The mechanism of whirlpools
What causes tides?
Tidal currents cause the whirlpools at the Naruto Straits. The tide flows in and out due to the gravitational effect of the moon and the sun on the Ocean. As the earth rotates around its axis each day, the high tide alternates with the ebb tide twice a day, making each cycle about 6 hours long.

What causes the whirlpool?
The high tide water from the Pacific Ocean comes to the Kii Channel and goes through Osaka Bay and the Akashi Channel due to the Naruto Straits being too narrow.
It takes about 5 hours to hit Harimanada from where the high tide water meets another tide that comes from the Bungo Channel to the Inland sea.

Want to see large whirlpools!?
When is the best time to see the whirlpool?
The highest tides called spring tides come twice a month when there is a full or a new moon.
During this period, the moon and the sun°«s gravitational effect overlap each other.
There is a great difference of water level between the high tide and the low tide so the flow of water will be fast. At Naruto, while the spring tide is coming in or out, the water flow can reach more than 20km per hour, especially in spring and autumn.
Spring tides from the end of March to the end of April are the best time to see larger whirlpools due to the large amounts of snowmelt water.