This damage shouldn't cause excessive weakness, but if you're concerned here is my suggestion for a repair that goes further than simply filling and fairing. sand the gouge a bit to form a bevel around the edge of the damage. The standard 8:1 bevel is not really necessary, but what you're trying to do is create a trough that you can partly fill, lay in 1 or 2 layers of fiberglass, then fair and finish. I'm assuming the rudder is not from a high tech one design or built with carbon. The model I have in mind is something like an Evelyn 32 as an example of a larger production boat with an outboard. With the site prepared, mix some epoxy and fill it to non-sag with either the 403 Microfibers or 406 colloidal silica and fill in the bottom of the gouge so you can a relatively flush surface to lay in a couple layers of glass cloth. The glass patches should be slightly smaller than the outside edge of the bevel so it can be easily faired. Laminate the glass in with unfilled epoxy right over the filler already applied. Once this begins to gel (it epoxy becomes tacky and is no longer a liquid, but is cured enough to have more epoxy applied without disturbing the previous layer)you can mix some more epoxy and add some 407 low density filler to non-sag to use as a fairing material. Slightly overfill the repair, allow it to cure, and sand fair.
