Were the Japanese mini submarines the Kaiten? Were the Kaiten successful?

No. The Kaiten were much smaller and were not developed and put in service until late 1944. They were developed as a modification of the Type 93 torpedo and had a crew of just one pilot.

The Kaiten were not very effective, although the Japanese certainly thought they were.  The first Kaiten operations were delayed until November of 1944. The attack at Ulithi on 20 November sank the USS Mississinewa (AO-59). Our ASW forces then spotted the other Kaiten and sank them.

The Japanese claimed great successes by the Kaiten in the last three months of the war. However, the claims of success were dubious: “[Japanese] Sixth Fleet Headquarters exaggerated the effectiveness of the Kaiten in the second Gen operation, especially before the loss of the I-48. Indeed, no ships were sunk, but as a U. S. naval intelligence officer noted, ‘The Japanese assessed the results as eighteen vessels sunk, including one converted aircraft carrier, nine large transports, one cruiser, and six other large ships including aircraft carriers, battleships and transports. Such optimistic estimates can only be the result of inexcusably poor staff work.” (“The Japanese Submarine Force and World War II” by Carl Boyd and Akihito Yoshida, page 170.)

This is confirmed in “Suicide Submarine” by Yutaka Yokota and Joseph D. Harrington, (also known as “The Kaiten Weapon”, page 256.) That text indicates in the Epilogue: “When World War II closed, the Japanese thought they had sunk some 40 Allied ships, including a British cruiser of the Leander class through their Kaiten effort. A check of all available sources reveals they sank only two U. S. Navy ships, the tanker Mississinewa in the Kikusui sortie and the destroyer escort Underhill. This second ship was actually sunk by friendly forces on July 24, 1945, after being hit by a Kaiten. Only one U. S. merchant ship, SS Canada Victory, was the victim of a Kaiten, going down on April 27, 1945. These three vessels, measured against the 8 submarines and nearly 900 Japanese lives lost in the Kaiten program, make the enemy’s sacrifice seem fruitless.”

Moreover, there was a great cost in development and use, “from the time of the first Kaiten sortie on 20 November 1944 until the end of the Okinawa campaign, Kaiten took 7 lives in training and 48 in action, without achieving any significant confirmed results.”  Further: “The cause of the failure was not the weapon, but rather the manner in which it was employed.”

(“The End of the Imperial Japanese Navy” by Masanori Ito, page 196.)