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Old 01-06-2020, 19:43   #1
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Steel hull how thin?

I have just been wondering about all those old steel yachts built in the 1970's and 1980's. How thin does the steel hull plating have to be before the yacht is scrapped?
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Old 01-06-2020, 21:01   #2
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Re: Steel hull how thin?

It's in the bilges and anchor well that the problems start. Steel boats rust from the inside out. My experience has been that corrosion in the bilges ultimately dooms them to scrapping. It's not because the plating is thin so much, but rather the sheer difficulty in gaining access to and repairing the structure within.



Plating itself is easily patched and a common "field method" is to try and poke holes in suspect looking areas with a spike although the technical purists would probably use an ultrasonic thickness tester and work with percentage loss of thickness as an indicator.
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Old 02-06-2020, 05:14   #3
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Re: Steel hull how thin?

They generally don't start leaking until the corrosion makes it's way all the way through and even then a hole may not appear until you beat all the rust of.
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Old 02-06-2020, 06:43   #4
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Re: Steel hull how thin?

Both posts above are accurate.

I don’t like the ultrasonic gages. They have their place but not as a good assessment for a hull. Insurance companies want them. A much better gage is a good hammer and a bad attitude.

The first time I replaced plate I was in a panic. Probably didn’t do a real good job as a result. Now I’ve done it or had it done a number of times it’s no big deal. The General before and after clean up and painting is a bigger deal.
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Old 02-06-2020, 09:14   #5
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Re: Steel hull how thin?

less than 25 percent wastage after 20 years is acceptable. More wastage and then plates need to be replaced. Both ultrasound and a hammer are the best ways to detect too much wastage. The most common areas of wastage are in the bow around the chain locker and aft around the rudder post on a steel or aluminum vessel.
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Old 02-06-2020, 09:15   #6
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Re: Steel hull how thin?

Quote:
Originally Posted by hpeer View Post
Both posts above are accurate.

I donít like the ultrasonic gages. They have their place but not as a good assessment for a hull. Insurance companies want them. A much better gage is a good hammer and a bad attitude.

The first time I replaced plate I was in a panic. Probably didnít do a real good job as a result. Now Iíve done it or had it done a number of times itís no big deal. The General before and after clean up and painting is a bigger deal.
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total agreement.... no big deal. Just don't burn your boat down while welding!
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Old 02-06-2020, 09:51   #7
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Re: Steel hull how thin?

Quote:
Originally Posted by hpeer View Post
Both posts above are accurate.

I don’t like the ultrasonic gages. They have their place but not as a good assessment for a hull. Insurance companies want them. A much better gage is a good hammer and a bad attitude.

The first time I replaced plate I was in a panic. Probably didn’t do a real good job as a result. Now I’ve done it or had it done a number of times it’s no big deal. The General before and after clean up and painting is a bigger deal.

Yes and no. I've spent thousands of hours doing UT testing on refinery tanks.

UT is a vital tool for:
  • General thickness. Structure and leaks are two separate issues.
  • Areas where the reverse side is not accessible.
  • Accessing the texture of a side you cannot see (the echo changes if there is corrosion)
  • UT can work better than a hammer if there are thick coatings. Requires experienced operator and A-scan ultrasound meter.
  • Plates thicker than 1/8". Thicker than that UT us generally better.
  • Fuel tanks. You really don't want to put a hammer through the side of a 1,000,000 gallon gasoline tank. By extension, a boat in the water! You can UT test through bilge water.
It must NOT be a thickness-only meter. You need the A-scan.

Hammer testing can give you an indication of soundness, but the proximity of braces can make that really tricky. And to do a good job you really need to pound on the whole boat, which is not going to go over well. In refineries, you hammer test the whole roof. It's loud.

However, UT tells you NOTHING about tiny pits (transducer is bigger than the pit) or areas very close to supports (can't get the transducer in).

Thus, a good assessment requires at least 6 things:
  1. 20/20 eyesight (corrected)
  2. Bright light
  3. Cleanliness
  4. Hammer (hard--will damamge paint)
  5. Scraper
  6. UT a-scan meter (not just thickness) and couplant
I agree that a hammer is vital. And a scraper.
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Old 02-06-2020, 10:14   #8
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Re: Steel hull how thin?

Yup, I’m OK with all that. But that’s NOT what you get on a standard hull survey.

But back to the question about hull thickness foe a moment.

Most smaller yachts are 10gage steel. That’s a tiny bit thicker than 1/8”. Larger boats tend to be 3/16”. Then you get boats with 1/4” keel and 3/16 topsides.

I have seen a 52’ boat made of 10 gage steel. Been 10 years since I saw it, and it was 25 years old then. Saw it for sale last year so it’s still around.

3/16” gives you more “wasting” time.

You can go pretty thin and keep water out, but the you collision protection goes to hell.
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Old 02-06-2020, 10:35   #9
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Re: Steel hull how thin?

Yes, I've worked on ships for quite a few years and steel needs to be maintained. It is in the inaccessible areas where rust and scale is difficult to remove and good preservation established is what dictates hull life. At a certain point the cost of maintenance becomes prohibitive.
But to the OP, hull thickness would be dictated and gauged by overall hull design such as frame spacing.
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It's in the bilges and anchor well that the problems start. Steel boats rust from the inside out. My experience has been that corrosion in the bilges ultimately dooms them to scrapping. It's not because the plating is thin so much, but rather the sheer difficulty in gaining access to and repairing the structure within.



Plating itself is easily patched and a common "field method" is to try and poke holes in suspect looking areas with a spike although the technical purists would probably use an ultrasonic thickness tester and work with percentage loss of thickness as an indicator.
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Old 02-06-2020, 13:44   #10
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Re: Steel hull how thin?

Scorpius was launched in 1982 and, as I've mentioned before, was completely flame-sprayed (zinc metallized - essentially hot dip galvanized) inside and out, then foam-sprayed inside down to the top of the box keel. She's hard-chined (a Colvin Gazelle hull) and 10 gauge steel down to the top of the box keel. The original box keel is 3/8" steel sides and 1/2" steel on the bottom. In 1989 we extended the box keel down a foot (long story) and moved the 6000 pounds of lead ballast down there. The fabricator of the keel extension didn't have any 1/2" steel kicking around so the bottom is now 3/4" plate (after all, it's ballast). The first time I hit a rock with it (I confess to have hit one or two. After all, I cruise the BC coast!) - and I hit HARD - I scrambled below and tore up the floorboards looking for water coming in. Nothing. The next time I hauled out it took me ten minutes with a file to take the burrs off the edges of the scratch in the keel before repainting.

Even now, after nearly 40 years, if I remove the foam to do something, the galvanized steel behind is pristine. After removing some tanks, the inside of the box keel is quite accessible and gets painted every five years or so. I have absolutely no concerns about structural rust anywhere - and if it does develop, as others have pointed out, it's easy enough to replace some plate or weld on a patch.

The point is, a well-built, well maintained steel boat can last pretty well indefinitely.

Go steel!
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Old 02-06-2020, 17:48   #11
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Re: Steel hull how thin?

Also be careful if you have a new computer controlled Tier III engine. The computer can be damaged from EMF caused by welding on the boat. Best remove it.
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Old 02-06-2020, 18:57   #12
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Steel hull how thin?

Quote:
Originally Posted by hpeer View Post
Both posts above are accurate.

I donít like the ultrasonic gages. They have their place but not as a good assessment for a hull. Insurance companies want them. A much better gage is a good hammer and a bad attitude.

The first time I replaced plate I was in a panic. Probably didnít do a real good job as a result. Now Iíve done it or had it done a number of times itís no big deal. The General before and after clean up and painting is a bigger deal.


Agreed. And to build on this a bit, you can have fantastic ultrasound readings all over the place, but miss that 1/4Ē area next to a bilge stringer or frame where the moisture has laid for decades, and reduced the playing to paper. That tiny region can be 1/2Ē long, or 24Ē long... depending on the shape and condition. Either way, itís damn thin and can sink your ass, if it lets go at the right (wrong) time.

When I brought my shiny steel cutter home to Nova Scotia from Florida in 2013, after an expensive (but in retrospect, ineffective) survey; we had the hull blasted to bare metal. It was time- the boat was 25, and the paint systems were tired. We blasted 5 holes thru the boat by accident, and I discovered over 200 horrifying corrosion pits and holes. Turns out we were being saved by the spray foam insulation on the inside, which can be surprisingly watertight!!

...We ended up replacing over 50 sq ft of plate.

Thankfully it was steel, and so a doable and reasonable job. But it sure wasnít what I had planned for!!

Like hpeer, once youíve sucked it up and done the deed, not much scares you any more. Kinda like having kids in a way.

But back to the point of this thread, there is no blanket Ďminimum plate thicknessí that is ok. You can get away with tissue paper in the right places, and need 1/2Ē in others. Depends on the situation, the design, the usage and the experience.
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Old 02-06-2020, 19:09   #13
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Re: Steel hull how thin?

10 guage I thought was .100" so less than 1/8". Wouldn't the thickness depend on the size of boat? I'm currently at a yard where a French guy has spent 3 years rebuilding a steel boat. Appears he's patched about 50% above and below the waterline. Not sure if that qualifies as "replating" or not. Never seen anything like it.
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Old 02-06-2020, 19:34   #14
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Re: Steel hull how thin?

Quote:
Originally Posted by Boatguy30 View Post
10 guage I thought was .100" so less than 1/8". Wouldn't the thickness depend on the size of boat? I'm currently at a yard where a French guy has spent 3 years rebuilding a steel boat. Appears he's patched about 50% above and below the waterline. Not sure if that qualifies as "replating" or not. Never seen anything like it.

Yes, it does. Delfin is 55', 1/4" plating, built in 1965. I've seen 1/8" on fairly small boats, like 30' or so.
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Old 02-06-2020, 19:43   #15
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Re: Steel hull how thin?

My mistake on the guage, but still an interesting discussion on how much of a steel boat can be reached.

I'm not an engineer but I would assume 144 1" pieces welded together is not as strong as a single 1 square foot piece? Or is the weld 100% if done right?
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