Propeller for water. May also work well in air.

thingiverse

I printed it laying down to get plastic fibers along the long axis for strength. Then as shown on the photo, I coated it with one layer of glassfiber and vacuum cured it. I will update once I test its efficiency in water! Print two times to get both two ends and glue together. -NOTE: the prop is unidirectional. I mean it is designed to only drive in one direction. So check your direction of rotation and mirror the model if needed.