Originally Posted by RichH
The outboard hung rudder can have one serious fault: at higher speed and when a boat is well heeled over such a rudder can easily 'ventilate' or 'suck air down along the lower pressure side of the rudder' and the rudder will/can lose ALL control depending on the amount of ventilation.
Couple this with a helmsman who doesnt understand 'weather helm' requiring more rudder angle to overcorrect for adverse helm; include the vulnerability to 'ventilate' ........ and you can expect an unexpected 'pirouette' / unexpected rounding up as a worst case scenario in your future.
Stern hung rudders can easily 'ventilate'.
Totally submerged rudders have great difficulty in 'ventilating'.
Advantage - submerged rudders
Yes that is related to the "end plate" effect that I touched on. Of course the scenario you describe really only comes into play when the boat is already more "out of shape" than it should be. In the case of the boat carrying excessive heel, and so much weather helm that the rudder is on the verge of stalling chances are the inboard rudder is also being exposed to air and turbulent surface water. In that case it too is in danger of "ventilating".
Having said that, it stands to reason that an outboard rudder could be more prone to that problem sooner than the inboard rudder.