At 0225 fired four torpedo spread, range 4,000 yards, from after tubes. At 0229 heard and observed explosion about 75-100 ft. abaft stack. At 0240 observed ship sink stern first. 0250 surfaced, proceeded to eastward, completing battery charge. Ship believed to be Rio de Janeiro Maru, or very similar type, although it is possible it was a larger ship, he was a big one.