A quick google for "free model yacht plan" will pop up a surprising number of hits. If you stick with hard chine designs from what is available, you can develop from the techniques learnt from the previous builds.
A nice size simple design here – http://www.sealevel.demon.nl/english/downloadsXL25modelyacht.htm –
Bears a remarkable resemblance to a design called "Akela" from the late '80s. Assuming that the object is pleasure sailing rather than formal racing, any of the designs suggested so far will do the job.
A massively important bit is the ballast – much easier to arrange on a slab type fin by using lead sheet, a large yacht with a long fin and tube type weight needs you to learn about casting lead, smaller versions can use reshaped fishing weights.