A boat prop. does not have an Airfoil shape and is Pitched extremely.I can Imagine with those characteristics it would Have a Spiral stream behind it.Don't know if your reffering to my Prototype.But I use that Downward Airflow to control Axis!I changed direction of rotation.Allowing me to Run stock rotation on Both wing motors with stock ptops. If you Rotate it in differant directions.You will see that 1st had to counter 1 side to get equal and desired airflows.Changing Direction solved that problem!!