Depends on the fish you're carrying.
First thing, S class sub? If so, you have MARK 10 torpedoes, slow speed only. If not, click the sub management button (the one that looks like a clipboard) then the torpedo icon, hover the cursor over the torpedo to see what it says. For the MARK 14 it will show 2 speeds, for the MARK 16 or MARK 18 only one speed. 16s are automatically fast, 18s are the electric wakeless and automatically slow.
|