Due to a lack of PY data for cats. Original UK conversions used the original classic tornado rating (1.00), took its PY rating (691 I think at the time) and used the SCHRS difference to give a PY to use for the cats. Not ideal I know but some clubs were happy using it. Now I think they use the F18 as the datum point for calculation purposes. You can calculate the same using your US portsmouth rating for the F18 as datum.
Cheshirecatman
There is a fiddle factor to convert SCHRS to PY of "multiply SCHRS by 686"; BUT remember you are not rating like with like.
PY rates the sailor too; I would thus suggest (as the RYA do) that you then convert the resultant PY to something meaningfull for your club and conditions.
