Foam core in hull exposed around saildrive well

[brianc]

We're replacing the saildrive on a 2005 Tartan 3700.

Here are the pictures.

The existing hole was pretty messy, and the saildrive was difficult to remove, so we decided to enlarge it, clean up the edges, and put a fiberglass plate over it in place of the rubber boot. The rectangle is where the plate will go. The idea is if/when we have to do other work that requires removing the saildrive, the procedure will be simple and there won't be any enlarging/modifying in the future.

However the hull is foam cored, and we cut in to some of the foam core along the edge of the saildrive well. We need to make sure it’s filled properly (primary question) and to make sure the screws to hold the plate are not going in to the cored area.

The yard already dug out some core, which is fine because they would have to anyway, and filled the area with 3m high strength repair filler which is vinylester with silica (or similar). I don’t think they knew it is an epoxy hull, however it's hard to tell what "epoxy hull" actually means, discussed below.

I think they’re going to have to remove that and use epoxy. I'm debating having them clean out the core all the way back from where it's exposed to the edge of the rectangle and having it filled entirely with epoxy so that it’s solid in that area (for leaks). That would also solve the screw problem because they would go in to the epoxy filled area instead of any core.

I didn’t realize the core was basically all the way to the very edge of each half of I probably would have pushed for doing something different than we did. Prior boat was a C&C 99 and the hull was solid glass around the saildrive, so I made an incorrect assumption that this would be the same.

I also can’t find whether the hull is “epoxy” or “epoxy modified vinylester” and whether vinylester sticks to “epoxy modified vinylester”, etc…need some help from some experts! I will email Tartan as well, but since I can't stop thinking about what to do, I thought I'd ask here as well.

Main point is, I do not want to find out 2 or 10 years down the road that the cored hull is soaked with water because this repair wasn't done properly. What are your thoughts?

Thanks! Brian

[Manateeman]

When you say screws, you mean bolts and back up plates ...right?

[brianc]

Originally Posted by Manateeman

When you say screws, you mean bolts and back up plates ...right?

No the plate I mentioned is basically a cover, just an alternative to the rubber gasket that usually is glued to the outside of the hull to allow water to flow smoothly around the sail drive well, so nothing that needs backing plates. Like the rubber piece pictured here: https://forums.sailinganarchy.com/th...fairing.83193/

[Matt Johnson]

The hull is vinyl ester. The marketing people from a few boat manufacturers realized the buzz word for buyers is "epoxy hulled" and ran with it. The "modified" part always slipped further and further down the brochure

Our boat is VE too. For areas like this we decore around 20mm, paint the core with straight resin, then backfill with a resin thickened with colloidal silica and 12mm fiberglass strands. The issue with straight silica is that it can become pretty brittle...this will lead to water ingress to the core.

[brianc]

Originally Posted by Matt Johnson

The hull is vinyl ester. The marketing people from a few boat manufacturers realized the buzz word for buyers is "epoxy hulled" and ran with it. The "modified" part always slipped further and further down the brochure

I knew it wouldn't be simple! What does "modified" mean in this case then? Does it have the properties of epoxy that would prevent vinyl ester from bonding correctly in a repair?

Originally Posted by Matt Johnson

For areas like this we decore around 20mm, paint the core with straight resin, then backfill with a resin thickened with colloidal silica and 12mm fiberglass strands. The issue with straight silica is that it can become pretty brittle...this will lead to water ingress to the core.

It looks like the product they used uses "short strands", not silica.

https://www.3m.com/3M/en_US/p/d/b40071937/

Is it necessary or recommended then to re-do the work with epoxy? I don't want to risk water getting in to the hull core here.

[Matt Johnson]

I forgot to add what they do on the factory boats for this type of thing.

What you do is create a solid laminate in the area around the opening by cutting back the skin on the inside of the hull 75mm. Peel off the inner skin, cut out the core all the way down to the outer skin - you now have just the outer hull skin and a 75mm border to the core. Lots and lots of layers of fiberglass go on the 75mm border to build back thickness, then the last few go up and over the old inner skin to connect everything together. This creates a solid laminate in the area you're worried about.

[Matt Johnson]

I'm sure the Wikipedia page on vinyl ester resin will explain it well. VE is modified epoxy. It sticks well to polyester resin and other VE, but not necessarily all epoxies. What you are using is great for this repair.

[brianc]

Originally Posted by Matt Johnson

What you do is create a solid laminate in the area around the opening by cutting back the skin on the inside of the hull 75mm. Peel off the inner skin, cut out the core all the way down to the outer skin - you now have just the outer hull skin and a 75mm border to the core. Lots and lots of layers of fiberglass go on the 75mm border to build back thickness, then the last few go up and over the old inner skin to connect everything together. This creates a solid laminate in the area you're worried about.

Thanks, this would be way more complicated (there isn't even 75mm of space inside the hole to remove the skin) but sounds like a significantly "better" repair. Well, glad I didn't open a can of worms with this one...hahaha.

[brianc]

Originally Posted by Matt Johnson

I'm sure the Wikipedia page on vinyl ester resin will explain it well. VE is modified epoxy. It sticks well to polyester resin and other VE, but not necessarily all epoxies. What you are using is great for this repair.

Thanks, your info was very helpful, and that led me to this link on the Tartan site which basically confirms what you've said. Not that I didn't believe you

Tartan Yachts Infused Epoxy Hulls and Decks | Tartan Yachts

But I'm still not sure whether or not what was done is sufficient

[boatpoker]

I believe a Google search will reveal law suits against C&C,Tartan for advertising epoxy hulls that turned out to be polyester.