The explanation is rather simple and is also described in Frank Bethwaites excellent book.

Yes, slender and long hull are predominately displacement hull. They may partly plane, but will never do so in an efficient manner. A small side step here : A planing surface is the most efficient when the area that is planing has the least amount or rim (edges) where energy can "escape". Ofcourse a squarelike hull has more are per length of edges than a long hull with long edges.

Planing on cats (long slender hulls) can be introduced partly but will never play a very big role and it must be regarded as refinement rather than a revolution. Boat like the Taipan are more likely to partly foil on their daggerboards, but then again the jury is still out on that one.

Okay, Back to the issue at hand WHY do cats go faster than what teh displacement formula allow them to go.

That is because the Froude law is often regarded as a absolute limit while is merely states that drag MAY significantly increase at that particular boat speed. It is just like sound barrier. 50 years ago everybody believed that that this was a absolute barrier untill somebody proved that it was just the shape the fusalages of the day that caused a jump in drag at those speeds, preventing the planes from going faster. They than altered the fusalage shape to the famous coke bottle shape and voila teh new plane with a very similiar engine puched through it (After the X-plane proved it could be done at all)

Froudes law states that for a given displacement (alot in monohulls , alot less in cats) the drag of the craft will increase significantly at hull speed. Later it was found that the creation of a wave system was largely to blame for this. Problem with cats is that at their calculated froude hull speed that they don't create a significatn wave system due to their slender hulls. Also the drag cats experience at Froude hull speed is rather small and a singnificant increase on a small drag total often amounts to only little drag just the same. Ergo, the Froude Law is applicable to cats but its predicted effects are simply to small to make an impact. The Frisian Engineeer Froude lived many centuries ago and didn't forsee the creation of long slender hulls. And ofcourse most sailors aren't maritime engineers and only repeat what others say in a dogmatic manner. Especially leadmine sailors suffer from this. Surfers, skiffies and catsailors just looked for escape routes and found them.

Wouter