Steel hull repair

Came across the following pic on YW and first impression was the boat must have sank or had soft plates from electrical currents. Anyways, my question concerns how the plates were repaired. Is this the correct way to repair plates on a steel hull?

Donsan said:

Came across the following pic on YW and first impression was the boat must have sank or had soft plates from electrical currents. Anyways, my question concerns how the plates were repaired. Is this the correct way to repair plates on a steel hull?

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Hi,

in that case, this type of repair is the only economically best option if the area is big and corroded thin and a small point of corrosion is filled, spreads on a thicker plate and professionally filled the filling that I do not have here. I think some of the welds look very poor in quality your picture, not professional welding and also missing rounding corners. The image correction welding should be welded into two batches, the first being the welding deep-cupped and the small bobbin, the second welding fills the rail evenly. The best way to repair if the plate is still sufficiently thick to cut the round edge hole and the corresponding flange and weld on both sides, this will give the original structure strength.


Are you buying this boat? If so, I recommend NDT measurement on all boat hulls, so you know the potential risks later on the base of any possible corrosion. Also think about what caused the bottom to erupt. Misplaced zinc anodes, electric leaks, or poor maintenance?

NBs

Exaples beter welding, On the second welding round the welding filler is applied, under this the first round of welding with which the plate structurally quite strong.

Proper way is to crop out rusted/thin steel back to where thickness is acceptable then insert new plate. Doubles below thw waterline are cheapest repair. Corners should be radiused.

North Baltic sea said:

Are you buying this boat? If so, I recommend NDT measurement on all boat hulls, so you know the potential risks later on the base of any possible corrosion. Also think about what caused the bottom to erupt. Misplaced zinc anodes, electric leaks, or poor maintenance?

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No, have no desire to buy the boat. I had read the recent thread on the Coot 35 in Singapore and seeing these plates aroused my curiosity. I kind of got the impression the steel boat I was looking at was kind of sloppily fixed and painted to fool would be buyers. The price outrageous too!

Just as an aside. If you own a steel boat then buy a very large anode, some chandlers sell special circular ones about a foot long. Connect a piece of stainless steel wire to your guardrail with a shackle and hang the anode over the side. Make sure there's good metal to metal electrical conductivity. Obviously inspect regularly and lift on board when moving.

I have built, owned and maintained a steel yacht for over thirty years.
There is nothing wrong with an external patch over a pitted area. I would not do this if the hull plate had been perforated.
With due respect to Irish Rambler , it is not a good idea to just hang an extra anode over the side.
Much better to buy a silver chloride anode and measure the boat's potential. If necessary then add or subtract permanent anodes. This is a simple and inexpensive process.
Anodes on a steel hull preferably need a small weld to ensure perfect electrical continuity.

D.Duck44 said:

I have built, owned and maintained a steel yacht for over thirty years.
There is nothing wrong with an external patch over a pitted area. I would not do this if the hull plate had been perforated.
With due respect to Irish Rambler , it is not a good idea to just hang an extra anode over the side.
Much better to buy a silver chloride anode and measure the boat's potential. If necessary then add or subtract permanent anodes. This is a simple and inexpensive process.
Anodes on a steel hull preferably need a small weld to ensure perfect electrical continuity.

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Not so fast there. An external patch or doubler plate is considered a temporary repair by all major classification societies and the USCG. A permanent repair would be an insert that crosses a frame space and is welded to the structural member. The minimum size of an insert is 18” x 18” and is commensurate with the original plate thickness.

In the case of the repair on the photograph, the doubler plate is quite large. This would NOT be considered an approved permanent repair.
Imagine if you can, the steel plate doubler is attached to the original hull plate only by the perimeter weld. If the original hull plate is thin, it can continue to waste away but now without observation. It is the original plate that is attached to the frames, not the new steel repair. So the repair is much weaker than original. One day it can just fall off and the boat may sink.

I have seen this many times while looking at vessels repaired without proper supervision. Subsequent drydock inspection caught them just before major failure mode sets in.

And finally, as a point of reference, the maximum diminution of steel plate thickness is 25% of the original thickness. Anything greater than this requires plate renewal. So if you find pitting, it must be gauged at the deepest point. I have seen surveyors gauge the area next to the pit where the plate is thicker and declare the steel “good”. And it isn’t.

I agree with some of what McGillicuddy says, with some qualification.
A large doubler plate would best avoided, but if necessary, some strategic plug welds would make it more secure.
I also agree that a hull with many doubler plates would be unacceptable, as would doubling over perforated plating.
I don't agree that corrosion will continue under a doubler, providing as I specified that the plate underneath not be perforated. With a continuously welded doubler, there is no oxygen access and thus can be no corrosion. I have tested this numerous times - sealed spaces can look like new after decades.
A case in point is when I was building our boat, I found after attaching one of the bottom plates, that it had significant pitting corrosion about 18 inches square where it had been touching the ground during storage.
Instead of cutting it out I doubled it.
We are abrasive blasting the hull at present (30 years on) and I looked at that area a couple of days ago after blasting but before painting. It is exactly as when I put it on.
Maybe commercial survey has more stringent requirements but for a pleasure boat, where plating thickness is by design much thicker than needed for structural integrity an intelligently placed doubler is acceptable.

Mr Duck, I am glad that you have had success with your methodology and it is obvious that you stay on top of maintenance on your boat. I certainly didn’t mean to pick on your post although it was the most convenient one to address some issues I saw in several posts in this thread.

Let me just comment on a couple of your observations for the benefit of others who are interested in steel hulls and might not be so hand-on as you are and will rely on shipyards to do their maintenance. Also, this might be handy for those interested in purchasing a steel hull.

On a large doubler plate, plug welds may indeed provide more adhesion of the doubler to the hull plate. But it still will not constitute a permanent repair with any (and I mean not a single one) authority in cases where there would be a need for inspection. So, if the same boat was used in charter, the doubler repair, if found, would be a reason to revoke a certificate of inspection if it had not been replaced with an insert within a year of installation. So the question should be, would you repair a boat to a lesser standard? If so, what would the standard be? Who sets that standard? If the hull was any other material, would a repair to a lesser standard than ‘as original’ be tolerated? If no, then why tolerated for steel? There is no reason other than it is ‘more convenient’.

I have seen corrosion continue after being doubled. There is such a a thing as anaerobic corrosion, especially in the presence of stray electric current. With a doubler, how would you know? It would be hidden behind the doubler, out of view. That’s why there is a time limit on how long a doubler temporary repair can be approved.

I don’t think commercial requirements are any more stringent than for non commercial. They are just published for all to see and they set a minimum standard for shipyard repair and they are enforced. Further, I don’t believe that steel hull pleasure boat hull thickness is much thicker than necessary for structural integrity. Mostly I see 1/4” bottom plate on vessels under 60’. This is significantly less strong that say 2” of well laid FRP. Further, pitting is usually not readily visible until it is about 1/8” deep and by that time it has eaten into 50% of the original scantling. Remember, the standard is 25% (1/16”) loss of thickness before the plate should be replaced, not perforated through.

If a steel boat were to go aground and become damaged, the insurance company would be required to replace the damaged steel, not fit doubler plates over it. If they tried that, owners would howl in outrage. So why should corrosion damage be treated otherwise?

Finally, remember that steel can corrode from both sides. The only prevention is to be well coated on a properly prepared surface. Paint on top of rust does nothing. Corrosion, where found needs to be arrested, dug out to the root, coated with appropriate primer for the environment, either internal or external, and finished with a tough topcoat.

There is nothing to be afraid of on a steel hulled boat and it’s main advantage in my opinion is the ability to be repaired in remote environments. If I had to have a boat for world traveling and it was going to be greater than say about 75’, I would certainly consider steel as this was my world for 40 years. But my choice for a 52’ coastal cruiser was fiberglass. It just made more sense to me. Plenty others have different opinions and that’s a good thing.

Interesting discussion as I am in the throes of a welding course. I will be largely welding aluminium and the comments about doublers are even more important with aluminium. As soon as you exclude oxygen you have a recipe for corrosion.

I worked in naval shipyards and had a small yard. Sometimes a patch like the one shown creates more stiffness in that area. When the hull flexes in heavy seas, cracks can develop in the surrounding plating because of the repair. It looks like a economy yard or an owner with high school welding did the repair.
Picture looks like pitting is throughout the hull. My guess he didn't have enough zinc protection. I suppose that's why there's a new zinc by the repair. Normally zincs on steel are mounted along the length, not just on one end.
Zincs on the Oriskany during overhaul. Another set in front of the outside strut. Even more near the bow. Note overlapping plates. I don't remember if they were welded or riveted.

Its so strange to me that so few of us are willing to accept the information presented by experts or even those people who have had lots of experience in a certain area. We are so quick to dismiss expertise with anecdotal knowledge. I found this:

"In the field of psychology, the Dunning–Kruger effect is a cognitive bias wherein people of low ability suffer from illusory superiority, mistakenly assessing their cognitive ability as greater than it is. The cognitive bias of illusory superiority derives from the metacognitive inability of low-ability persons to recognize their own ineptitude; without the self-awareness of metacognition, low-ability people cannot objectively evaluate their actual competence or incompetence.[1]

Conversely, highly competent individuals may erroneously assume that tasks easy for them to perform are also easy for other people to perform, or that other people will have a similar understanding of subjects that they themselves are well-versed in.[2]

As described by social psychologists David Dunning and Justin Kruger, the cognitive bias of illusory superiority results from an internal illusion in people of low ability and from an external misperception in people of high ability; that is, "the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others."[1]"

McGullicuddy is correct:

As best as I can tell from the pics, that looks like they installed doublers.
Lapping a plate over top of a damaged plate. The best way to check the welds would be to have them MT (magnetic particle test) or pull a vacuum on the welds.
In the US this is illegal the CG won't allow it if they were to inspect.
Ships have doublers on them but is only allowed to reinforce the hull plating in a area that would be required for a offset of a heavy object say around the anchor housing to the hull.

I was on the DD881... prior to my arrival, they were chipping the bilge and poked a hole in the hull.
The nuc subs I was on had zinc inside too.

Xsbank said:

Its so strange to me that so few of us are willing to accept the information presented by experts or even those people who have had lots of experience in a certain area. We are so quick to dismiss expertise with anecdotal knowledge. I found this:

"In the field of psychology, the Dunning–Kruger effect is a cognitive bias wherein people of low ability suffer from illusory superiority, mistakenly assessing their cognitive ability as greater than it is. The cognitive bias of illusory superiority derives from the metacognitive inability of low-ability persons to recognize their own ineptitude; without the self-awareness of metacognition, low-ability people cannot objectively evaluate their actual competence or incompetence.[1]

Conversely, highly competent individuals may erroneously assume that tasks easy for them to perform are also easy for other people to perform, or that other people will have a similar understanding of subjects that they themselves are well-versed in.[2]

As described by social psychologists David Dunning and Justin Kruger, the cognitive bias of illusory superiority results from an internal illusion in people of low ability and from an external misperception in people of high ability; that is, "the miscalibration of the incompetent stems from an error about the self, whereas the miscalibration of the highly competent stems from an error about others."[1]"

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I had to read that three times to understand this. So psychologists would say in the field of psychology I have low ability with a cognitive bias of illusory inferiority? Or something like that. Good luck with the welding course. Anytime you can cut the metal you are working with a Skilsaw, life is good. I’ll tell you something strange that I can’t explain. I have been seasick three times in my life. And each time it was on an aluminum boat. Now I have probably 100 or so voyages on aluminum boats under my belt so maybe it’s just a coincidence. But I have been in heavy weather on steel, wood and fiberglass boats and never felt a twinge. Strange huh?

"Sometimes a patch like the one shown creates more stiffness in that area. When the hull flexes in heavy seas, cracks can develop in the surrounding plating because of the repair."

All boats will flex and squirm in heavy conditions.

Welding can restore metal to near new , I question weather a GRP boat can have a huge hole chopped in its side to speed an engine change and be restored to flex equally as when new.

McGillicuddy said:

. I’ll tell you something strange that I can’t explain. I have been seasick three times in my life. And each time it was on an aluminum boat. ?

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Or, you were scared $hitle$$ that the newly installed Al wouldn't hold up.

BTW, for cognitive disparities venture to TF's own OTDE.