Spent some time talking with Ralph Narranjo at a yacht club event last winter.
His take was full keel 'traditional' boats tended to sail less at anchor due primarily to keel design and higher wetted surface area. These boats do not typically need riding sails
Fin keel boats both moderate and 'racer' types sail to varieing degrees depending all a host of factors including keel length, keel chord, rudder depth
, skeg, etc. Also contributing is windage. typically longer waterline boats with moderate to longer keels sail less than more race
type keels. All of these can benefit from a riding sail.
For what its worth the Moody sails
some at anchor but less than my Frers 36 which preceded it.
aft had no effect.
We typically sail at anchor less than 20 degrees even if blowing 25+. Riding sail reduces this to 5 degrees or so, but is a bit of a hassel to rig given the size of the boat.