Dark Blue Moon (The water monster) makes a whirlpool via his physical strength.
Stand Scaling[]
First we gotta get his forearm length.
His Stand User is around the same size as Jotaro Kujo (if not a few cm taller) and as such I'm going to get Jotaro's forearm length and use that as DBM's forearm length as it'll be the same size more or less.
Jotaro is 195cm tall at 412px (obviously not counting bottom of shoes and top of hat) and his forearm is 40px at 30.7+cm.
Dimensions of whirlpool.[]
I made a perfect circle and a line that goes to the exact center of the circle. Lining up the center of the whirlpool to the end of the line and the edge of the frame to the edge of the circle and treating that as the edge of the whirlpool then scaling off his forearm gets the whirlpool's diameter to be 4994cm+ and halving to get a radius of 2497cm.
(it's stated that he is in the exact center of the whirlpool, if there was any doubt)
Now this takes place very close to the sea floor and can be seen from the surface so I'm gonna use a depth of 62.5m for high end and 30 for low end.
Plugging the radius of the whirlpool and the depth into this calculator and treating it as a cylinder gets a volume of 119067977700.24cm3.
Low end gets 57613537596.892cm3.
Speed of the whirlpool[]
video It takes Star Platinum from 00:00:00:02 to 00:00:01:19 to make 3/4th's of a full rotation (unfortunately it zooms in to where you can no longer see star platinum so we cant get an exact time for a full rotation in that scene, we gotta use math to get the last quarter of it). A full second is 0:24 (24 frames) so it takes from the two frames above, 1.708333333333333354 seconds for 3/4ths of a full ration. Divide by 4 to get 0.4270833333333333385 seconds for the last quarter and add it to the above number to get 2.1354166666666666925 seconds, that's how long it takes to get a full rotation.
We need the rotations per minute though so 60(Seconds a minute)/2.1354166666666666925 (Rotation per second) gets 28.097560975609755757649rpm.
Mass of whirlpool[]
Taking the volume (119067977700.24cm3(high)-57613537596.892cm3(low) and multiplying it by the density of sea water of 0.001027kg per cm3 for a mass of 122282813.09814648kg for high end and 59169103.112008084kg low.
Moment of Inertia[]
Using the MOI of a cylinder, plugging our radius and depth of 2472.44538834951454759326394822006472803cm and 30m (3000cm) and our mass of 59169103.112008084kg we get Mass moment of inertia about Z axis - 18084995534.6842 kg*m^2 for our low end.
And for the high end, 2472.44538834951454759326394822006472803cm and 62.5m (6250cm) for our radius and depth and 122282813.09814648kg for our mass. Plugging it into the above calculator again gets Mass moment of inertia about Z axis - 37375657438.3462 kg*m^2.
Rotational KE.[]
Taking our found RPM and plugging it into this along with the found MOI we get 78285781417 joules for our low end.
And 161790614929 joules for our high end.
Results[]
18.71 tons of tnt (Low-end). 38.67 tons of tnt (High-end).