C.E. Smith Boat Trailer Bearing Protector with Bra - Boat trailer hubs get warm while traveling to the ramp. Warm hubs backed into cool water during launching are suddenly cooled which contracts any air inside the hub creating a vacuum. The vacuum pulls water and road grit inside the hub contaminating the grease. The contamination causes corrosion and rapid wear. Perfectly good bearings can be ruined in less than one season.