I would suggest that you sleeve any through holes in the fiberglass that are wallered out with stainless steel tube to prevent it from happening again. I believe the large hole at the top is where a bolt that allows the tiller to lift up goes.

 

@Brad O you need to be careful sleeving holes with stainless especially if you are thinking about epoxying them in place. in an oxygen free/low environment you can induce crevice corrosion. Encapsulating the stainless creates this environment

@rcoleman67

Not sure I've ever seen a keel hung rudder on a catalina 25. I think you mean transom hung.

1. a proper sized dremel bit goes a long way here I like the small carbide tile bit. one of the cutting wheels would work as well. go slow and only go as deep as you need to. if the scratches don't go below the gelcoat, then all you'd need would be refill with gelcoat.

2. if you can undercut the top layer of gelcoat on each side a smidge and then fill/pack the hole good, the top layer of gelcoat will help hold the puck of filler in place, less chance of it popping out. tape around the hole with painters tape, and then a small piece of plastic to cover the hole old credit card, hotel key, clear plastic blister pack cardboard cover with packing tape. then tape it down real good, lay the rudder this side down and then fill the hole ensuring that you pack it full. once it cures, removes everything sand smooth, drill you hole size, and cover any exposed epoxy with paint/gelcoat

3. never seen a bracket like that could it be for a tiller to wheel conversion? Autopilot? most tillers are attached to the top of the rudder that are transom mounted. careful heating with a heat gun may help release the grip. other wise an oscillating tool with a sharpened edge (think paint scraper edge) could work if that doesn't work then you might need to get a serrated edge which could damage the gelcoat and make fore more repairs. Is there a cutout in the transom for this bracket?

4. good plan same as 1 only go as deep as you need to until you see no damage. if that boat has been in the water for a while. a hole drilled at the bottom of the rudder into the core could help any water that is inside get out, that is if you feel the rudder is water logged.

5. the large hole on the top would be for the tiller the fitting would look like this

 

The 1989 C25 rudder is foam cored. All rudders will generally deteriorate quickly if water gets inside. In the winter the water freezes and splits the fiberglass, creating more openings for water intrusion. It's usually wise to remove a transom hung rudder for the winter and store it indoors. Without seeing your rudder, it sounds like it already has a number of openings. I suggest you take it inside and store it in a warm place. Drill a hole near the bottom, as Marcjsmith suggests, to drain water out and encourage it to dry inside. In the spring I'd repair it as best you can. Meanwhile, I'd start looking for a replacement. Reasonably priced used rudders show up occasionally on the Catalina 25 facebook group Log into Facebook and on the Catalina 25 National Association website Association Forum. New replacements can be ordered from Foss. Replacement Rudders and Centerboards | Foss Foam Products

 

If it has taken more than 30 years for the pintle holes to become wallowed out, filling them with epoxygoop and redrilling them will likely be enough to not have to worry about them again for another 30 years.
If the tiller arm bracket (for an autopilot?) is stuck in place but also has wallowed out holes you could still refill the holes with epoxygoop and redrill them without removing it. Just be careful not to get goop on the bracket - maybe use a syringe to fill the holes.

 

As sailormon inferred, the cracks sound like a rudder that was saturated and froze. It’s common up north.

Drill a 3/8 hole near bottom, see if the core is wet. If it is, drill more holes and start blowing in air and sucking with a vacuum. The idea is to dry the core. Then fill all holes and cracks. Most of your small cracks don’t leak, but your seam does, particularly at the top.

 

That bracket on the side of the Catalina 25 rudder is a from the factory tiller to wheel conversion attachment for the Cat 25. If you look at the Cat 25 boats that shipped from the factory with wheel steering that is what they look like.

I'd be tempted to just fill the holes out with thickened epoxy, wait for it to harden and redrill, but that's just me.
You do need to address the wet core. I would recommend https://catalina-capri-25s.org/ going here and seeing what others have done. The association members are quite good.

 

If all else fails, you could just contact these guys and buy a new one; Standard Rudders

For reference, I ordered a new rudder & rudderpost for my O'day 35 in 2020. They had the mold and the materials in stock. They shipped the finished rudder in two weeks to my location, and it was <$3k. I was VERY happy with the rudder from Foss Foam.