Aluminium vs Steel

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

  1. yago
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    yago __

    Wynand, very interesting, first time I hear that. I have seen plans for alloy or ply decks on steel, of course, but never thought of FRP.

    I am not sure if that is a commercially valid combination, might not be easy to communicate, but there are lots of advantages: light topweights, easy schaping, rust happening mostly on chipped corners on deck and fitting etc. actually many the advantages of the FRP boat with the simple strength of a steel hull... worth thinking about...
     
  2. MikeJohns
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    MikeJohns Senior Member

    Jim
    To the last part I would reply Often !

    Regarding concreting Al bilges.........Dont. Aluminium doesn't get the protective layer from a cement coating that steel gets, it actually leads to corrosion problems.

    Epoxy the bilges and wells. Then if prone to abrasion , coat liberally with polyurethane sealant (not paint) we specify building grade poly gap filling sealant, sika make one .
     
  3. D'ARTOIS
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    D'ARTOIS Senior Member

    I second that. The schooner Neorion was internally epoxycoated in such a way that even copper pipes could not have the chance to cause any damage.
     
  4. Wynand N
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    Wynand N Retired Steelboatbuilder

    Hello Gerd,

    All calculations checked and Dudley was happy with the combination. I always wondered what would have happenend if my company did not went down the plug.

    Maybe old Dix and myself were a few light years ahead in steel boatbuilding in 1990 :confused:

    BTW, here is an advertisement of the boat in question. A looker you would agree. Oops, did I mention she was frameless as well :cool:
     

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  5. D'ARTOIS
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    D'ARTOIS Senior Member

    It is not weird at all the combination of a FRP top and steel hull, perhaps you have to pay a bit more attention to the hull-superstructure connection/joint but as far as I see nothing wrong to the idea. On the contrary you gain a lot: less topweight, better insulation, easier manufacturing of rounded corners etc.

    Wynand, Oceanco is laying up their building activities in S.A. - apparently economics are turning the wrong direction?
     
  6. yago
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    yago __

    very sweet modern classic, fully battened main ansd all, I like Dix a lot, nice designs and an open mind - a pity :(
     
  7. Wynand N
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    Wynand N Retired Steelboatbuilder

    I believe the strong ZAR to US$ is the problem. A few years ago the exchange rate was about double what it is now. AAMOF, that is the same reason so many goldmines are now in dire straits and closing...........
     
    Last edited: Feb 28, 2005
  8. boby boy
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    boby boy owner of cla boat design

    go with aluminum!

    It takes over 60,000 pounds per square inch (psi) to tear apart a chunk of mild steel, and 30,000 psi to deform the same piece; to make it yield. With aluminum, around 45,000 psi will tear it apart, and around 35,000 psi will deform it. Yes, you read that correctly: size for size, aluminum has a higher yield strength. In these facts lie the extreme benefits of metal for hull construction: The "plastic range" of either metal is quite high, so the material can take a terrific beating without failure.

    Aluminum is light, strong, corrosion-resistant, non-sparking and weldable. Because aluminum is not abrasion-resistant, it can be cut with carbide tools. Aluminum is subject to electrolysis, pitting and crevice corrosion, but these liabilities can be managed as long as the installation of dissimilar metals and electrical items are correctly done.
    In terms of seakindliness, some boat shapes may be better if built in steel. Aluminum's extreme lightness can introduce a faster pitching and rolling motion in some hulls. For example, very beamy boats will exhibit a gentler roll if built in steel. Fairly narrow or light-displacement boats, which tend to have a narrower waterplane and less inherent form stability, will benefit most from aluminum construction. These are of course generalizations.
    An aluminum bare hull, built to the same strength standard, will weigh roughly 45% less than the same hull in steel. As a result, if high strength is of the highest priority, the aluminum boat can be built to the same structural weight as the steel vessel, and then be considerably stronger.
    The aluminum to build a bare hull costs just under twice as much as the mild steel to build the same design. But aluminum is faster to work with, so the savings in labor helps even the score. The labor saved can be substantial since aluminum can be cut with common carpentry tools and is welded much faster than steel.

    Another significant advantage with aluminum is that there is no need to sandblast or paint the interior. You do have to insulate an aluminum hull, but that won't ordinarily require sandblasting. Painting the exterior of an aluminum boat is unnecessary, representing another big savings.

    After you've factored in the added costs of painting steel, the margin for building an aluminum hull drops to being a very minor amount when compared to building in steel.
    In the end i would go with aluminum because of it being easier to work with (I built one steel boat and it took much longer). But the decision it is up to you.
     
  9. MikeJohns
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    MikeJohns Senior Member

    Bob

    Offhand I haven't checked your figures but..

    Remember yield is only part of the story, UTS is the ultimate safety.

    However what happens to yield strength in the weld zones !

    Structural faiure (eg bending or collapsing of frames) involves buckling and the characteristics are quite different there.

    Fatigue strength must also be factored into the equation.

    Lastly most steel producers for a relatively small increase in raw material cost can supply steel ready cleaned and pre-primed.
     
  10. D'ARTOIS
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    D'ARTOIS Senior Member

    The main source of concern is located in the comparable weakness of the welds in aluminium. The number of possible deflects are large and one of the main is porosity in the weld. Second to that is the heat transfer in the metal during the welding process. A third is the difference in potential currents between the particular sheets that are welded together. It happens when aluminium is coming from different suppliers or are having different batch numbers.

    During welding high loads of currents are moving through the welded hull and charging in some way the hull sheeting.
    There is a huge alloy vessel in the shed of Hakvoort, an American Sportsfisher in size xxxxxllllllll but that vessel needs a tremendous overhaul, including the hull, to get going again. If I compare that to a Frans Maas AC hull, a bit more than 20 years old, that is untouched, and I compare that again with the unblemished aluminium hull from WWII from Blohm & Voss, than I arrive to the conclusion that the way of building, the quality of building is crucial to the quality of the hull.
     
  11. MikeJohns
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    MikeJohns Senior Member

    Here are a couple of pictures that show stress cycle fatigue in marine alloy.

    One due to the load from the self steeering gear support strut, the other from a combination of chainplate load and flexure due to wave impact, cracking at the hardpoint at the stringer frame intersect , note the secondary failures occuring on the plate.

    Vessel was a sailing vessel to DNV standard, had circumnavigated.
     

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  12. trimarandan
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    trimarandan Junior Member

    from a shipfitters (possably biased) prespective i would choose steel. just for ease of repair and general tolarance to damage, maintanance (rust)as said before is the only downside as far as i can see. statistical strength tolerances dont meen as much as you may think unless you plan on running it into stuff, my point being... fiberglass boats make it around the world just fine, just be carefull not to crash into things, and even abnormaly thin steel is stronger than fiberglass if welded correctly. plus with aluminum even if you are in a situation where you find the right kind of aluminum for a repair in some small country (not all aluminum is the same) the next challange is finding a knowlegable welder that wont screw it up, unless you can weld and have your own equiptment you might be SOL as far as damage goes. if you can't weld, learning to arc steel is easy, anyone could do it with a little practice, but TIG is annother ball game for the inexpearinced welder. and on the other hand you can patch regular carbon steel with any other kind of steel (even stainless) with minamaly compromising weld strength ware as aluminum would fail easily not to mention the distortion and warping if its not heated properly.

    ...just my opinion, formed on a more extensive metal fabricating background than boatbuilding (i like fiberglass)
     
  13. MikeJohns
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    MikeJohns Senior Member

  14. MikeJohns
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    MikeJohns Senior Member

    There seems to be a major problem with alloy vessels corroding through from inside the watertanks. I have seen 3 of these in the last month that failed ultrasound due to massive wastage in the integral tank hull plate. It appears many builders are not coating the inside of the tanks and not aware of the problem. One vessel pro built in Denmark did not even have inspection ports. Designers and or builders beleiving the hype associated with alloys corrosion proof properties?
     

  15. Cheesecutter
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    Cheesecutter Junior Member

    You cant beat aluminium.Its easier to work,you can use your home power tools and having build both in steel and alloy the cost is about the same.Steel boats are generally built upsite down .Makes it easier to sandblast the interior.But the you have the expense of turning it over.With alloy boats you build them the right way up and there is no need to paint them.If you can weld steel with a mig then you can weld aluminium.Its no harder.Perhaps a bit quicker.But the main thing when you get old like me its a hell of a lot lighter to handle.
    Fred
     
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