Here's an interesting Portsmouth calculation:
What is a "uni"? A sloop, but sailed solo and without a jib. OK, take any standard sloop 2-up rating and apply two modifications. First, sailed "light", you take a weight modifier. Since the F16 has no class weight minimum, Portsmouth requires you to take the L4 0.97 modifier (a recent and very wise rule). This says "lighter is faster". Second, remove the jib and apply the "US" 1.026 modifier. No jib will slow down the faster light boat. Now, these 2 are combined...
0.97 x 1.026 = 0.995
A very small change indeed! So Portsmouth says a uni is SLIGHTLY (0.5%) faster than a sloop. I agree, in 16 foot boats weight is a HUGE factor and the small jib only works upwind.
Bottom line, Portsmouth says uni and sloop are nearly identical. My thinking is the separate numbers for the F16 uni vs sloop flies in the face of normal Portsmouth reasoning as the F16 numbers, 67.1 and 65.2, says the uni is 2.8% SLOWER than the sloop.
If you register F16 or F18, no modifiers should be be allowed. If F16 says uni = sloop, so be it. If you register for Portsmouth "open class" as Taipan or Blade, then you use modifiers and suffer the 0.5% modification (big deal). Basically, uni still equals sloop.
Dump all specialized numbers! Each boat gets one sloop number, then apply the modifiers. Modifiers are required for everyone else so why should F16, Dart 18, or the Supercat 15 get special numbers?
Dump the special ratings. To use Portsmouth you have to believe in it and let it work. "Let us proclaim the mystery of faith..." Portsmouth is God.