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This post briefs on the symptoms, likely causes and fix of a potentially hazardous issue on an original-fit Raymarine depth-sounder. As I know for certain that one other person has experienced similar issues I'm detailing it as part of the forum's archives.
Earlier this week I aborted a trip to the Isles of Scilly, where there's often a need to take 'overground' passages which are only passable at high tide. The wind had been fair with everything working well but I needed to motorsail to make a tidal gate in good time. Soon after starting the engine, the depth sounder started reading from 2 to 65m and every depth in between. Charted depth was 65m and I'm sure there was nothing fouling the sensor: no bubbles, weed or submarines below. The integral speed log worked properly throughout. This had happened twice before in previous years, both times again in the region of the Manacles rocks but the issue had ceased immediately on isolating the bowthruster and windlass. While this indicated that the cause was interference with the HT cables which shared some of the route it didn't explain the long periods of full functionality in between in waters far away from the Manacles. However, this time the issue remained despite our best efforts; the gauge worked fine with the engine shut down but never with it running; we tried several times. Although we've actually got a lead-line I wasn't willing to trust the boat to our skills with it around the Scillies so we diverted and enjoyed a fast sail to Plymouth with everything reading properly. To our surprise, starting the engine to enter a lock the wretched sounder behaved perfectly, mocking our earlier efforts.
Starting to wonder if there was some sort of electronic 'Bermuda Triangle' around the Manacles, on later tracking the route of the transducer cable I found that no less than about 5m of cable had simply been stuffed behind the inaccessible forecabin locker where it was loose to jumble about in close proximity to all other cables, including the leads to the windlass and bowthruster. The end was wired into a grey box under the fuse panel where it terminated in a number of spade fittings, instead of the more modern Raymarine STNG fittings with which all but the (2008-vintage) Airmar transducer had been replaced in the past 3-4 years. This presented a number of potential causes for the random symptoms experienced: poor connectivity, excess cable length, electronic interference and corrosion being but a few.
I got a new transducer with an appropriately-sized cable which plugged straight into the STNG system and - so far - all looks good. The acid test, however, will come on returning to the Manacles where, quite possibly, 'there be (electronic) dragons'......
I can only conclude that the excess cable had been present from build and was lazily installed, something which seems even with brand new boats to be a not-uncommon failing. Anyone experiencing similar symptoms - or anyone just curious - may care to identify the transducer cable where it passes under the port-side chainplates and give it tug.
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H400 (2008) 'Wight Leopard' Gosport, UK
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