A boater (also basher, skimmer, katie, or sennit hat) is a kind of hat associated with sailing and boating.
It is normally made of sennit straw and has a flat crown and brim, typically with a ribbon around the crown, which is often in colours representing a school, rowing crew or similar institution. Boaters were popular as summer headgear in the late 19th century and early 20th century, and were supposedly worn by FBI agents as a sort of unofficial uniform in the pre-war years. Nowadays they are rarely seen except at sailing or rowing events, period theatrical and musical performances (e.g. barbershop music) or as part of old-fashioned school uniform, such as at Harrow School.
Inexpensive foam or plastic skimmers are sometimes seen at political rallies in the United States.
Being made of straw, the boater was and is generally regarded as a warm-weather hat. In the days when men all wore hats when out of doors, "Straw Hat Day", the day when men switched from wearing their winter hats to their summer hats, was seen as a sign of the beginning of summer. The exact date of Straw Hat Day might vary slightly from place to place. For example, in Philadelphia, it was May 15; at the University of Pennsylvania, it was the second Saturday in May.
The boater is a fairly formal hat, equivalent in formality to the Homburg, and so is correctly worn either in its original setting with a blazer, or in the same situations as a Homburg, such as a smart lounge suit, or with black tie.