Hi-

The D CLASS -- I think the original ISAF classifications for cat design were the A B C and D classes based on basic graduated length beam sail area specs.
This was the most logical way to compare and race various new cat design .

The A class still exists in its nearly original form though believe the original A class sail area was 170 sq ft instead of 150 now --the larger sailors would be more partial to the original -basic A specs are -single handed
150 SA 18 FT lENGTH 7.5 FT BEAM -now a 160 LB min boat weight added -
The B class -10 ft beam --20 ft lENGTH -230 S A WAS TAKEN OVER BY THE EXCELLENCE OF THE TORNADO DESIGN -for Olympic trials and has been the Olympic cat since -

C Class cats --Little Americas Cup -etc developed wings and numerous innovations in design --basic specs are
300 sq ft sail area -25 ft length -14 ft beam

The D Class specs are 32 ft L -and max 500 sq ft sail area

--The benifit of developement type classes based on basic specs are obvious in the innovative creative aspects and wonderfull boat design created as result as the current A Class exemplifies ,-
Similar Formula type class categories are a return to this basic classification for racing ,--though more limited developement in scope per each classes rule variations ,-

MFG one design classes are great too -but I for one prefer the innovation of design and modifications allowed per individual preference and creative innitiative per developement classes . The main reason multihuls were and are built !!to innovate and make them faster and safer for sailing enthusiasts that enjoy speed that multihulls can achieve.--
If OD is your main consern or objective ,-race a mono that limit speed similarly and go slow all the same --this is also indicative of the problem with Porthmouth system handicap rating ,-it works for monos to some extent due in part to limitations of monohull speed ,-but speed differences are far greater on cats and again multihulls main reason historically for being built as innovative speed sailing craft that can be developed to be continually faster and safer for those sailing them .-

In other words ,-the old Hobie 18 is great ,-but much prefer A Formula 20 with spin snuffer and my own innovations and preferred set up .

sorry -a little too wordy and opinionated --as usual


all the best
Carl