Aluminium vs Steel

Discussion in 'Boat Design' started by Wynand N, Dec 3, 2004.

  1. mabelsgift
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    mabelsgift Junior Member

    I have spoken to many people and debated the question of steel VS ally many times and the thing I like most about sailboats is still the fact that we can have opposite opinions and still both be right. It all depends on what your'e looking for.
    I chose steel because:
    1) My air cooled welder could be kept on board and run off a genny if I wanted to do a smallish repair job far from anywhere. I know a guy who cut a piece out of a steel bulkhead to make up a patch.
    2) Drop a few bronze coins and if they end up in the wrong places on an ally boat you have cheese.
    3)I have seen a few aluminium hulls that were built by top pro's break along the weld lines but I am confident that, even in spite of my welding, my steel hull will not break up.
    4)I'm a Japie so tough + simple to fix is the way I like things to be.
     
  2. ivansalasj
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    ivansalasj Junior Member

    And i would use aluminium when,

    1) A higher level of performance is required together with a smaller weight.
    2) Money is not a problem.
    3) Good finish!!!

    But then there is the question in why not use a composite material?

    A good day of thinking a couple of sheets in advantages and disadvatages for both materials and you have your anwser.

    At the moment i am designing a high performance cruising sloop, (32m Loa) and the material i decided to use is Aluminium but purely for my demand on a small weight of 91 tons.
     
  3. mabelsgift
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    mabelsgift Junior Member

    The only reason I did not consider composites is I know very little about working with them but I have now become very friendly with a chap who knows plenty and designs for major users of composites, so my next boat will be... you guessed it.. just to learn about the product and see what I can do with it. But then it might end up as a junk rig and spoil the experiment for some folks, but that's life. My test will be a 25/26 footer
     
  4. MikeJohns
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    MikeJohns Senior Member

    Composites:
    Non Isotropic materials that change their properties over time.

    An engineers nightmare. You have to design for a life span with composites, the polyester resins in GRP are unstable in water! they break down when the water content gets too high in the layup. It tends not to be a good option for work boat designs.

    The projected life span of many modern grp boats is very poor. Ironically the life of modern steel hulls is considerably higher than GRP, steel adds the possibility of cheap effective re-building down the track and some hulls may well come close to imortality. BUT the steel hulls need to be designed for easier access to the inside of the hull, without this access it is not so good.
     
  5. mabelsgift
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    mabelsgift Junior Member

    Thanks Mikejohns, I agree with you but I am building this boat just to learn more about the properties of the materials. I still uise steel for my cruising boat. Once I was in a storm and I saw a wave "eat up" a fibreglass fishing boat. It came out of the wave looking like alphabet noodles in a soup bowl. We manages to rescue all the crew but my steel hull never complained at all.
     
  6. yago
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    yago __

    I am building a steelboat myself, it's the fifth hull I build now, but I have also build in ply and helped on lots of projects in many other materials.

    I believe that the argument for steel is very often based on simplifications and emotions though.. something like

    steel=heavy=strong=safe
    and cheap on top of all that...

    - strength can not be measured as an absolute factor, we have to decide between impact, abrasion etc, and boats can be built very strong in any material really. Obviously, building a boat to knock around corals or going into pack ice for prolonged periods has other requirements for strength than a boat that would be build for open passages between marinas..
    - weight is not a safetyfactor per se, in almost all conditions a fast responsive boat is preferable. Adding weight also tends to weaken strength: for example, a heavier boat will offer more resistance when a wave hits superstrucure, bullseyes etc...
    - Safety depends on too many things and last but not least on having as light and fast a boat as possible to avoid critical situation. In the end, design, scantling and quality of execution decides on safety for a given usage, not the material itself

    Money: with rising prices for steel, and having done the overall calculation for the entire boat, you usually find that the total budget is really not that much different, and depending on whom you ask it might even be in favor of alloy because all equipment, rig, winches etc could be one size smaller and that would clearly produce a lower budget for the same size and habitable volume.

    The reasons for my personal (repeated) choice of steel:
    - lower startup funds required for tools and materials, you can literally finance a smaller steel hull from your pocket money as you go along, building outside. The first time you really have to find larger amounts is when you sandblast and paint, but then you have already a finished shell and something like 30 % of the work done.
    - easy strength - you really have to try very hard to build a weak hull. Scrappy welding may produce aesthetic problems, but steel for smaller boats is always so much overkill that in a quarter of a century I have never seen any steel hull fail that was properly framed and stringered for keel, skeg & rudder.
    - Low tech and KISS philosophy - building in steel with minimal equipment is a good mental training for everything that comes later, and will help to keep away from the chipchandlers. Most alloy projects I have seen were in the end much more expensive than most of the steel ones, NOT because of the material but because the alloy guys had a different mindset to begin with and would add more equipment and of higher prices than their steel counterparts.
    -finally, steel is nice to work with - but that is very personal, and in the end you will choose what feels best to you.

    I also would like to build in alloy one day, and if I had the time, some cold molded WEST would also be lovely ;-)
     
  7. JimCooper
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    JimCooper Junior Member

    Solid grp is the only way to use grp. the foam cored hulls are so much landfill after a few hard years. Even then as MikeJohn says the material is prone to ageing. Alloy loses its strength. I worked on an alloy fishing boat that was past its lifesapan, kept cracking round the stress points. The patches cracked around the welds in the end we had large riveted plates cause the welds kept failing. looked awful. Steel is just a cut and replace weldup job with no re-work problems.
    As for GRP patching a hull is not so good either.
    Nah give me steel any day laddie
     
  8. MikeJohns
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    MikeJohns Senior Member

    Yago


    If you do any higher altitude sailing then Gales are a regular occurance that you will often encounter.

    There is no boundry condition for lightness and safety; the heavier the boat becomes the larger her scantlings and the stronger she is, the more gentle her motion,( and the greater the expense to build), whether she is wet and or unsafe depends also on her design . I have sailed with 50 tonne 60 footers that were brisk comfortable and dry.
    I find my own 16 tonne 45 footer too light in heavy weather for comfort .

    Confined to a bunk for 10 days with severe sea-sickness is a critical situation too.

    A heavier stronger boat much better survives groundings, towings, and collisions and the survival technique of lying-ahull.

    It is good to be able to have faith in the boat, In Austrlia alone we have seen 3 keels fall off so called cruising yachts in the last year or so with several resultant deaths.

    Internal ballast in steel hulls is so preferable to an anorexic strut holding a bomb shaped lump of lead attached to a foam cored grp hull.

    Jim Cooper
    I have seen similar problems on alloy work boats, problem is the patches are always weaker around the welds and you need doublers, I have seen a large alloy yacht hull split along several of the seams where the welds failed after a long period of rough weather, the boat was ok the failure was not catastrophic but the crew were a bit shaken .
     
  9. cyclops
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    cyclops Senior Member

    2 cents and very unscientific, but usefull. I once took equal sized pieces of various types of steel and Aluminum put each piece in a large vise and with a piece of the same metal on each side, bent it back and forth till a complete fracture occured. Is it necessary to say which metal takes more flexing by a factor of 5 to 30 times than the other. Not lab. standards, but still undeniable, and it proves the overall safety of steel. the best Aluminum came close to the worst steel. That is just the way it is. Welding will kill it ( Aluminum ) in a flexing enviroment.
     
  10. yago
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    yago __

    Mike, the point I wanted to make is that it would be correct to seperate weight and strength. For a given material, it's the other way round, the larger her scantling the stronger she becomes, but she can well become heavier without gaining strength (placing weight elsewhere). Also, by using different materials you can very well build a boat just as strong but lighter. Think airplanes. Or build a boat from carbon and vacuumed epoxy to the same weight as a traditional steel hull and then try to destroy it ;-)

    Also, increasing scantlings is not a guarantee for ultimate strength. Advocates of frameless construction (which I am not) might argue that a boat with lighter framing will have a skin that will not puncture as easily near a hard point, the lighter frame bending away and the skin just denting in.

    Gales: I would say that any hull AT SEA, from whatever material, has to be built strong enough not to split at the seams or have the superstructure washed away, otherwise shoot the designer or builder ;-) Weak points are then usually the openings, hatches, etc. That's why discussions about steel versus alloy use corals, rocks, containers and other nasty hard stuff for proof of strength.

    As for comfort, yes, I agree, a boat with more mass (but not automatically more strength now...) would be felt as being more comfortable by most people. Still, depends where the masses are distributed, both for comfort and safety. For me, a reasonably heavy boat with modern hull, high Cp and wider U-section seems the most comfortable - I do not like the movements of typical traditional hulls with long keel, deep wineglass and low Cp. But that is very personal, I admit.
     
  11. D'ARTOIS
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    D'ARTOIS Senior Member

    If somebody would offer me an alloy (read aluminium) hull from Huisman, Jongert, Blohm Voss, Lurssen, and the like, I would never doubt there craftmanship and accept this boat in the blind. I have seen a Blohm & Voss boat, with an alu hull in pristine condition, almost 65 years old.
    Huisman's aluminium boats from their early years are still (mostly) in excellent condition. Some years ago I saw a Bloemsma (Makkum) casco that was almost the non plus ultra, iceclass rated etc.
    Aluminium requires topwelders. The main problem is porosity in the weld. Only highly trained and very skillfull craftsmen, artisans, can supply good hulls.
    It is not the price of the material - and certainly not with aluminium - any comparison between the two isotropic materials is therefore not in place. If you choose, you have a reason and that is that.
    I always see that "Fliegerrettungsboot" from WWII made by Blohm & Voss ( Bismarck et al) aluminium, all wood inside "verrotted" but the hull as sound as the a** of a 16 y/o girl.
     
  12. cyclops
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    cyclops Senior Member

    Memory tells me Blomm & Voss did make the largest 6 engined flying boat that actually flew in service. Aluminum, had to be shot out of the sky. Boats, Flying Boats all the same to B & V.
     
  13. water addict
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    water addict Naval Architect

    Mike Johns postings seem to be spot on.

    There were a couple of links posted on this thread pushing obviously slanted data favoring aluminum. From my organizations' research performed over multiple decades, you can expect about a 20-45% weight savings using aluminum, but you pay much more, and you have lots of fatigue problems. Also, there is a lot of nitty-gritty in the details depending on the application and grade of both steel and aluminum used, making it hard to really say exactly what if any weight savings will be had using aluminum. And if there's a fire of any size, kiss aluminum goodbye.

    If you need lightweight for performance and can pay the price for a short term structure, alum. is generally better. If you want toughness and durability for less money and more weight is ok, go with steel.
     
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  14. water addict
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    water addict Naval Architect

    forgot to mention that 20-45% weight reduction for Aluminum is the weight of structure savings alone. The actual displacement reduction will be less depending on your payload.
     

  15. D'ARTOIS
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    D'ARTOIS Senior Member

    That is not a fact. A steel boat requires less ballast than an aluminium one where the ballast ratio is higher. Take this in consideration and you see the distance between alu and steel shrinking. Next to that: for an alu hull you require more stiffeners: bulkheads, frames and longitudinals.

    A given 37' displaces in steel 19.000 lbs; in alu 15.500 lbs - difference 3500 lbs
    that is about 18.5% difference in weight in favour of aluminium; - AND the difference in buildingprice - is that restricted to only 18.5%?
     
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