A boat trailer wheel hub is provided with a flexible diaphragm which encloses the exposed end of the spindle and which has a peripheral portion in sealing engagement with the open end of the hub. When the boat trailer hub is subjected to a sudden temperature change, such as sudden cooling by being backed into water, the diaphragm prevents water from being drawn into the hub. In one embodiment, the diaphragm is carried as a part of the hubcap and in another embodiment it is retained separate from the hubcap.