The optimum angle for the shroud
to mast connection is around 12 degrees. Any more acute angle, the compression
load skyrockets. Larger angle, the compression
load drops off. On a mono hull
you are limited to how far outboard
you can put the chainplates and still get that 12 degree or more angle at the masthead. To keep that angle at the masthead and the resultant compression load on the mast at a reasonable level, masts are built with spreaders. On a multihull
, you aren't limited by the width as severely. Designers can get that magic 12 degree angle or even larger by mounting the chain plates out on the hulls without spreaders.
Strangely, the most complicated rigs I've seen have been on multihulls. To keep from having shrouds angling out to the hulls and the mast in column, they've gone to spreaders and wire that are anchored to the base of the mast with only a spreaderless cap shroud
to the masthead to hold it up.