Larry,
The SC20 came out in 1978. This was a 20ft by 12ft by 276ftft of sail. It had a DPN of 62 back in the early 80s. Add a spinnaker and it is 59.5.
Today there are new boats with the same sail area that are 8.5ft wide and require two big guys to hold'em down when the wind blows over 12 knots. The SC20 with its 12ft beam made it possible for two average size sailors, a guy and his girl friend, to sail competitively in 15 to 20 knots of wind. It takes a given amount of righting moment to drive 270 to 290ftft of sail area efficiently. You can get this torque with two big people on a short lever arm or with two average size people and a long lever arm. The choice is yours.
There was a light air version of this boat called the SC20TR, tall rig. It had about 325ftft of sail area with a 38ft tall mast. This boat developed a DPN of 60 without spin. Add a spin and you get a DPN of 57.5. Today the ARC22 has a DPN of 57.3. So, Larry, what's new??? Where's the new technology? Where are the breakthroughs? All I can see is that boats are lighter weight today because of improved materials properties. That is not improved boat design. Spinnakers have been added. Spinnakers have been around for 50 to 75 years or more. That is nothing new. Boats with unirigs and spinnakers are sailboats/sailplans tailored to the windward leeward race course. Go back to the triangle race course and the sloop rig with spinnaker is faster. Larry, I'm having a hard time finding any real, true, pure improved boat design. What do you want to see????

As far as an 18 footer goes: Take a SC19 and move the transom forward 1ft. This was done with much success years ago in the 18 sq class.The boat had a higher top speed than other 18sqs and it was much harder to pitchpole. If you want it, it was there 18 years ago. Where were you?
Good Sailing,
Bill