To Amos & others who might be able to help me out on this. I'm ex-navy too, 60's $ 70's,and the underwater sound equipment then was good enough to detect the noise of the exhaust bubbles from a divers demand valve, at some distance too. Even if the screws were stationery the sound print from the trawler must have been extensive with the operating noise of diesel generators, compressors, air-con & and refrigeration machinery, sundry pumps etc. Now, the vast leap in technology in the last 30 years would lead me to believe that the sound detecting equip. on US subs should be able to pick up the sound of my goldfish farting in his tank in the kitchen.Well, nearly. How come they didn't "hear" this trawler cranking out all this noise ?

Regards / John Gray F.M.E.