The mechanism behind the whirlpools
The Naruto Strait that runs between Shikoku and Awaji Island is home to one of the world’s three major tides, alongside the Strait of Messina between the Italian peninsula and the island of Sicily, and the Seymour Narrows between the North American West Coast and Vancouver Island. Topographically speaking, there are tidal currents coming into the Naruto Strait from the Kii Channel and the Harima Nada part of the Inland Sea, but with the times of high tide from those two areas being different, the change in water level can be as high as 1.5m, so with the water rushing from a higher position to a lower position, the center of the water flows faster while the water on either side of the center flows more slowly with the change in speed causing the whirlpools. Furthermore, directly below the Onaruto Bridge, there is a V-shaped underwater ridge which falls to a depth of 90m, and with the Naruto Strait having a distinct concave topography as the southern edge of the strait falls to 140m while the northern edge is 200m deep, the flow of the seawater further contributes to the whirlpools. In a day, high and low tides occur twice each every 6 hours which is when the whirlpools appear. Also, twice a month, with the arrival of spring tides at the times of the full moon and new moon, the water flow becomes faster, and especially from the end of March to the end of April, the spring tides produce whirlpools of up to 20m in diameter. Please refer to the homepage in advance for the peak times to observe the whirlpools.
Observe the approaching whirlpools
To appreciate the whirlpools, you can view them from the Tokushima Uzu-no-Michi bridge observatory 45 meters above which has a glass floor directly above the whirlpools or the sightseeing boats which actually approach them. The Wonder Naruto is one such large boat which can hold up to 400 passengers and allows them to view the whirlpools over a leisurely 30 minutes. The Aqua Eddy is a smaller fast vessel with an observation cabin that goes 1 meter underwater from where you can see the appearance of the whirlpools in the ocean itself for a different thrill. Advance reservations are required for the Aqua Eddy. You will want to experience the exciting roaring currents as you approach the whirlpools.