We've positively identified the submarine.

Most modern warships are equipped with over-the-side torpedo launchers stocked with light-weight torpedoes. These are the same torpedoes that are dropped from aircraft--they have a limited range and relatively small warhead compared to the heavy-weight torpedoes carried by submarines.

Outside of this training scenario, closing with a submarine with the intention to attack it using light-weight torpedoes fired from a surface ship is not recommended.

Against modern submarines with wire guided torpedoes, these light-weight torpedoes are little more than self-defence weapons. The most common use of these over-the-side launchers is to fire a snap shot down the bearing of a detected incoming torpedo before commencing evasive manoeuvres. The chances of destroying the submarine are slim, but the possibility of forcing the submarine to break its guidance wire is quite high and this increases the ships chance to evade the torpedo.

With that said, the submarine we're about to engage has all sensors and weapons removed and is no threat. Engage and sink the submarine with torpedoes from your frigate. Just remember that this is not how you should be routinely engaging submarines with surface ships.

You may also wish to practice bearing-only launches, simulating the response from a detected incoming torpedo. Press Ctrl+F1 and click on a position within torpedo range of your ship. A weapons allocation dialog will appear with the position as the target; this is where the torpedo will commence its search pattern so set the position as close to the suspected position of the submarine as possible.