Should I be concerned...3.2mm steel thickness

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tomdmx

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I've managed to get my hands on an ultrasound from one of the chaps in the boatyard (he is a surveyor but works for the boat yard also)...so I sounded below and above waterline...
In the keel I got a consistent 4.5-5mm thickness but after scraping off 2-3mm of pain on the sides below water line, I got a consistent 3.1/3.2 mm thickness of steel (and above waterline with paint I got 4.5mm). I tested in approx 20 spots...

Where I could access the tested areas on the other side, the steel appeared very solid etc as I got concerned about thickness. I then talked to the surveyor chap and said that 3mm is not much but it may be likely that this yacht has been built in part a different type of steel named kardan which fits the years and the fact that the thickness would not be much beyond 3mm..No other signs of rust (ie no pitting etc) is evident either (some on the paint but removing two coats shows nice clean steel)...

This is a sloop built in Harlingen boat yard in Holland, by Van Der Vlis under Van Der Stadt design (well the plate states Cumulant, Harlingen boat yards, Van D. Vlis) in the 70s...

So is this thickness a problem?
 
Steel Thickness

From what I've been told in the last few weeks - no. I certainly hope not!

The thicker the better. It would've been 4mm probably but over 3 is acceptable. I was panicking about thickness but if you can find a good welder, the great thing about steel is it can be patched and if it's done right you won't even know. If the welder starts talking about putting a patch OVER the steel run away - it can be done properly. The only hassle is removing flammable stuff on the inside. I'm not worrying about my 3.2mm bits as I'm in the land of the steel boat and they say it's ok.
 
We are wanting to cross an ocean or three and going to remote areas but my mate can weld well and we're taking equipment (ie a very good portable welder etc) with us....
My issue was that both sides of the tested area appeared very good..yet thickness wasnt there (which got me thinking on corrosion within the steel...) not sure if that makes sense...
 
I don't think you have any problem at all. You don't say what size your vessel is but boats around the 30 foot or so length are usually built of about 3mm except for the keel. Anything thicker and the boat would be heavy and slow and wouldn't be able to carry much stores and supplies.
My boat, a Dudley Dix Hout Bay 33, has 5mm keel and 3mm everywhere else. Provided the plating is well supported by the internal framing this is plenty. If the ultrasound gives consistent results and you haven't seen any worrying rust inside or out you need have no fear. Remember that the loss of 1mm of steel produces (I think) approx 8mm of rust/scale, so any rusting going on under paintwork is pretty evident. The areas that tend to give problems are interfaces between steel and any timber, such as hatch frames or teak decking.
The Dutch are fine steel boatbuilders and are well respected for it, so I think you can be confident your boat will carry you through.
 
Sorry should have said, its a 31 footer, no teak deck :) and I've no idea what its original thickness was, but thank you for your words of confidence, its much appreciated and logical in explanation so I do feel relieved :)

And yes Corten is the right term :)
 
No problem with thickness at all. Whilst the standard for UK narrowboats has been the 10,6,4 mm (base, sides, top) my old De groot only ever started with a 5mm hull thickness. When it got below 3 mm at the waterline (where most degradation occurs) I had a 6" wide overplate placed continuously around the waterline welded top and bottom.
 
The 10M Tucker design steel boat I built has thickness of 4mm plate for the hull, 3mm for the deck, 6mm for the sides of the twin keels and 10mm for the keel feet and tops. This was what I ordered from the steel supplier. When surveyed it was found that the decks and the hull thicknesses were about 10% thicker than this. Having never had a welder in my hands before building my boat my biggest concern was the boat falling apart in a sea because if my **** workmanship.

Ten years later my greatest concern upon taking the boat out of the water annually is any wastage in the hull (no probs so far). Welds are not a concern any more.

I am grateful that I have more hull thickness than recommended by a superb designer. However if I checked my hull thickness below the water line to-morrow and found it about 3mm I would be worried. I am no expert but I assume there is a rule of thumb as regards hull thickness for a vessel which you are talking about. I just get a feeling that the thickness you have found are not the original spec of the designer.
 
I think your statement "I got a consistent 3.1/3.2 mm thickness of steel " should put your mind at rest. If you get corrosion in steel it is usually inconsistent.

Also Dutch builders of steel vessels have always been masters at welding thin steel without buckling. Many builders use thicker plate because of this problem.
 
If my memory serves me right, I think 3.2mm is what the Americans call 10 gauge.

If so it's the ideal material for a 31 foot boat. 4mm is way too heavy. Its only appears a little difference, but works out as quite a lot over the full area of a boats hull plating.

Seems odd though that they would use the thicker 4mm plate on the topsides. Anyway, 10 gauge was the standard steel for hulls up to 36 ft.
 
My understanding is that steel boats less than 40 feet have a steel plate thickness of 3mm. Otherwise the boat is too heavy. 40 feet + and it is likely to be 4mm. Our steel boat is 38 feet and 3mm. I'm surprised you got readings on the keel. Anything behind the steel such as a frame, or the lead ingots in our keel give a dead reading.
 
We tend to talk about plate thickness in an abstract form.
Just recently I was going to buy some 4mm stainless plate to do some small scale fabrication.
I chose 4mm because I thought it was the same as some plate I had inherited.
I was quite suprised to find the plate I had was only 2mm.
Not suggesting this for a hull but so long as its a professionally built hull with adequate framing;welding etc. 3mm should be no problem
Remember Van de Stadt was a pioneer in developing construction methods akin to the mirror dingy stitch and glue process where thinner preshaped hull plate sections were welded up first and frames stringers and bulkheads added afterwards-in this form the hull plates themselves are part the of the overall structure unlike traditional designs where the frames;bulkheads and stringers were the structural form and plates did no more than keep the water out so to speak.
Latest design plywood Wharram cats use similar techniques.
 
I'd suggest that if you want to know what the original thickness should have been, Van Der Stadt would be the people to ask http://www.stadtdesign.com . They may well have a construction schedule that will tell you exactly what thickness of plate was specified.

I do note that 1/8" is 3.175mm, and also that nominal 3mm plate could be 3.1 or 3.2mm and still be within manufacturing tolerances. If 3mm or (less likely) 1/8" plate was specified then you're fine. If 4mm is what you should have, then your readings show wastage of 20%+, and that would be a problem, but I don't think that's likely.

I'm currently looking to buy an ultrasonic thickness tester that can give accurate measurements through paint that ignore the paint thickness. They're not cheap (the cheapest quote I've had was £1100 plus VAT), but they do make surveying steel boats a lot easier (without one I'd either have to get permission to grind back to bare metal (unlikely to be given), or buy a paint thickness gauge and subtract the paint thickness from all of my readings, and I'd still have to shell out for a fairly expensive thickness tester to get a reading through paint at all).
 
>frames stringers and bulkheads added

If you are looking for a steel boat and it has stringers walk away, they trap moisture and are not needed for structural integrity. Also the frames should be spot welded to allow any moisture to run round the weld, although this is unusual unless built professionally by a Dutch yard and perhaps other steel boat yards around the world.
 
KellysEye said:
>frames stringers and bulkheads added

If you are looking for a steel boat and it has stringers walk away, they trap moisture and are not needed for structural integrity. Also the frames should be spot welded to allow any moisture to run round the weld, although this is unusual unless built professionally by a Dutch yard and perhaps other steel boat yards around the world.
Click to expand...

Having the shell plating only spot welded to the frames is unusual, because it means that the shell plating is likely to be inadequately supported. There are also plenty of properly designed steel boats that have stringers.

Moisture traps on steel (or indeed pretty much any) boats are avoided by having limber holes in frames, stringers, and anywhere else that could trap moisture, and keeping them clear.
 
DanBurrill said:
Having the shell plating only spot welded to the frames is unusual, because it means that the shell plating is likely to be inadequately supported. There are also plenty of properly designed steel boats that have stringers.

Moisture traps on steel (or indeed pretty much any) boats are avoided by having limber holes in frames, stringers, and anywhere else that could trap moisture, and keeping them clear.
Click to expand...

Agreed

I am struggling to recall seeing a steel cruising yacht without longitudinals; either notched or backed to frames.

Providing the limber hole is smooth and elliptical there should be no problem with drainage.

An alternative is to spay foam insulation flush to the top of the stringers.
 
chinita said:
I am struggling to recall seeing a steel cruising yacht without longitudinals;
Click to expand...

My previous boat, built in the late 50's or early 60's had angle frames at 16inch spaces, 3mm plating for all the curved hull and 4mm in the keel. Only longitudinals were the keel, deck edge and cabin sides. A James Mcgruer 8 metre cruiser racer, the first owner bought plans for the wood design, and had it built of steel in Belgium.

Unfortunately, she rusted away from the inside, and is on her second rebuild.
 
Hey All,

Thank you for the words of wisdom, whilst I got no further info on her build I'm reasonably confident she'll be fine...anyway time is running out for us to get across the channel to the French canals so need to complete a few more jobs and get going very soon :)
 
srm said:
My previous boat, built in the late 50's or early 60's had angle frames at 16inch spaces, 3mm plating for all the curved hull and 4mm in the keel. Only longitudinals were the keel, deck edge and cabin sides. A James Mcgruer 8 metre cruiser racer, the first owner bought plans for the wood design, and had it built of steel in Belgium.

Unfortunately, she rusted away from the inside, and is on her second rebuild.
Click to expand...

I know that they exist, as do frameless builds.

I was reflecting on somebody who commented 'if you see a boat with stringers - walk away'.

If that is the case, there may be a lot of walking to do!
 
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