This is another debate that will run a nd run. Just checked boards Forum to see what opinion was on there. Rik (of Rik`s Windsurfing) posted this;
Originally Posted by Rikster
"A RDM sets a sail flatter so immediately you have less power from a standing start. In use the softer relative flex of the smaller diameter spar prevents the sail from delivering the same amount of power as the SDM. The extra deflection causes draft instability when sailing heavily powered-up.
Like with so many things it’s all rather subjective to the user, the conditions they sail in and the equipment combinations being used. I know many sailors swear by the softer more forgiving flex the RDM provides but that is often making up for other deficiencies (technique or equipment set-up). In general terms however, easier conditions that do not require the extra strength of the RDM benefit from the extra power and wind range offered by the SDM. "
This is the NP line on issue. I use NP sails ...nufsaid.