Scallop & collar plate

Discussion in 'Boat Design' started by Vinassman, Jun 1, 2012.

  1. Vinassman
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    Vinassman Junior Member

    Hi everybody,

    I would like to be sure about this so I ask your opinions.

    What is the purpose of collar plates? Is it to break the unsupported length of a stiffener? If yes, then why we can see some design with only some stiffeners fitted with collar plate and some not? Looks like the load should be the same so unsupported length as well.

    Thanks in advance.

    Cheers, VM
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    A collar plate allows a structure member, such as a longitudinal stringer to be continuous through watertight boundaries. Continuous members when they pass through a web of a frame often has a cutout larger than required, but not always. A typical example of a FB running through a frame is shown here:

    typ. collar.jpg

    So there is now another small bit of plate which lays over the cutout hole created (the small mouse holes in the frame) and extends past it by some 15mm or so. The shape of the collar plate is generally in 2 parts so it can be welded and fitted easily .

    Thus the collar is a small watertight boundary from the FB to the web of the frame but allows the long.t to be continuous. Which is important in monohulls for example.
     
  3. Vinassman
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    Vinassman Junior Member

    Dear Ad Hoc,

    That is very clear indeed. Thanks a lot for the explanation!

    However, if I have a look at this snapshot (detail of the floor in a double-bottom arrangement), the layout of the collar does not make any sense to me, if I think about it in view of watertightness...

    This is why sometimes I am a bit confused regarding the purpose of collar plate... Or I am missing something?

    [​IMG]
     
  4. Ad Hoc
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    Ad Hoc Naval Architect

    Those are not collars.

    They are called "fish plates" or "shear tabs". The purpose of them is to increase the available shear area (for load transfer) in locations of high load. Those angles look like what 150mm deep?...thus there is a cutout of roughly 160mm in the web it passes through. If you add a fish plate of what looks like roughly 100mm between the web of the angle to the web of the frame, the amount of cutout has been reduced from 160mm locally down to just 60mm (160-100).

    So on that 10mm frame, you have only lost 60x10mm = 600mm^2 of shear area. Without the fish plate you have lost 160x10mm = 1600mm^2. It can be a significant difference.

    Where as on the angle, the amount of weld would be roughly 130mm (less the mouse holes) to the 10mm frame. This joint would be welded both sides of the angle to the frame. So the available shear area is 130x10 = 1300mm^2. But this is the only shear path available. If you then add the 100mm width fish plate, you have increase the amount of shear load available from the web of the angle to the frame by 100x10mm. What this does is reduce the stress concentration (since the shear stress is reduced by the increase in area) as well as adds more load paths which is important.
     
  5. Vinassman
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    Vinassman Junior Member

    Well Ad Hoc, thanks a lot for the clear & quick explanation! Now I fully understand.

    It is funny, here in Indonesia / Singapore, I have never seen anybody making a difference between fish plate / shear tab / collar plate. This is where my confusion comes from.
     
  6. Ad Hoc
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    Ad Hoc Naval Architect

    Perhaps because they are unaware of the actual functions of each?

    Summary:

    Collar is to keep a watertight boundary watertight, whilst allowing the long.t to be continuous, through a frame web (or Tank plate)

    A Fish plate/Shear tab, is the gain back lost shear area in the frame web by the cutout for the long.t, locally with the addition of the small plate connecting the 2 togther.

    The two are different. But you can use a collar as a shear tab, but not the other way around :)
     

  7. Vinassman
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    Vinassman Junior Member

    Yes indeed, this is why I asked those "stupid" questions so that now I know the functions of each, thanks to you.

    Cheers. VM
     
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