Bonding to cement?

I really don't know if I'm doing this repair correctly, and that's why I'm here. Hoping to find a little guidance and advice.

Quick background. The boat was slamming the bottom while on a permanent mooring during a storm. This named storm came with astronomical tides and gale force wind that the mooring field is very exposed to. The result was a destroyed rudder and I wouldn't find the rest of the damage until a few weeks ago when I started the rudder repair. I hadn't visited the boat all winter, and when I got to the yard, the laminate was blown out just forward of the rudder shoe. I cut away the dead laminate and crumbling concrete came out. I cleaned out the area using a wire brush until I got to healthy material.
I then trowled in hydro cement over a period of a few days. I took approximately 4 gallons to fill.

Now is where I'm stuck. I'm going to bevel the area 20:1 to scarf in the repair. I'm wondering how I'm going to get the glass to bond to cement properly without worry of future delamination. I was thinking I would need to grind it down, cover it with thickened epoxy, then a single layer of thin glass so it contours seamlessly. I can't find anything anywhere on how to execute this repair the right way, so I'm guessing. I'm also not a professional.

The other issue is that same thing is happening on the port side- except with this side, the laminate is healthy. With percussion testing, you can hear the void/delamination. I would rather not cut away all of the laminate if I didn't have to. Could a drill holes mid keel and inject epoxy?

 

Who was the manufacturer? The skin of the rudder may not be epoxy, but something else. A coal tar epoxy should adhere to the concrete.

 

There really isn’t much of a bond during construction, the hull is built and then cement is poured in place.

So there isn’t much of a concern for great adhesion in the repair. And for the most part, as long as the material you reshaped it with is dry, any of the resin systems will provide an adequate bond.

 

It's a 1985 Sea Sprite, so it's all poly. I've actually already finished the rudder repair. Fairly easy since it's foam core. I was planning on using epoxy for the laminate, so the coal tar epoxy could be a good choice.

 

Got it- so you're saying the port sides laminate release to cement isn't an issue? If I were looking at buying a boat, and percussion tested that area, I think I would drop the hammer and run away as quickly as I could. But maybe I'm totally wrong since the laminate is healthy.

 

If you are worried about that, just drill a pinhole in the skin and inject a coal tar epoxy. Unless the rudder has too much volume and tends to float, this is an un-issue. FWIW, steel rudders are often filled with pine and pitch just so they don't get that pinched "hungry dog" look due to water pressure. I like building a hollow rudder then filling it up with compressed foam so even if the head fails the rudder does not sink out of tube.

 

Sorry for the confusion- the rudder is already repaired.

This is the keel. The boat is full keeled with encapsulated lead in the forward portion of the keel, and concrete filling the voids underneath a glassed bilge.

 

For reference:

 

How is the concrete attached to the hull structure? Just the laminate skin? I would be concerned with the compromised laminate failing at some weak point and the weight of the ballast "unzipping" the glass over time leading to a much bigger problem than an unfair keel.

 

It’s encapsulated. The molds for these boats were build in two halves. The lead ballast was fitted into one half, then the other half joined. The voids in the keel that remained, aft of the lead were filled with concrete, then glassed over to create the bilge.

 

Methyl methacrylate will bond to concrete. You may need to produce a fiberglass panel, bond via methyl methacrylate, then lay up to the fiberglass panel and scarf it in accordingly.