What is this spot on our keel?

[Make_My_Day]

In conjunction with maintenance to our boat, we recently had the hull blasted back to the steel to check for any weak spots. It was then covered with a light coat of epoxy as protection from corrosion until the maintenance work is complete. It was then that we discovered these spots. See photos.

Our boat is a 1980 Carena 40 long keel steel ketch and the 200 liter diesel tank is in the keel.

I don't see any diesel on the ground and can't really smell any either. The shape of the spots seem also regular and even symmetrical.

I still haven't heard from the yard that did the work what they think it may be nor any proposed solution.

Any ideas as to what this might be?

[JPA Cate]

Hi, there, Make my day,

I have a sinking feeling that in your heart you know what people are going to say about this, and I feel for you. Our keel is a lead filled steel shell that also is used as a fuel tank. When we hat it blasted, because the paint systems wouldn't stay put, the blasting opened a spot and diesel came out. We had to weld it up and re-coat the keel.

What you have happening there looks to me like diesel seepage, that you have a number of tiny holes. After all, what else is there in there to come out? If it were some kind of problem with the system you put on, there would be blisters forming on the new coating.

Ann

[IdoraKeeper]

Yep, you have a hole. I saw something similar on both boats that I have owned. You need a patch welded on. One my first boat it was glass work, on the second it was a plank. Consider that you may have a larger corrosion issue in that area. Anything can be fixed.

[Make_My_Day]

Thanks Ann and IdoraKeeper for the input. It is what I was thinking as well.
The unusual thing is that it is on both sides of the keel, opposite of each other and almost symmetrical. That's what got me thinking. Symmetrical holes on both sides? Hmmmmm

[Jim Cate]

Originally Posted by Make_My_Day

Thanks Ann and IdoraKeeper for the input. It is what I was thinking as well.
The unusual thing is that it is on both sides of the keel, opposite of each other and almost symmetrical. That's what got me thinking. Symmetrical holes on both sides? Hmmmmm

Is there a weld line under the row of spots? That seems an obvious potential cause: porous welds...

Jim

[UNCIVILIZED]

It would really help us a lot to know where in the keel the tank(s) are located. That way if the spots are near where the fuel is, then the conclusion is much more solid. Though I'm inclined to agree with Ann & the others on the tiny leaks idea.

Might we please know how thick is the plating in those areas, & where were/are the plugs down low for drainiage & cleanout? Assuming that there are/were any. Since sometimes they're optional, & also get welded over.

Were it me, I'd like to think that before I got much further in terms of work on her, I'd have her audio gauged, along with some other expert (non-destructive) inspection. Especially since if the plating is getting thin down there, then it makes sense to ascertain it's health elsewhere. Such testing & inspection would probably reveal most prior repairs or alterations done to her, which would also be good to know of.

[Make_My_Day]

Thanks Jim for the feedback. The little spots on the top might be under a weld but the larger ones by the rudder, the welds are aft not above.

I never noticed these spots until we had the hull blasted. No problem with the antifouling staying put or otherwise.

[Make_My_Day]

Thanks Uncivilized for the input,
We had the boat surveyed when we bought her last year. The surveyor used a Cygnus 4 ultrasonic meter to measure the hull thickness and concluded that it was sound. There was an exception at the bow where it was just under 3mm thick. The keel is 4.5 mm thick from an original construction thickness of 5mm. The hull surface is still 4mm.

[Pete7]

How about tackling one of the spots with a hair dryer. If it dries you know its water, if not its diesel, though I would have thought you could tell diesel by the smell. Are the bilges clean and dry or full of oily water?

Pete

[rustypirate]

Check for rust in the bilges along the stringers either side of the transition to the keel. That is most likely the cause of the row of spots at the top of the keel.

Open the fuel tank(s) and have them drained. check the bottoms of the tanks for rust as any water that gets into the tanks, (including condensation), will settle below the fuel and start rusting the steel tanks.
Most likely is that there are pin-hole rust spots that have started leaking fuel into the keel between the ballast and the plate. The liquid runs down until it can find any imperfect welds where it appears on the surface as in your photos.

I had a Frantz Maas 43 that had welded in tanks and this happened to me too together with the rust along the stringers in the bilge.

An external patch can be applied to the hull to seal the leaks, but the internal damage really needs to be addressed.

[s/v michaela]

I too have a steel boat and have learned about the wonders of an industrial polymeric compound called Belzona. Utility companies use it to seal leaking transformers, the Oil and gas industry uses it to rebuild offshore platforms. It will seal oil and water leaks,,rebuild shafts ,threads and replate metal. It truly is amazing and will seal your leaky keel without welding . There are many you tube videos you can look at. We have used it and sing its praises. It's been around since the 50's but is definitely not on the typical consumer's radar. Read up on it and your sick stomach will feel much better.....

[buzzstar]

Originally Posted by s/v michaela

I too have a steel boat and have learned about the wonders of an industrial polymeric compound called Belzona. Utility companies use it to seal leaking transformers, the Oil and gas industry uses it to rebuild offshore platforms. It will seal oil and water leaks,,rebuild shafts ,threads and replate metal. It truly is amazing and will seal your leaky keel without welding . There are many you tube videos you can look at. We have used it and sing its praises. It's been around since the 50's but is definitely not on the typical consumer's radar. Read up on it and your sick stomach will feel much better.....

While I have no way of knowing, I looked at the Belzona website and came across photos that made me wonder if the "spots" show the keel/tank had previously been treated with a product called Belzona Superwrap II. Take a look see. This might explain both the color and the lack of diesel smell.

[knockabout]

the small spots look the same colour as the large spot on the bottom of the skeg / what was used to wipe down before spraying

[stillbuilding]

I am not clear if you have integral tanks or a separate fuel tank set in the bilge.

If you have an integral tank then drain the tank ( by drilling into base from exterior keel if necessary),
grind back in the affected areas to track the defects, cut out the affected parts and weld in new plating as required. If you think there is inaccessible rust then clean the tank, fill with heavy oil and seal closed. No oxygen = no more rust. Just need to find another place for fuel tank.[emoji38][emoji38]

Ultrasound may not be as useful as you hope for pin-point defects. Ask me how I know.[emoji38][emoji24][emoji24]