Aluminum Hull - Advice Needed

Discussion in 'Boatbuilding' started by We're Here, Jul 8, 2009.

  1. mydauphin
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    mydauphin Senior Member

    If an engineer designs a truss out of 1/2 steel to hold a certain weight and force, and a third of it is removed, it can not handle the same weight and forces. Aluminum boats like an airplane or soda can are held together at all sides. Remove one side, replace it with something else and it unknown what the results are.

    I work next to a marine salvage yard, and I see what they do with burnt boats. They get boats with severe burnt fiberglass, reglass and gelcoat over it and sell it. The fiberglass is all like sawdust at this point and lost most of the structural quality. Don't assume they did a good job on repairing boat, assume they did a bad job and try to prove otherwise.
  2. Guest62110524

    Guest62110524 Previous Member

    MSDAUPHIN thats good advice
    I am not a metallurgist, but I have seen and felt hot metal:))
    there is only one way really, IMO to tell if the metal has been so hot it has lost temper, and that is to take a scriber or some such thing and try scratch it, , hard, if it peels easily and soggily it may have
    Unless one was used to how the metal saws, works, then this would be of little use Example, when I saw something either in the bench, or bandsaw, i can tell by the chip what hardness it is, soft metal sticks to the teeth and so onSomeone said the bottom was immersed, so it would not get hot
    I guess thsi post is of little use, but it at least describes how to tell a soft or hard alloy
    Sometimes(quite often actually) the mill gets it wrong, particulary on extrusions
    You order a mill run, for frames, stringers and the meatal come brittle and when you try to form it breakes This is annoying paricularly when they have issued a mill cert, saying it is ie 6061 t6
    Have learnt to try bending a pice now, before working it into a build
    The big Euro mills always get the plate right, Perchenay , Kobi in Japan But even then I always get a mill cert and I mark on the plan where I have put each and every plate
    if a boat has been locally very hot, like hot enough to worry about ,you would also expect lots of distortion
  3. wardd
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    wardd Senior Member

    the scriber may not tell anything

    there can be a difference between hardness and strength of a metel
  4. Ad Hoc
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    Ad Hoc Naval Architect

    scribing is very similar to a "standard" hardness test.

    Hardness is directly proportional to the yield strength of the material.
  5. wardd
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    wardd Senior Member

    but not to brittleness

    and how deep is that hardness?

    in lots of situations hardness may be only a few thousands deep

    and we are talking about a metal that may be different than it was when first used, ie subject to being heated
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    Be simple:

    - Stay away from a yacht if it has a history like fire, hard grounding, hit by another boat etc... Good boats are like happy people, nothing to tell or explain. Would you buy a car rebuilt after being wrecked in an accident? what's the need for taking any risk?

    - Mixing materials is often ill fated. So a sandwich polyester on a aluminum hull is highly suspicious, specially after a fire. It has been a fashion of plywood decks on steel yachts: not good.

    - I bought some boats (never for myself...) and never a broker asked me a deposit to permit a survey and I never gave a buck before the buy, except a proof of funds. The expense of a surveyor shows by itself that you are interested. For sea trials I always managed a one day charter refundable in case of buy and I always asked the proof that the boat is insured... If the broker does not accept your reasonable conditions, forget it; you're the customer, thus the king. There are plenty of boats in sale specially in these hard times.

    Do not focus on one boat.
  7. apex1

    apex1 Guest

    Will you grant me the right to choose Paul?;) (and I was´nt suggesting, I said wild guess!)

    I still cannot see any sense in a GRP deck on a metal hull, and much less on a Alum. hull. If there was any, it was at least not related with, what I call, a good executed repair.

  8. Ad Hoc
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    Ad Hoc Naval Architect


    You said "..there can be a difference between hardness and strength of a metel.."

    No, hardness is directly proportional to its yield strength.

    "...but not to brittleness.."

    Didn't say it was!
  9. Guest62110524

    Guest62110524 Previous Member

    there are a lot of very good yachts with timber decks and or houses My first ally yacht had a timber house
    the advantages are many,
    First you get a strong hull, but then you save a huge amount of time on finishing , if you use timber or glass
    later I,ll dig out the pic
  10. wardd
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    wardd Senior Member

    that is if the hardness is all the way through

    all im saying is scratching the surface only says something about the surface
  11. Ad Hoc
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    Ad Hoc Naval Architect


  12. apex1

    apex1 Guest

    the deck was in question here, not the wheelhouse! And there was a suspicion of fire damage.
    Wooden decks on steel boats were quite common during the first half of the last century (and ALL had problems), but that again is not the topic here. A GRP deck on a Al newbuilt would be at least a unnecessary complication and additional cost (and I doubt there are any). On a repair I call it crap, and I will not leave this point of view (maybe except the repair was done at the outer reef of Motuarohia with nothing else at hand).

  13. We're Here
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    We're Here Junior Member

    The broker and the owner have been quite difficult to coordinate with but it appears that tomorrow's the day. Thanks to to generous advice given here, I feel well-armed. BTW, I have come up with yet another theory regards the non-plate work deck that I'll share when I get back but at the moment I need some serious shut-eye as this is a solo drive - five hours each way plus the time spent on the boat (I'm an old fart who just can't put on the miles anymore). I will take photos but apparently they can't be posted here so specific shots should be available as an e-mail attachment should you be interested.
  14. Gilbert
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    Gilbert Senior Member

    Just a footnote about aluminum combined with fiberglass construction. Ray Wadsworth's Kodiak Marine Construction built a herring seiner called ORDER OF MAGNITUDE that from the keel to the chine was half inch aluminun plate and from the chine up was fiberglass. It also had a 3,000 horsepower gas turbine and a diesel engine coupled to a water jet, an either or propulusion mode not both engines at the same time. Google it if you are interested.

  15. Guest62110524

    Guest62110524 Previous Member

    very interesting, think outside the box
    I am doing a boat now, I considered heavy ply houses, but at the end of the day I stuck with al al keel to truck
    thing is with al al, your shape is ltd unless you have the skills to work in shape I should add, that these skills are not black magic at all .I could teach any reasonably competant person to wheel plates in a day Knowing where to stretch and form takes longer:))
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