Is fairing compound reasonable for a hull crack

[4lane]

I have a couple cracks to fill around the driveshaft strut. I ground down to glass through a 1/4” of some softer material. I was planning to fill the cracks with 5200, use fairing compound (FGCI Gelcoat Putty) over the top to build up the 1/4” gap, then antifouling paint over the top. Reasonable approach?

 

[Davidasailor26]

If the cracks are still there the surface probably won’t be stable enough and the fairing compound will fail eventually. The best course of action would be to grind the cracks down to solid fiberglass, rebuild the area with new glass and epoxy, then use fairing compound just to smooth the repair. Also below the waterline an epoxy based fairing compound like TotalFair or a West System product would be better than that polyester based putty. Check out the Boatworks Today YouTube channel for some examples of how to go through those steps.

 

[dlochner]

The crack is probably caused by the use of polyester putty/fairing compound to close the gap. Over time polyester fairing compound can shrink causing the crack. Had a similar problem on one of my boats.

The strut is probably through bolted to the hull, if it is, then the fairing compound is more cosmetic than structural. Remove the existing fairing compound. If the strut is solid, don't remove the strut. Fill the gap with thickened epoxy. This will probably be a deep gap, so fill the gap in layers so avoid any exothermic problems*. The last layers should be an epoxy fairing compound like Interlux Watertite or TotalBoat Total Fair.

*Epoxy develops a lot of heat as it cures. Most of the time this is not a big issue as the epoxy is spread out over a thin layer which allows the heat to dissipate. In an application like this there is very little surface area over which the heat can dissipate. The epoxy can over heat and foam with a lot a gas bubbles. This significantly weakens the epoxy. In this application, apply epoxy not more than a ¼" deep and let it cure for several hours but not fully cured and apply another layer. If the epoxy fully cures you'll need to wash the amine blush off and sand the surface for the subsequent layers of fill.

 

[4lane]

@Davidasailor26 @dlochner The strut is solid, bolted down, and not leaking. It does seem like compound is purely cosmetic because it sands down very easily. I don’t really understand why there is a 1/4” of the stuff all over the strut. What would be the normal way to ‘cover’ the base of the strut, which I could do if I removed the entire 1/4” surrounding the strut base. Epoxy fairing compound?

 

[colemj]

The bronze strut is installed in a hull recess, then fairing compound is put over it to make everything even, so there is no grinding back to fiberglass and rebuilding with new glass. The first thing you should do is make sure the strut is not loose and causing the cracks. Otherwise, the existing fairing putty held for 40yrs and isn't structural at all.

It isn't clear that there was ever any glass over this, as your grind marks show paint, gelcoat, filler. The only glass I see is where you went into the hull itself. If you want to refair it, do not put 5200 underneath fairing putty. There just isn't any need for epoxy here. If the existing fairing is solidly attached, you could just use the FGCI putty you already have to fill the gaps. Maybe grind them into a Vee a bit for better fill, adhesion, and tooling. Maybe even some light cloth fiberglass tape or CSM around the join then a skim of putty to fair the weave. You could also just fill with something like 5200 and tool it smooth.

If the existing fairing isn't solidly attached, then you will need to remove it and refair. Again, epoxy isn't the thing I'd use here. You could even just leave it unfaired if you don't race it.

Mark

 

[dlochner]

The strut casting are slightly imprecise, so the well for the strut base is a little oversized. This allows for any variation in engine placement hull thickness, etc. At least with Sabres of the same vintage the strut is set in a bed of polyester putty, aligned with the shaft log and bolted in. The fairing compound is then used to to get a smooth bottom.

It would be best to clean out all the shrunken fairing compound until you get to something solid. If you go below the surface of the strut base, use thickened epoxy unto the strut base. That epoxy will strengthen the strut mount. Over the top, use an epoxy fairing compound designed for below the waterline use.

This photo is from a Sabre 30 which shows how the strut is set into the hull with the fairing compound over the top.

 

[4lane]

I believe “the existing fairing putty held for 40yrs and isn't structural at all.” I was a bit scared to grind all the way through the compound but I see now that if I did I would hit the bronze base of the strut. I will just grind it out a bit more so the edge of the strut base is clean and clear (the crack you see in the photo), and just build it back up with compound. Maybe I just won’t make it as thick as it was originally.

 

[dlochner]

I believe “the existing fairing putty held for 40yrs and isn't structural at all.” I was a bit scared to grind all the way through the compound but I see now that if I did I would hit the bronze base of the strut. I will just grind it out a bit more so the edge of the strut base is clean and clear (the crack you see in the photo), and just build it back up with compound. Maybe I just won’t make it as thick as it was originally.

Try chipping it away with an old screwdriver and hammer. The old polyester isn't very strong and it is brittle. Then just clean up with a small grinder or sand paper. Much neater.

 

[4lane]

Thank you all for the great direction!

 

[marcham]

Based on the photos, it is impossible to provide a remote diagnosis. You'd have to remove the material that covers the strut base plate. We can't tell what's been removed, what remains, what's structure, what's cosmetic.

If you have never worked on a structural GRP part of a boat, you would be well advised to consult a boat fiberglass repair expert to provide an on-site assesment.

If I were on-site, I would ask and try to determine what is causing the cracking??? Just filling and painting is not solving the issue. Misaligned strut? Loose strut? Bad cutlass bearing? Prop imbalance? There's likely much more to this than filling and painting a crack.

There appears to be rust no the drive shaft???? A stainless 316 shaft should not have any rust on it....

 

[4lane]

As must suggested, the old fairing compound just shrunk which developed the crack. I chipped it all out, cleaned it up and filled it back up with epoxy fairing compound. All good. Thanks everyone!

 

[marcham]

That bronze is not looking in good shape...can you polish it a bit? From the photo, seeing a lot of dezincification.

 

[colemj]

That bronze is not looking in good shape...can you polish it a bit? From the photo, seeing a lot of dezincification.

If it is bronze, it cannot dezincify, and I'm pretty sure that is bronze.

I see the red that I think you are pointing out. That is actually the raw bronze color, and the areas of green patina are making it stand out. It was previously coated with fairing compound, so wouldn't have a chance to oxidize.

Mark

 

[dLj]

None of us can know what that strut is made from by looking at pictures. It is common to find these struts made from leaded red brass.

The pictures given on this thread look as though there is still some foreign substance on the strut. Without cleaning it down to the bare metal, photos aren't telling us much as far as dezincification or any other degradation processes that may or may not be present.

It is highly likely this strut is just fine and the OP simply needs to find the best solution to addressing the cracks as discussed.

dj

 

[4lane]

Many of you are really reaching into your shadenfreude reserves. The strut is bronze, it has antifouling paint on it, it was treated with a barnacle buster which has a residue, there is nothing wrong with the strut, driveshaft, etc. As I mentioned the old compound in the strut recess was simply cracking. That is all.

 

[jssailem]

The strut is brass,

Unless you sought out a brass strut and installed it, that would be highly unlikely on a C&C boat. The issue is that brass would corrode quickly in a marine environment. When polished, bronze can look like brass, but its metal properties differ.

Brass is an alloy primarily made of copper and zinc, while bronze is mainly composed of copper and tin. Each has distinct properties and uses, with brass being more malleable and suitable for decorative applications, and bronze being stronger and better for structural uses.​

 

[colemj]

I looked up propeller struts and found that most are made of either manganese bronze or aluminum bronze. Manganese bronze is technically a zinc-containing brass and it can dezincify.

Mark

 

[jssailem]

Yes. But more slowly than zinc/copper brass or zinc/tin brass. It is also a less malleable metal form.

This would be an issue because struts can and do break when made of soft, corrosionable metal. If this occurs when the boat is under auxiliary power, it can quickly become a serious problem.