I have modeled screws and springs which have similar features. I would recommend you make a cylinder, then a helix. Then make a sketch of the cutout (cross section of one flute) you want to make at one end of the helix. Next, sweep the "cutout" through the helix and subtract that from the cylinder. This should give you what you need. For multiple flutes, make multiple helices and repeat sweeps for each one (4 flutes will require 4 helices).
Caveat: You must use the advanced sweep so that your swept cut stays parallel to your helix axis throughout. Choose Insert/Free Form Feature/Swept. Consult the CAST help for questions about sweeping this way. Good luck!
Hope this helps.