The Suntory Mermaid II successfully completed late Friday night a 4,350-mile trip from Honolulu, Hawaii, to the Kii Channel off the east coast of Japan, marking the longest known voyage by a wave-powered boat.

The journey was undertaken by a Japanese team to demonstrate that an environmentally sensitive propulsion system powered exclusively by waves can operate in real-world conditions.

The bow-mounted mechanism, which harnesses wave power to provide a dolphinlike tail kick from two independently mounted flippers, was designed and built by Dr. Yutaka Terao of the department of naval architecture and ocean engineering at the Tokai University School of Marine Science and Technology in Japan.

The design team originally estimated that the 31-foot-long, three-ton catamaran would average 3-4 knots and arrive off the east coast of Japan about 60 days after its departure on March 16. But, unusually good weather and calm seas resulted in the boat traveling an average of only 1.5 knots, and the Mermaid’s maiden voyage ended up taking 111 days. Nevertheless, Dr. Terao and his team were satisfied with the result.

“We were able to prove that our propulsion system delivers a 7,000-kilometer voyage,” Dr. Terao said in an e-mail interview from Japan. “And we can easily improve the speed. In fact, the improvements have already started.”

Kenichi Horie, the ecologically minded sailor who captained the Mermaid, has set two world records for piloting environmentally sensitive boats, the first in 1993 for the longest distance traveled in a human-powered pedal boat (4,660 nautical miles) and the second in 1996 for the fastest Pacific crossing in a solar-powered boat (148 days).

At a dockside celebration on Sunday at Shin Nishinomiya Yacht Harbor, Mr. Horie told the gathering: “The time has come for us to shift from fossil fuels. I hope this voyage will increase awareness and interest in natural energy.”

Mr. Horie, 69, appeared energetic if noticeably thinner after his three and a half months at sea.

“I had some food left, so I could have enjoyed the trip a bit longer,” he said with a smile. “But I think I’ll save it for the next voyage.”