i think there is a guide to tassie sailing spots knocking around somewhere...
in summer there is always somewhere to sail in summer either frontal wind or seabreeze...but not always wavesailing.
if you want to sail marrawah specifically, you need the front side of a dry high which give a southerly airflow and produces a southerly seabreeze later in the day
or you are looking for a south westerly wind/swell from a low in the bight which produces big swells and ssw-sw winds which put you on nw facing beaches.
theres patterns will produce 90% of DTL sailing to be had at marrawah
as fluro says, Dec/Jan have been a superb year for sailing marrawah... but this current east coast low has interfered with that...
i prefer the marine weather wind forecast from the BOM
www.bom.gov.au/marine/wind.shtmlgood luck