Sometime later in the game you will get electric propulsion torpedoes, which leave no trail on the surface. Actually until then, you will probably be as comfortable with the steamers to not bother with them.
With Destroyers and sometimes Merchants detecting your torpedoes, you'll have to expect and account for it in a way, but (sadly) there is no "magic" solution for this problem.
Anyways, you usually should be able to find juicier targets. Not to discourage you of killing Destroyers when possible. Heck, I was looking once for the Battle of the Coral Sea and all I found where a couple patrols of 1 Destroyer and 1 Gun Boat. Killing these Destroyers backfired when I got ordered to photo recon the carrier presence at Raboul. Finding two stationary carriers and a load of patroling destroyers, made me curse about using most of my torps prior.
Oh and, welcome to the SubSim [Insert Dramatic Music]
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"No Boom?" "No Boom, today. Maybe Boom tomorrow. Things always go Boom!"
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