Hollow Glassfiber Propeller 31,5x18'' The optimum propeller for our 2,8:1 reduction gear, light grey color. The glassfiber propellers are hollow and have a spar through the middle, these are similar in construction to a glassfiber wing and are lighter than a wood propeller and have a lower gyroscopic effect. The glassfiber propellers are balanced and ready bored for our 2,8:1 reduction gear.