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Old 11-07-2011, 18:10   #1
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Hull Thickness Too Thin ? I Paid the Deposit - Oops or Not ?

I would really appreciate some professional advice here. I have just been to view a Dutch style steel cruiser (30ft) today and put a deposit down to buy her.

I was concerned about the hull thickness being to thin; the survey showed the keel to be 6mm and the bottom and sides to range from 2.8mm to 3.2mm. however the hull survey was positive and said there were no signs of damage or corrosion and that the boat was built well. Bulk heads and stringers were in good condition too.

Acting on the survey's very positive tone I put a deposit down. I am now feeling though that 2.8 to 3.2mm could perhaps be to thin. Some people seem to think it is normal (especially if there is no corrosion and the structure is sound) others think its not. The boat by the way was floated in 1989 but took about 6 years or so to construct.

Some advice would be much appreciated. Thanks.
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Old 11-07-2011, 18:17   #2
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Re: Hull thickness to thin? I paid the deposit, oops or not?

Three mm is .11". That is too thin, IMHO. Unless she's a lake cruiser...
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Old 11-07-2011, 18:27   #3
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

My 35-footer uses 12, 8, and 6 mm steel for keel/hull while decks are 5 mm and superstructure 4 mm. It might be considered a bit over-built, but there is more than thickness to strength.



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Old 11-07-2011, 19:40   #4
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Not professional advice...

If you want professional advice then find the appropriate professional and pay them to advise you. This is a discussion Forum, so we can discuss your question but I'd strongly caution against thinking that its professional advice.

That said, the deck on Boracay is 3mm steel and the hull is 5mm, on a 44' yacht. The deck has been fine for the 5 odd years that I have owned her.

I've had some corrosion and some electrolysis, but none of it looks to have gone over 1mm.

Steel is a high maintenance material. If you were to buy this boat you'd need to constantly inspect her inside and out, replace electrodes on a regular basis and to keep the maintenance to a high standard.

The upside is that repairs to steel are within the ability of many professional welders and some shipwrights and the cost is not enormous. The structural integrity is almost always kept.

I've heard that some owners just use Blue Tack...
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Old 11-07-2011, 20:00   #5
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

Contact the original designer or an owner's association to see what the plans called for in the first place. Was she built to the correct specifications? For some reason lots of amateur builders think they can modify designs to their heart's content, though they usually way overbuild and end up with something too heavy.
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Old 11-07-2011, 20:04   #6
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

I would echo above, there are many factors that contribute to strength of steel hull. Curverature, spacing of structural members, ... How thick was it manufactured?? Only a professional, and you can determine if the boat meets your needs. Good news steel is very repairable, bad news it will cost.

If I didn't buy a plastic boat, I would buy steel. And spend much money on anodes, and high quality coatings. Steel is a high strength material, but basically soluble in sea water.
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Old 11-07-2011, 20:36   #7
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For a 32' boat your hull thickness is just fine. Sometimes a bit harder to keep if fair when building but strength-wise is no problem. Corrosion will eat thru anything but you are not stupid enough to overlook that. For smaller boats it is crucial not to overbuild with steel and many smaller boats have been done in 3mm or less. It is not a 44' boat. Just takes more welding skill. Go for it if that is your only worry.
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Old 11-07-2011, 20:42   #8
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By the bye, I built a 26' steel yacht in 3mm for hull and 5mm for keel. No problem but getting it fair was tricky. A professional would handle it easily and European steel boats have been routinely done in 2mm for smaller boats. Just more skill required.
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Old 11-07-2011, 22:56   #9
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Rust on the top, electrolysis on the bottom...

I found that I only got rust on my topsides, where there's lots of oxygen. Below the waterline there's no rust, as there seems to be little oxygen in sea water, but shinny metal on the occasional haulout, especially if the anodes have "vanished".

Twixt wind and water is my big concern...
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Old 12-07-2011, 00:21   #10
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

I am assuming that since you just looked at it, this is an old survey done for someone else.

So Boracay’s advice on finding a good steel surveyor is exactly right!

Keep in mind that steel boats rust from the inside->out, so get the proper equipment to inspect every inch of the inside structure frame spaces (especially the inaccessible areas fore and aft, where water can lie) and focus on the quality of inside coatings as a key indicator.

As well as construction, Type of steel also makes a difference in required scantlings (thickness)

My own SG is made of the higher strength, self-sealing Corten Steel, which allows for thinner plate but the Dutch builder overbuilt (as they usually do)

After 29 years the bilges are like new but I was worried about electrolysis so last year I had the hull sandblasted to white steel for detailed exterior inspection.

I first scraped off the years of antifouling to identify any areas of poor adhesion before blasting a side each night

Perfect, except for this small dime sized spot near the stern quarter. It was mostly cosmetic but I burned rod anyway.

Found out that under the lazarette were about 8 large zinc anodes that had been used for ballast.

One anode had shifted and its corner was touching the exact spot where I found the blemish. (after 28 years)

Point I am making is that you should research what you have and know that steel is quite easy to live with if it was constructed well and you follow basic maintenance.

Good luck with your search!
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Old 12-07-2011, 01:55   #11
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Quote:
Originally Posted by Boracay
.....

The upside is that repairs to steel are within the ability of many professional welders and some shipwrights and the cost is not enormous. The structural integrity is almost always kept.

I've heard that some owners just use Blue Tack...
The proper repair of a 2pack painted steel vessel like the Dutch build is more skilful then GRP repair and often needs a complete repaint. The process is more akin to car body repair. It's certainly not a job for the unskilled.

The thickness is typical for such a small steel boat. If well built it should be fine

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Old 12-07-2011, 02:45   #12
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

Some good advice above. It will cost you about £500 for a survey in the UK for her which would be money well spent before you complete.

Worth doing because the sentence that she was launched in 1989 and then took 6 years to complete is slightly worrying. Dutch made steel cruisers have a good reputation for quality but would be completed by a Dutch company and sold, why then did this one take 6 years or was it a DIY project? in which case caution is required and a full survey by someone experienced with steel boats.

Got any photos?

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Old 12-07-2011, 04:36   #13
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

Greetings and welcome aboard the CF, cilrath.
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Old 12-07-2011, 04:58   #14
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

That is the about the right hull thickness for a thirty footer. In fact, some reputable designers argue that 2mm would be sufficient but far to hard to weld and build fair.

For a newcomer to steel, there is a $12 book (download) that provides lots of good info and advice. You can get it here: Marine books and documentaries by Scott Fratcher
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Old 13-07-2011, 12:01   #15
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Re: Hull Thickness too Thin ? I Paid the Deposit - Oops or Not ?

My Roberts 38 is plated with 1/8 inch plating on the hull. That's 3.2mm. She's been around since 78 and is still there. Compared to my last boat which was 3/16ths on a 30 foot length she seems a bit thin but so far no problems. I agree, overbuilding is a problem on steel boats.

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