We've done that, or similar at least:
Our last yacht had no echo sounder that'd been installed within an oil-filled tube, inside the hull - no hole through the bottom, yippee! Sadly the oil
had leaked out, so the transducer wasn't working; no doubt because the plastic tube had been glassed-in in the centre of a locker space, so restricted use of said locker and also left it vulnerable to damage.
I'd heard a similar anecdote/rumour to yourself, though it was to set the transducer in epoxy
, but not wanting to risk damaging/losing the transducer if it didn't work as implied, I originally tried setting it in silicone to 'give it a try'. All I did was clean-up a section of hull - because you're not trying to drill holes, or do glass fibre work, you can pick an area of wasted-space, in a really out of the way, well protected location (ours went under the heads, behind the pump) then pumped perhaps 1/3 of a tube of cheap
silicone into a big blob in the spot chosen, left it for fifteen minutes, then pressed/screwed the transducer vertically down into the blob of silicone and left it alone for a few days.
It worked fine for the next ten years and we never did get around to making the job 'permanent' with epoxy
, in fact, we re-fixed it after about eight years again with silicone - no problem with the original silicone, but it was 'in the way' when we wanted to replace a seacock, so we cut the whole thing out with just a modelling knife, cleaned off 90% of the old silicone and when the job was done, just stuck it back in place.
Given the choice, I would always install one this way (it's one less hole in the bottom!) and the only slight disadvantage I would report is that the unit will lose itself at a lesser depth
than it might if you used a through-hull transducer, though that may be just a presumption on my part; comparing a vey old and much new transducer? Our current
yacht has exactly the same make, though later model of echo-sounder fitted, but in a through-hull installation
and stops reading at around 120-130m/400-450' depth, whereas the old one sat inside on its blob of silicone and reading through the hull, rarely read beyond 90-100m/300-350'; once the water's that deep, who cares?