Steel dinghies

Discussion in 'Metal Boat Building' started by Skippy, Sep 21, 2005.

  1. Skippy
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    Skippy Senior Member

    Many people seem to believe that steel doesn't work as well for small boats as for larger ones. Why is that? Are there any books or web pages out there that discuss problems with steel in smaller sizes? Thin sheet stock less than 20ft long should be both strong and manageable, so what are the arguments for or against it?
     
  2. yago
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    yago __

    for: don't know really, unless you have a stock of thin plates lying around and time to wate ;-)
    against: weight and the difficulty to get fair results when welding thin plate.

    I have seen some 20 years ago open flat boats ("scows?") in steel, in teh 4 to 6 meter range in Holland. Maybe d'Artois has some info?
     
  3. Skippy
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    Skippy Senior Member

    Thanks Yago. Just to present some pros for refutation:

    • If you're interested in adventurous cruising in a 15-20ft/5-6m dingy/daysailer, you might want steel to protect against shoals.
    • As for fairing, if you keep the ratio thickness/length constant, why would a larger size be any easier to fair than small?
    • Likewise, why would weight be any more of a problem with a smaller boat than a larger one? General scaling principles favor the smaller size, since weight increases as linear dimensions cubed, whereas strength scales only with the dimensions squared.
    • Another advantage of the smaller boat is that it's easier to pull it out of the water and store dry, reducing the corrosion issue.
     
  4. yokebutt
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    yokebutt Boatbuilder

    Skippy,

    If you build a small dinghy, couldn't you just throw the whole thing in the galvanizing tub?

    Yoke.
     
  5. yago
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    yago __

    It's simply for practical reasons. Try to stick weld anything lighter than 3 mm and you see what I mean.
    On my current project the hull is 3 mm and the deck 2. (check out my site at http://www.justmueller.com/boats/) and the deck is really difficult to work, difficult to fill gaps, and welding deformation is impressive.

    Alternatives for a smaller boat would be TIG welding, or riveting maybe.

    Also, the thinner you go, the less elastic stiffness you would have, first wave you go through will plonk your hull panels inward, and steel likes to deform permanently.

    For an open boat that you would like to drag up a beach you really don't want it to be heavy. We also have built a Murray Isles designed 5.5 m open skiff, spritsail yawl, very nice boat and perfect for raids and camping out. It's plywood/epoxy, with a glassed bottom panel. But even that is too heavy to move around alone, you need two people to get it up the beach.

    for tough, light and easy to build and maintain I would go for alloy.

    Gerd
     
  6. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    In large boats the global stiffness of the hull regarded as a beam is the dimensional factor, therefore ships are built in steel. In smaller boats the local panel stiffness is the important factor. You can step on 3mm of steel or 6mm of plywood. The steel weighs 23kg/m2 while the plywood weighs 3kg/m2.
     
  7. Skippy
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    Skippy Senior Member

    yokebutt: If you build a small dinghy, couldn't you just throw the whole thing in the galvanizing tub?

    Sure, why not? There must be something that doesn't work as well at that size. Does a standard welding torch produce too much heat for the thin stock? Heck, you should be able to make a scale model if you can join the pieces properly.
     
  8. Skippy
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    Skippy Senior Member

    yago: Try to stick weld anything lighter than 3 mm and you see what I mean.

    So I guess it's hard to manufacture a small welder in oxy-acetylene or whatever they use these days.
    Edit: Oops, I just remembered that's used for brazing.
     
  9. Skippy
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    Skippy Senior Member

    Raggi Thor: You can step on 3mm of steel or 6mm of plywood. The steel weighs 23kg/m2 while the plywood weighs 3kg/m2.

    But if you jump on the steel or slam it into a rock, it will deform without breaking. Whereas the plywood will shatter. I've read that 2mm steel would be sufficient for a 20ft/6-7m boat.
     
  10. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    OK, so you could build a dinghy of three layers of 12mm plywood and some layers of 450g glass and it would still be lighter than steel :)
     
  11. Raggi_Thor
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    Raggi_Thor Nav.arch/Designer/Builder

    A small, say 18 feet or 5-6m, boat will need maybe 20m2 meters of sheet material.
    That is 300kg of 2mm steel or (for example) 120kg of 12mm plywood.
     
  12. CDBarry
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    CDBarry Senior Member

    Little steel dinghies are pretty common around shipyards. They are easy to handle with a crane, usually built out of scrap, etc.
     
  13. MarkC
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    MarkC Senior Member

    Ian Nicholson's book 'Small Steel Craft' has some information about steel dingys - building and design tips - he proposes a really nice pram-style. He shows photos of the Dutch-style (scowish type) too.

    I have been trying to 'collect' designs and photos for small steel boats - the material and the size interests me too.

    There were lots of smaller size steel craft. If you go back into archives you see quite some craft. It is an absolute shame that they seem to be forgotten. (We are being made to forget them?)

    Aluminium has probably taken their place now.

    The argument usually runs - that they are too-heavy or require too-much maintenance. This is debateable. It is a matter of their design, purpose, their 'crafting'. Just when someone says that, for instance, a guage of steel can not be welded - I find someone who has done it. 'Tricks of the Trade' abound - and are not given away lightly!!

    Check out Al Sorensen and his 'mini tugboats' - he used tempering on light-guage steel to keep the weight down. (I dont have a link anymore since Ken Hankinson's site is no-more).

    - rivited launches and old speedboats 'auto cars'

    Check out Mullins Boats (Mullins Pressed Steel Antique Boats of Salem Ohio) http://mullinsboats.com/Sub.htm. Just amazing!!!

    Check out Dennis Ganley's smaller designs. www.fairmetalboats.com

    Weston Farmer has some (one below, originally from Sam Rabl) www.duckworks.com?

    The Atkins have some.

    I dont really 'buy' the plywood is better simply because it is lighter argument. It is the concept/design overall that is important. When you examine all the designs, look at the ideas, know how much steel ist a 'craftable' material - then you realise that the sky's the limit.
     

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  14. Thunderhead19
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    Thunderhead19 Senior Member

    Essentially steel is stronger than aluminum, it's stiffer too. A well engineered 18' boat made from steel will also be as light as a typical welded aluminum boat, but you have to be very particular about how it's designed. Unfortunately there are more steps involved in making a steel boat than an aluminum one. Material handling, preparation and finishing are more expensive, and labour is the highest cost in any job these days. That is why you don't see little steel boats around a lot, but they are out there. Little row boats, for example. With no internal framing.
     

  15. Milan
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    Milan Senior Member

    Dutch waters are full of steel boats of al shapes and sizes, from about 3 meters. I would say that aside from low cost their main advantage is their incredible toughens. They can be just left in the crowded waterways without worries. They tolerate rubbing and bumping into the sometimes rough wharf sides or other boats like no other material. They have very long life in the fresh water. (Lot of lakes here). Because of the weight of the material you can't build high performance planing dinghy from steel, but for heavier, workboat types it works very well. Standard rowing /sailing boat of Dutch scouts is about 5 meters long steel dinghy, gaff sloop. Boat is made unsinkable by building steel watertight compartments at the bow and stern.

    Milan


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