Not directly weather-related, one of the major causes of ship under-performance can be Hull Fouling. This usually occurs when a vessel is kept in warm water for long periods waiting, loading or discharging.
If Anti-Fouling Paint or Anodes on the hull are defective growth can soon occur. In one case a tanker off an African port for only a matter of days subsequently had 80 tons of barnacles and other accretion scraped off; in another a vessel off Bandar Abbas for two months suffered a 6 knot reduction in speed immediately after leaving.
These days with modern cleaning techniques and mobile units the worst effects can be lessened but provision should be made in the charter as to the responsibility for fouling if claims are to be avoided.