Saffir-Simpson Scale

Is the classification of a hurricane only based on wind speeds? I know that pressure and storm surge or often mentioned as defining parameters.

I have some friends in the insurance business that are trying to assess future risk. I'm trying to convince them that although the winds were typical of a cat 4 in Katrina the surge and pressure met cat 5 criteria.

Can 2of3 or 1of3 get you too a cat 5 classification?
Interesting... I assume the scale is based solely on wind, as that's the easiest thing to measure when the 'cane is way out at sea.

There is also another "catagory" - "hypercane". It's only a theory, but it is something to the effect of a hurricane or tropical wave developing over superheated water (i.e. from an underwater volcano)... Such a hurricane would be working with water in excess of 120F, so it would theoretically become quite a monster while over that region of hot water. I haven't researched it much, but it sounds interesting.