OK...The improvements have been evolutionary rather then revolutionary. (That was my point in posting the picture in post #56) Before the 1970's, it could take a 3-4 fast designers weeks to generate a stability curve for a single design and it would not be terribly accurate since it did not include the impact in stability that resulted from changes in trim. The first reasonably successful Velocity Prediction Program (VPP) came out of the work of Jerry Milgram and George Hazen at some point in the early 1970's and filtered into the yacht design world at some point in the early 1980's
But the computers of the day were slow and so the analysis results were not especially detailed, granular or accurate. I remember hearing that a mini-computer could be set up to run and would run over night and into the next day, but could produce stability curves and a VP. That seemed miraculous.
Since then there has been a vast amount of research on developing more accurate calculation methods and validating the data produced. (Validation in this case is comparing real world measured values to computer simulated values, and incrementally improving the accuracy of the computer simulations.) The research has certainly looked a being able to quantify performance oriented characteristics such as lift to drag, and load paths for real world forces. But it also has looked a motion as well. The ability to quickly compare the impact on motion from perhaps moving a battery bank or making the stern a little fuller is a pretty recent tool (I would think less than 5-10 or so years) and can be done in minutes and not days on a laptop computer, so the designer can tweak to their hearts out, until they have the best design that they are willing to keep looking for.
But its not just about computers since there are other non-computer driven innovations. As these newer boats, within a boat's normal usable service range, have greatly increased stability and reduced drag (as compared to similar displacement older designs) , sail plans have gotten much more efficient allowing smaller sails to accomplish the same thing as larger sails and in turn be much easier to handle across a much wider wind range. Better sail handling gear has added to the making sail handling easier as well.
In other words, there is no one point in time that there was a sudden change, and it is not one single thing that suddenly changed, it has been a slow and steady improvement, incremental improvement to hundreds of smaller components that make up a boat.
I will add this one last example. You and I sail boats that are roughly the same age, and while the hull forms are very different and your boat is nearly 50% heavier than mine, the rigs are very similar. Our fractional rigs were state of the art rigs in the 1980's for performance cruising boats. Compared to modern fractional rigged performance cruising boats, they were pretty small fraction (7/8 or so) rigs as compared to 15/16th or 16/17th rigs of today. So what changed? When our boats were designed, performance cruising boats only used panel-cut broad seemed Dacron sails. There was a limit to the aspect ratio before leech stretch made those sails ineffective, That meant that there was a limit to the aspect ratio of both the mainsail and jib, and as the foot of the mainsail became bigger, and the foot of the jib became smaller on a fractional rig, the luff of the jib became smaller relative to the height of the mainsail.
At some point better stress mapping in the sails allowed the fiber orientation to be rotated on panel cut dacron so that there was less leech stretch and that allowed the jib's aspect ratio to increase and the fraction to get bigger. At the same time small improvements in the fabric itself reduced stretch so the forestay crept further up the mast. Then cruising laminates came on the scene and once again the aspect ratios increased. That increased aspect ratio required more stability, and coincidentally stability was increasing during this same period as well. And in combination these changes produced boats that were wildly easier to sail, much more forgiving in changing conditions, cheaper to own, faster, and so on. And that is just one small slice of the myriad changes going on behind our collective backs.
So what is the year of "modern designs" if this was... and I accept and agree an evolutionary process. I am not an observer of production yachts... so my impression is mostly what I see where I moor my boat and pass close enough to see the "design"... sailing somewhat... but I don't get a close look. Can a date be fixed for the beginning of the modern cruising yacht design?