best depth for hydrophone

[rosentorf]

Hi y'all,

I have a question regarding the best depth for listening with the hydrophone (e.g. checking your surroundings for surface ships when you're in your patrol area). I usually go to 100 ft and then do my check.

I wonder is it necessary to go that deep (100 ft) or is periscope depth just as good. If it is, then there is no need to go deeper which is less of a hassle if you have to do it frequently (it's the reason why I ask). If it is better to be deeper than that then I would like to know:

Which does apply? (always being higher than thermal layer)

1) the deeper the better
2) at least deeper than x and higher than y

and also

if there is a best depth, what do we gain from that? I am assuming greater range, but how much approximately?

Of course any suggestion made above always include being higher than the thermal layer as being lower would make no sense right?

Thanks

rosentorf