Where are all the little guys?

Discussion in 'Metal Boat Building' started by Outboard Dave, Dec 12, 2011.

  1. Outboard Dave
    Joined: Nov 2011
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    Outboard Dave Junior Member

    You som gums are jest to smart for fellas like me. I have red your posts and have come to garl-dern conclusion you fellers are way to smart for me. Talk about some fancy posts. You guys have the market cornered there. It hurts my brain reading these posts.

    If your a little guy like me and love restoring old aluminum boats, post back.

    This is like being lost in a whirl wind.

    Just saying......
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Do you have a question little fella?
     
  3. Outboard Dave
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    Outboard Dave Junior Member

    Well, I thought I knew a lot about boats untill I stumbled onto this site. Then, I read, read and read some more.

    Is everyone on this site a master ship builder?

    Dont get me wrong, I will continue to read and learn but, hokie smokes, you guys are a smart crowd. I feel like a Zebra running with a herd of Quarter Horses.

    Kinda like being back in school.

    Post Script:

    Is there really such a thing as day/night cycles for aluminum stress?
     
  4. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Konichiwa Dave :)

    If you're referring to thermal stresses created by the difference between night and day, the day time having the heat of the sun on the ally, then yes. This linaer expansion can exacerbate a flaw in the structure. Aluminium, in case you're interested, the thermal conductivity is 244W/mK for the temperature range 0~100°C. Additionally, for the common commercially used wrought aluminium alloys the coefficient of expansion varies from 23.5x10-6 /K for the 4.6% Cu aluminium alloy and to 24.5x10-6 for the 4.5% Mg aluminium alloy. In plain language, it is twice that of steel.
     
  5. Outboard Dave
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    Outboard Dave Junior Member

    So, what your saying is Aluminum is stong but heating/cooling will cause a crack to "run"?

    Did I get that right?
     
  6. michael pierzga
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    michael pierzga Senior Member

    Aluminium certainly moves when heated. Ask any welder.

    On a hot day, the proximity sensor that signals DOWN position on my retractable bow thruster fires off daily because of the movement of the dark blue hull. Its also very difficult to keep paint on the sheer clamp joint...too much movement and the paint film breaks at the blue white transition
     
  7. hoytedow
    Joined: Sep 2009
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    hoytedow Carbon Based Life Form

    Here's a classic ring and ball experiment to illustrate how much metal expands thermally.
    http://www.youtube.com/watch?v=hX2Y2IDjuGI

    P.S. I'm in your category(one of the little guys). Some of these posts make my head spin too.
     
  8. Submarine Tom

    Submarine Tom Previous Member

    And some are just plain ridiculous.

    -Tom
     
  9. lewisboats
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    lewisboats Obsessed Member

    Does that work with chemical HEET 'cause I sure could use.....
     
  10. OFFSHORE GINGER
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    OFFSHORE GINGER Junior Member

    I agree 100 % . ;)
     
    2 people like this.
  11. hoytedow
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    hoytedow Carbon Based Life Form

    Especially mine. :D
     
  12. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    If the heat/cooling cycle is continuous or frequent, and the crack or flaw, is pre-existing, then yes. The constant expansion and contraction will induce a strain cycle into the structure. This also assume that some part of the structure is restrained, or held.

    If you have a sheet of aluminium with a hole/crack in the centre, and it is simply supported on its lower edges, then the whole plate will expand and then contract uniformly. If you clamp one edge, then you are introducing a "stiffness" to the plate, that prevents a uniform expansion. That shall gives rise to internal non-uniform strains, and act as the driving force for crack propagation.
     
  13. daiquiri
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    daiquiri Engineering and Design

    And to give an idea about what it means for the structure, take a case of a 1 meter (3.3 ft) long aluminum-alloy bar constrained at it's ends. If it is heated from 15° to 60° C (a not uncommon situation in areas where sun is hot during the day and the night sky is clear), you get a 45 °C (or 45 K) thermal excursion.
    It means that, if the bar were not constrained, it's length would vary during a 24-hour period by 24.5x10-6 x 45 = 1.08x10^-3 meters, or 1.08 mm.

    But the bar is constrained, so there are counteracting loads on it's ends, and their intensity is such to keep the bar's length unchanged.
    Since elongation (strain) and stress are related by the equation stress = E x strain (where E is the Young's modulus, equal to 70 GPa for aluminum), it follows that the (thermally induced) stress due to constraints will be as high as 70x10^9 x 1.08x10^-3 = 75 MPa.

    If the bar was made of 5086-O alloy, which has a yield strength equal to 130 MPa, it would be loaded to 58% of it's yield stress just due to this thermal expansion/contraction, which adds to eventual other mechanical loads acting on the bar.

    Though a case of a bar rigidly restrained on both ends is not so common, it gives a clear idea about how fatigue carves it's way after a sufficiently big number of comparable cyclical thermal+mechanical loads on the structure during the years of service.

    Cheers
     
  14. Poida
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    Poida Senior Member

    And now comes the rediculous post.

    It is common for people with aluminium dinghys to turn them upside down on the beach.

    Because the sun is heating up the shell of the craft and the bracing inside the boat is in the shade (because it is inside the boat) wouldn't expand to the same degree.

    Does this make the problem worse?

    Poida
     

  15. hoytedow
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    hoytedow Carbon Based Life Form

    It shouldn't be too problematic because metal conducts heat as well as it conducts electricity. Inside bracing would increase temp and expand just due to contact with hot outer surface.
     
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