It is not just a large single whirlpool - though it is generated by vortices from a single pinacle of rock, the vortices vary in strength and move with the current.
I have pinched the following from:
"A brief outline of how the 'whirlpool' works follows. 'Maelstrom' would be a better description.
The whole of the Sound of Jura is a tidal anomaly. The tide is low in Lochgilphead when the tide is high in Crinan. It is absolutely diametrically opposed.
This is curious when only six miles separate the two, on land at least. However, the Mull of Kintyre, a peninsula over sixty miles long, forms a wall opposite the island chain formed by Islay, Jura, Scarba, Lunga and Rubha Fiola.
As the tide ebbs south past the gaps between these islands, it sucks the tide up the Sound of Jura, flooding and rising north. It then flows west, out through the Corryvreckan, The Grey Dog and north through Fladda.
The maelstrom in the Corryvreckan works differently when the tide is ebbing from when it is flooding.
On the ebb, water is flowing in relatively undisturbed from the open sea.
On the flood, water has flowed up the Sound of Jura and has been agitated by the topography of the seabed.
There are innumerable humps and holes and reefs in the Sound and these create terrific tidal flows, up-thrusts and eddies all over the place until finally in the Gulf there is a huge hole down to 219 metres before being confronted by a pinnacle of rock off the Scarba shore which rises to 29 metres from the surface.
The steep east face of the pinnacle forces a massive upthrust of water to surface in pulses which are then swept away westward by the tidal flow and these dissipate into vortices or whirlpools moving west.
This is all clearly visible when there is no wind and the turbulent patterns are fascinating to see in mirror calm conditions.
However when there is any serious wind strength, particularly from the west, the up-thrusts at the pinnacle fold into the oncoming waves and accentuate them. Thus building, in gale force conditions, standing waves that can be 8, 10 or 15 feet high. A truly awesome sight!
When the wind is from the South East or from N or E of North, then the Gulf is in the lee and it tends to be less rough - one can see the pure tidal turbulence. The relative state of Springs to Neap tides also affects the strength of the flows and hence turbulence. So as you can imagine there are many, many factors which affect the performance and so one can never be entirely sure how it will be behaving before one gets there. "
It is a very special place and well worth a visit.