I think my initial post was unclear.

The following resource does not use the 1.34 constant, and seems to go into some depth about the actual factors used to calculate drag for catamarans:

http://members.xoom.virgilio.it/mfcatamaran/hull.html

After citing that resource - and I can't vouch for it's accuracy, but I can say that it looks detailed and careful - I then attempted to say that a straightforward application of the hull speed formula using the 1.34 constant - which is NOT advocated in the URL above - would give very low speeds indeed for my boat.

Jonathan