Wonder if the primary effect wouldn't have been detection via the noise.
You could fake this with a very narrow cone hydrophone with limited range directly aft of the ship. Make it not sensitive to the silent running state of the target if that is possible. Maybe it could be active without a ping noise (or change the ping sound for it to a scraping noise.
Again, the only point here is detection. The trawler would hear the scrape, then immediately drop a DC.
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