Depends.
Day? Night?
Sub damaged?
Are they heading towards you? Away from you?
Do you have radar? (Easier to do an end-around if you have)
Any escorts?
If night, where is the moon, relative to your planned approach route?
Too many variables to give a simple answer, need more details.
Initial consideration - 17 knots is awfully fast for a merchant, perhaps they are troop ships? This is the only category of merchants that can go that fast, I think. Unless they are headed for you, you won't be able to intercept them submerged.
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