Hi -
good thread ,-always been the major problem with large cats and tris and why many monohullers look down on catsailing deeming it unsafe .

Multihulls are safer in most regards ,-positive floatation ,-not sinking if rolled over as many monos will with their lead mine keels to pull them to the bottom once evan partially filled with water. -being the biggest single advatage over them . Multihulls are faster ,-can run up on beaches if designed to do so, and can float if inverted keeping crew safe for days if needed and designed correctly.

As boats or most anything scale up --become larger,-- there are a whole series of compounding engineering problems to address .
Interesting ,also --if one doubles the size of a cat design ,--say from 18 ft to 36 ,--is the 36 faster ?? ---NO often not as fast . ! Why ,--again scale and measurement we use does not corespond in direct proportion to the engineering realities .
We use math ,-but different units of measurement to form our comprehension ,weight units of measurement , strength of materials ,-stress in bending aspects,--adhesive and expoxy .f g strengths ,-metal strengths and fitting ,--hardware .- basic scantlings descibing boat construction in numerous forms ,--metric -imperial measurement ,-weights ,-volume ,-buoyancy ,-moment ,-S A forces ,---none of these corelate without applied engineering calculation relative to one another . These do not uniformly scale with size ,---they are all very different .
Going from scale models to full size the --law of mechanical similitude is applied . ---Here is an example applied to planning craft ,--- http://naca.larc.nasa.gov/reports/1936/naca-tm-795/

-Getting back to the speed question ,-a 36 is often slower than the 18 at half the size ,--why ? --
For many of the same reasons sticking a huge mast float or airbag at the top of a mast does not work ,--why ?

Many that build small scale models ,-be it sail speed trial models , then converted to large scale models fall apart of break up ,-like the resent double masted cat design for the RACE round the globe ----why ? - Scale ,mechanical similitude ,-engineering strength problems that increase rapidly and compound with size disproportionally with other design features that do work in smaller scale . --
These 120 ft cats had to solve these types of engineering problems which had never been directly tackled at that huge scale ,--They were experimental to some extent .--
Recall the bulkhead crossbeam and other related stress -structural problems during the RACE .This is HOW WE LEARN grow and achieve ,-the benifits in future engineering and design applied to help create future safer faster craft far outweigh the setbacks and problems initially overcome in these fantastic cat designs raced round the globe .--
Can't wait for the next one ..

--Added for larger cat design . --
Along with the A FRAME rig --per post above ,--the design had larger bow sections above the sealed crossbeams and an extended aft crossbeam hull section as well again extended above the crossbeam . When inverted the design still floated on the extended inverted hulls.. Simple concept ,-basically build hulls with entended rounded tops for headroom and place crossbeams and hatches below , --add the open stern design features .--This type of design is safe when inverted indefinately and could be powered and brought into port ,--along with the righting system described .

now about this rating system ,-- ---
have fun
---Carl