Steel Hulls

Discussion in 'Boat Design' started by ScottK, Feb 1, 2010.

  1. ScottK
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    ScottK Landlubber

    Hi all,

    Can someone give me a rough idea on the thickness of steel hulls for sailing yachts that are 30', 40', 50' & 60' long? The boat would be used for circumnavigation so must be able to stand up to anything nature throws it's way. I understand that there are many variables such as how many people is it going to carry, is it going to have engine and how much fuel, etc. But I just want a general idea please.

    And while on the subject, what percentage could a person reduce the thickness of the steel if they went from mild to corten, or monel?

    I see the 40' Arctic Wander http://www.arcticwandering.com/the%20craft.htm has a 5/32 mild steel hull.

    Thanks,
    Scott
     
  2. gonzo
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    gonzo Senior Member

    It largely depends on the hull shape and framing design.
     
  3. Wynand N
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    Wynand N Retired Steelboatbuilder

    As Gonzo mentioned, it depends largely on design and framing to an extent.

    That said, small steel boats are generally stronger than larger ones.
    Here are some thicknesses of steel hulls I had built in the past;

    Tom Thumb 24: Hull = 3mm, Deck = 3mm - Frameless design - multi chine
    Tom Thumb 26: Hull = 3mm, Deck = 3mm - Frameless design - multi chine
    v/d Stadt 34: Hull = 4mm, Deck = 3mm - Frameless design - multi chine
    v/d Stadt 40: Hull = 4mm, Deck = 3mm - Frameless design (Norman) - multi chine
    Dix 38: Hull = 4mm, Deck = 3mm - Frameless design when built(I commisioned this boat 1991) radius chine - with frames nowadays
    Dix 43: Hull = 4mm, Deck = 3mm - Framed - radius chine
    Dix 57: Hull = 4mm, Deck = 3mm - Framed - radius chine
    Dix 65: Hull = 4mm, Deck = 3mm - Framed - radius chine
    Roberts 40 Spray: Hull = 5mm, Deck = mm - Framed - multi chine
    Roberts 45: Hull = 5mm, Deck = 3mm - Framed - multi chine
    Roberts 38: Hull = 5mm, Deck = 3mm - Framed - Old offshore design - multi chine
    Lavranos 40: Hull = 4mm, Deck = 3mm - Framed - multi chine
    Moekli 36: Hull = 4mm, Deck = 3mm - Framed - multi chine

    Corten in my view is over rated for small boat building and except for a bit of more stiffness and negligible amount of copper in it (barnacle prevention?) well over priced. Other problem is thicknesses available - thinnest is 4.5mm which put it out of contention for a lot of boats. And you need to weld this steel with a low hydrogen welding electrode (E-7018) that is very temperamental (hot box etc etc)and need a serious welding machine with an OCV of at least 70, putting it out of the home builder scope.

    Monel on the other hand is an excellent material and is largely used in the manufacturing of petrol tanks but very costly. What is the point having these steels if you are going to paint it anyhow...?
    Mildsteel, shotblasted and painted with a compatible epoxy paint system with airless spray unit, will be as good, if not better than any of the above at an affordable price.

    Hope this help.
     
    Last edited: Feb 1, 2010
  4. ScottK
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    ScottK Landlubber

    If they all looked like the one I have a link to then.
     
  5. ScottK
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    ScottK Landlubber

    Thanks Wynand. Thats what I was looking for.

    And if anyone else out there would like to post what they have used that would be great to.

    Scott
     
  6. gonzo
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    gonzo Senior Member

    That boat has an average plating for a cruising boat. They are usually a bit heavy. With hard chined boats, the plating is thicker than with round hull to keep it from oil canning.
     
  7. jehardiman
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    jehardiman Senior Member

    Also, as Wynand's data alludes to, for small steel boats plating is more often selected for fabrication and welding ease than for absolute minimum strength and weight. It is faster, easier, and cheaper to use a slighty thicker plating and use it's SM to keep the skin fair than to use a lot of internal structure and much thinner plating like aircraft do. Easier to weld with less distortion also.
     
  8. Crag Cay
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    Crag Cay Senior Member

    In the USA you also have the option of plates in 'gauge' sizes. Sometimes it's better to find out what steels are available and at what cost from your steel stock holder, plus what can be best worked by your prospective yard and then do the scantlings using the best materials availble for that project.

    Many of my designs where recalculated to be built in 10 gauge in the USA with the framing adjusted accordingly. It sits quite nicely between 3 and 4mm, which is actually a jump of 33% in shell plate weight.

    Any adjustment can be made just as easily for hard chine as well as round bilge - you have to consider the whole structure as an entity.
     
  9. CDK
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    CDK retired engineer

    5/32 or 4 mm mild seems a bit thin to me for a 40' hull, but with the right shape and a lot of reinforcement to look smooth and stay that way, it is a weight saving construction. The shipyard I go to when I need material uses mainly 6 and 8 mm, but they build and repair "small" fishing ships. With a thicker skin there is less welding distortion and rafters can be further apart.

    The material strength is not the issue here, just good looks. You don't want to pay the bill if the boat you ordered doesn't look good. Using thinner but stronger material has no advantages but is much more expensive in both purchasing price and handling.
     
  10. capt vimes
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    capt vimes Senior Member

    1 person likes this.
  11. Wynand N
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    Wynand N Retired Steelboatbuilder

    Here is a photo of a Dix 65 I had built in 1991 (my client commissioned the design) and she was built with the following;
    Keel Plating: 10mm (8mm? - long ago to be totally sure)
    Complete hull: 4mm mild steel plate
    Deck: 3mm mild steel plate

    Photo taken at shotblasting / epoxy primer stage and the hull is perfectly fair and smooth without nasty fillers etc. The hull and deck is welded inside and outside.
    In fact, it is very easy to build a fair hull if one uses patience and correct welding procedures. It is unnecessary to go for thicker steel plates to keep plating free from distortion when welded :confused:
     

    Attached Files:

  12. Crag Cay
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    Crag Cay Senior Member

    Not only unnecessary, but also mustn't be done without consideration of the stability implications!

    Too many steel boats work on the maxim that if steel is good, more steel must be better! Not so.
     
    1 person likes this.

  13. capt vimes
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    capt vimes Senior Member

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