i think tha tube concentrates that water to the prop or keeps air from entering and causing cavitating until the boats on plane....sounds similar to something i saw on MHZ....could also be a type of jet drive too...

eyekandy, could you explain to me why subdrives are better?

my SW (surface drive) jumps wake and gets the prop completely out of the water a lot and almost never cavitates, but i also had a sub drive boat that every time the prop came out of the water it would cavitate when it came back in untill i let go of the trottle and let the air escape...