if your boat is a TheMightyHobie18, i am faily certain that the rudder pin can not make it's way out the top as it will hit the lip that runs around the perimeter of the hull. in fact the pin can't come out the bottom with the drain plugs in place. therefore no cotter pin is necessary except to keep the pin out of the way of the drain plug during hull draining.
for a pin to come out the top on a H16, you need to turtle, have the rudder cam come unlocked, and have smooth working rudder pins. it has happened, and that's why they started putting the cotter pins on both ends at some point.