Aluminum vs Carbon/Nomex!?

Discussion in 'Metal Boat Building' started by TealTiger, Aug 31, 2014.

  1. TealTiger
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    TealTiger Junior Member

    I appreciate your input rxcomposite; thanks.
     
  2. TealTiger
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    TealTiger Junior Member

    I appreciate your input Mr Efficiency; thanks.
    I figured some might not know of Gunboat, hence the link.
     
  3. TealTiger
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    TealTiger Junior Member

    If I understand your concern(s) PAR;
    I promise I won’t ask the same question twice,
    and assume everyone understands participations is voluntary.
    Thanks.
     
    Last edited: Sep 2, 2014
  4. TealTiger
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    TealTiger Junior Member

    When Gerr said ‘Derecktor’s extreme lightweight method’ “gives hull weights comparable to high-tech composites”,
    I assumed that he meant per weight otherwise it seems like a pointless ‘non-statement’.
    Like saying an aluminum foil box is lighter than a steel one.
    Do I understand, however, some to be saying that:
    1) Only compared to extreme racers, of “high-tech composites”, with negligible reserve strength that Derecktor’s method is almost as good (~ 85%).
    2) Derecktor’s and the Strongall (http://www.meta-chantier-naval.fr/we...-du-strongall/), methods have the exact same strength per weight.
    3) If greater reserve strength is desired, plating *must* be thicker and only because of that, frame spacing can be greater.
    4) Finally, Derecktor’s method doesn’t have enough reserve strength for cruising.
    Even though the overall structure is up to the extraordinary strains of offshore racing.
    That the hull would breach (rip or crush) upon almost any ‘point load’ like grounding, deadheads, or docks.
    Thank you.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    That is EXACTLY the point. Only those that are not familiar with boats, structures and weights will fall for the sales line. It sounds "WOW"...but as noted several times now (may be the endless repetition is working) there is nothing mystical or magical with it at all, it is very simple engineering. Span x Length with an applied load, that is it.!
     
  6. TealTiger
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    TealTiger Junior Member

    I appreciate your input Ad Hoc; thanks.
     
  7. TealTiger
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    TealTiger Junior Member

    Does anyone know if the reverse is true?
    If a Gunboat (www.gunboat.com), of 100% carbon/Nomex, built with sufficient strength for cruising,
    would only weigh 15% less than
    a 'best practices', 100% aluminum boat, built with the same strength?
    I doubt Peter Johnstone, spent all that money on carbon/Nomex, for only 15% less weight.
    I'm not sure, or sure why; or has carbon/Nomex just become that 'affordable'?
    Thanks.
     
  8. upchurchmr
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    upchurchmr Senior Member

    You first have to know just how close the original design/ material represents the lightest design.
    A "better" design in a lighter material might show more than 15%. Also as discussed in another thread, one design might have a smaller excess margin. This shaves weight fast but the consequence will not be known until you have lots of weather conditions and miles under the keel.

    People are always improving designs and material use - unless the claims are mostly just advertizing.
    Hard to tell without lots of engineering experience with the vehicle you are actually talking about. That means that me talking about aircraft is not directly applicable to boats. It can only give general guidelines.
     
  9. TealTiger
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    TealTiger Junior Member

    I appreciate your input upchurchmr; thanks.
    I suspect we're miscommunicating though.
    Put a different way:
    If best practice carbon and aluminum
    at the same minimal spec's (like a wing, or racer)
    are with 15% weight of each other;
    then would best practice carbon and aluminum,
    at the same but heavier spec's (like an ocean cruiser)
    also be within 15% weight of each other?
    Or are those materials know to 'scale' differently?
    Thanks.
     
    Last edited: Sep 9, 2014
  10. upchurchmr
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    upchurchmr Senior Member

    I can't give you an honest answer.

    My suspiscion is that the technology used in an aircraft is significantly more expensive than that used (typically) in an ocean cruiser.

    A very minor change in quality of the parts could provide a large change in strength and stiffness for an ocean cruiser. That change in manufacture (%resin) would probably be more acceptable if you were making a graphite part - since the raw material is automatically more expensive.

    I wouldn't worry about scaling for equal quality composite and aluminum designs, but do worry about amount of effort expended in the design and life prediction.

    Sorry I can't give you a straight answer.
     
  11. TealTiger
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    TealTiger Junior Member

    No worries upchurchmr; I appreciate your input; thanks.
     
  12. Ad Hoc
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    Ad Hoc Naval Architect

    That's the cart before the horse!

    The loads imposed on a vessel scale far greater than you image.
    As an example, if you have an Lwl 8m vessel that is say 8 tonnes, that then becomes 10m it is now 16 tonne. So a 25% increase in length, which is scaled, doubles the displacement. If you wish to keep the response (structurally) of the 10m the same as the 8m, clearly the effect on the structure is significant.

    It is not (again) a simple apples for apples comparison.

    I'm really struggling to understand what is your motivation behind such Qs, other than trying to elicit a one liner that satisfies a preconception that you may have heard or read or would like?? Because that's how the Qs are worded, in a very binary fashion.

    Every boat is different, and must be treated as such. If the boat works, it has satisfied its SOR. Will it satisfy a different SOR...who knows, and who cares, so long as it satisfies the one it is designed for!
     
  13. TealTiger
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    TealTiger Junior Member

    I appreciate your simple request for clarification Ad Hoc; thanks.
    I'm sorry, but I've brought my question down to simple repetitive binaries,
    because others’ erroneous assumptions are as frustrating for me,
    as my lack of clarity seems to be for others.
    I understand that cost/weight/load go up approx. exponentially with length; thanks.
    Background to question:
    I like aluminum.
    I understand that aluminum, at typical cursing strengths, but significantly below ~40’, isn't usually as light as typical polyester construction, but it does start to come into it’s own over ~40’.
    My root question is:
    In larger cats (>~40'), with typical cruising cat strength,
    how close can ‘best practice’ aluminum construction, come to best practice FRP (probably something like carbon/Nomex) built to the same size and strength?
    As an example, I’ve been using the atypical example of a 60’ Gunboat cruising cat.
    It’s well over 40’.
    It (has to) have typical cruising cat strength.
    But it’s atypical in using extreme lightweight materials and methods.
    If you built ‘the same’ cat in ‘best practice’ aluminum, does anyone know how they’d compare?
    FYI The Gunboat 60 is “Lightship 15,500 kg / 34,172 lbs?
    I’m not asking about dissimilar strengths.
    I’m not asking about cost at all. If any readers’ of this post don’t know the answer, I fine without a response.
    Thank you.
     
    Last edited: Sep 9, 2014
  14. Ad Hoc
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    Ad Hoc Naval Architect

    I have no idea what that means?

    Again, your're comparing apples with oranges again ie. different SORs, as such you wont get a simple yes/no response.

    Very very simply... a boat, any boat can have the weight broken down into 6 categories (there are more). One of which is the structure and another the outfit.

    For a given length and draft, the design waterline will yield a displacement. Thus will the "design" meet the draft in other words, when the boat is finished and sits in the water is the draft the same as the design waterline (DWL) and hence the displacement at said water line? usually they are, or should be, that is called design!

    So if you now change the material, the structure weight may or may not change. Why??..well after you have run the numbers, does the modified structure boat still sit on the design waterline..i.e same displacement?

    If the material is "lighter", does it mean the structure will be lighter...not always. But, for sake of simplicity let us say it is lighter. So does that mean the boat is no no longer on the same draft....again for sake of simplicity, let's say yes, the draft is less.

    So with a lesser draft the displacement is less. So will the boat still perform as designed (on the previous DWL) at the lesser DWL..usually no. There is a change in the performance...sometimes good sometimes bad. The designer will inform the client of the effects of the changes. Thus the client will either accept or not accept the changes.

    So that is one route..it is now different.

    Another is that because the draft is less, the hydrodynamic loads imposed onto the vessel are also less. This means the amount of structure required to satisfy the same response has changed, i.e. less structure required.

    So in this case, already again as above, you can see if the loads have changed, the structure changes, as such it is not a simple apples for apples comparison.

    So what about if you maintain the same draft...i.e. same displacement even though the structure weight is less, again, as a simple example, so the performance is the same. The client can now add more outfit to the same boat.

    Well a lighter material usually means its 'E' is less too. In other words it is more flexible. So if it is more flexible, you need more structure to ensure the stiffness of the structure is not compromised by too much flexing, either by fatigue or simply having hulls in a cat bouncing up and down in a rough sea when they should be near stationary to each other!!. As a crude example, image if the hulls are made from rubber...they would be flexing independently!!

    So you now make the lighter structure stiffer...this adds weight. The weight ends up roughly the same as the previous material or in some cases can be greater if the amount of deflection is too much.

    So, in a very very rough and simplistic review, you can see it is not a simple case of comparing apples and apples of 2 boats being the same but with different materials used for construction. It affects far too many things. Design is greater than the sum of its individual parts..i.e focus on one thing..and you loose the bigger picture and the design is seriously compromised!
     

  15. TealTiger
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    TealTiger Junior Member

    Yes, aluminum apples to carbon/Nomex oranges.
    A second SOR? From where? I think you’re teasing :)
    Exact same LWL, Beam, & Draft? Now I know you’re teasing :)
    If someone asked for a bulkhead to be changed from ply to composite, I suppose the whole boat really should be redone ;)
    Anyway, thanks for your input Ad Hoc.
     
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