog transducer and the DT800 was networked ONLY to the ST70 via an STng backbone. The Tridata is now entirely out of instrument topology.

The first ST800 immediately gave erroneous readings and was replaced by our Marina promptly.

The new ST800 works most of the time. However it will occasionally, and almost always while under way by either sail or power, stop reading the actual depth and display readings such at 2.3, 2.5, 2.7 feet. It will do this for a long time, many minutes to an hour or more. Power cycling the ST70, unplugging and reconnecting the DT800 and terminators or varying boat speed does not have any effect. Occasionally - the readings will return to normal. As this happens, the depth display will flash several times - and then the correct readings will appear. Once in a while - this flashing behaviour will occur, but the correct depth will never appear and the display will return to "2.7" and stay steady. The 2.7 reading will stay even if the ST800 is unplugged - so I wonder if this is a failure mode reading? Or latent signal of some kind?

I have checked for any external problems with the xducer (dove the boat with Scuba) and all appears fine.

The one action that DOES appear to solve the issue (it's worked four times in a row) is to slow the boat when motoring - REVERSE - and then engage forward. Viola! The depth readings return to normal!

We sail in the perilous waters of Georgian Bay and need to have reliable, trustworthy depth readings all the time.

I've worked through your excellent xducer troubleshooting posts and nothing has worked so far.

Anything more to check? I did see a mention of transient, ultra-short voltage drops possibly forcing the DT800 to reboot - but that doesn't seem to explain this behavior?

Further - when the depth readings are wrong - we DO seem to still see temp readings on the ST70 - so unless those are being held in some sort of cache somewhere - the DT800 seems to be "on" all the time.

Thanks for any insight!

Hello RKonGB,

What that sounds like to me is an echo off the keel. We've run across a few cases. The attached extracts from our Dealer training give a little background. This is, in my experience, the usual cause for a consistent, false, shallow depth reading on instrument systems and happens because the beam on these transducers is so wide that it can easily reflect off boat structure. Backing the boat up directs aerated water across the transducer, blocking the echo and breaking the 'bottom lock' on that false shallow depth. When the aerated water goes away, the system can then reacquire a true bottom lock afresh.

Fascinating and seems likely. The image you attached depicts our keel exactly (A Beneteau 351).

Save for changing our new xdcr or keel (lol!) we'll adopt the SOP of reversing before heading into perilous waters if we're "locked on the keel".

Is there any chance that rotating the xdcr could move the polar pattern to avoid the keel? Any reason that would be contraindicated?

Thanks for the insight.

Ah yes, that's the bit I meant to include in my last reply, sorry.

In the last 2 cases I've been involved in, we recommended changing from an 'instrument-type' transducer to a tilted-element fishfinder type connected directly to a compatible multifunction display, and using the tilt of the element in the transducer housing to orient the beam forward and away from the keel. For example, one of the boats had an eS98 MFD and the installer changed the instrument P319 transducer to a CPT-S 12-degree tilted, which has the same cutout, and that solved the problem.

What specific part number transducer is it that you have, though? If it's A22112 then that is a tilted-element type you're absolutely right, rotating it so that beam points elsewhere should solve the problem. In that instance you'd want the keel arrow on the transducer cap to be pointing forwards instead of to the keel.
If you have a non-tilted DT800 (e.g. A22165) then rotating it won't help - according to Airmar's acoustic info for that element it's symmetrical/circular - so it would be a case of fitting an A22112 into the same housing, or replacing with a CPT-S, B60 or SS60 if you have a compatible MFD aboard that can connect to one of those transducer types. They're all available in tilted versions.

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