[Bushwacker]

The fin works like an airfoil, so it depends how you use it. If you trim the motor down so the airfoil has a positive angle of attack, you'll get stern lift which is why it helps you plane at lower speeds. If propshaft is parallel to surface, you won't get any lift, just a little drag which will knock a couple of mph off your top end. If you trim motor out/up, the foil will have a negative angle of attack and stern will probably dig in.