They are formed two times a day and the size varies by season, moon phases and by hour. The strongest whirlpools are formed from the end of March to the end of April, during full moon, and the second strongest period is in autumn. During the highest tides, the whirlpools are 20 meters in diameter and the speed of the water reaches 26 km/h!
Travel tip: On the Uzu-no-michi official site you can find out the time of the fastest currents so you can plan your trip to Naruto.
EXIF info:
Nikon D700
Lens: 24-70mm F/2.8G
Focal Length: 70mm
Aperture: F/6.3
Shutter Speed: 1/1500s
ISO Sensitivity: ISO 200