Hull speed is indeed the square root of the waterline X 1.34. The trick is to calculate the actual power required to move the boat at hull speed. A very good book to give you some background on this is "Skene's Elements of Yacht Design" by Francis S. Kenny. He has offers a very good explination of how to calculate power needed to move a yacht at hull speed. Another book on yacht design by Dave Gerr also discusses this.
Power need is a function of hull shape, prop size, and some other element. On also allows for a reserve. There is great deal of difference between power needed in flat water and that needed if powering into a headsea or tide/ current.
You will be quite surprised to see how little power one needs to move a Hatt 53 at theroetical Hull speed which is about 9.4 knots. My guess is 250 HP would be more than adequate. Take a look at some of the old British Steel displacement yachts built in the 1960s. I was on board one that was 106 feet, powered by a pair of 200 HP engines and was only burning about 14 gallons per hour at 11 knots.
Dave Gerr has designed some interesting yachts with low resistance hulls that are easily driven. He presents one in his book that is around 15 feet in beam, 85 feet in overall length, and cruises at 12 knots with a 340HP engine.
I think if we really put our minds to it, a Hatt 53 could be repowered with a set of efficient smaller eingines that generate power at lower rpm and might really enhance the ability of owners to use the boats more efficiently at today's higher fuel prices.
Engines such as the John Deere / Lugger with the proper reduction gear and prop would be a good start.
