When the southwest monsoon arrives in Thailand, the weather is typically better for sailing, since the wind speed rises up to 30 knots. However, the monsoon brings also some rain. Normally it can rain heavily for two hours in the afternoon, which won't actually hinder your trip. Be aware, though, that during this season you cannot anchor at the west coast of Phuket. The Similan islands are closed for visitors at all. Keep also in mind that heavy downpours are highly possible throughout September and October, often lasting up to five days in a row!