Carbon Fiber compression plate question

Discussion in 'Fiberglass and Composite Boat Building' started by jorgepease, Jul 8, 2015.

  1. jorgepease
    Joined: Feb 2012
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    jorgepease Senior Member

    Hi, I am making a compression plate which clamps to the anti ventilation plate of an outboard motor.

    Any pointers appreciated including what cloth (weave, uni, etc and oz) to achieve a very rigid part but keep thickness under 1/4 inch.

    I will be infusing with expoxy.

    This is what the plate looks like with the notch cut out but I will infuse it solid.

    Thanks

    [​IMG]
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Why do you call it a "compression" plate ?
     
  3. Jim Caldwell
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    Jim Caldwell Senior Member

    I would make out of something cheap, ALUM., first see if it works.
     
  4. jorgepease
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    jorgepease Senior Member

    It compresses water around the prop on motors mounted on jack plates. When I am running full up the skeg of the motor is only a couple inches under bottom of boat. I didn't invent the name, it's been called other things

    They work, I had a fiberglass one I bought for $300 on my last motor. It worked really well to keep prop from blowing out on turns and even helped steering around dock. I could build it out of glass but it was a good 3/8 thick and I would like to keep it thinner

    But they are built a stock size and I wanted one customized to my prop. Haven't found a good aluminum shop to work with yet down here in Miami, too expensive, too lazy, too booked up etc etc ... anyway the mold is already built and ready to go.
     
  5. SukiSolo
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    SukiSolo Senior Member

    It is however only 4 straight bends and a bit of routing if in sheet alloy!.

    Worth getting only a half hard grade or even less temper though, maybe local annealing. If you do go the carbon route, glass is a LOT cheaper, you may still require 10mm (3/8") thickness just a lighter core. The section size still counts more than the e modulus ......
     
  6. jorgepease
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    jorgepease Senior Member

    Im not too worried about the cost as it's a small part but I'l just make it out of glass if I can't get the right stiffness out of 1/4 inch solid carbon fiber, no core.

    The one I bought for my last motor was solid glass.
     
  7. gonzo
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    gonzo Senior Member

    Water is not compressible. Do you mean it creates dynamic lift?
     
  8. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Water is incompressible but can be pressurized. With increasing pressure, the vapor pressure increases, and the air bubbles take longer to occur.
     
  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    It's the typical anti-ventilation plate seen on a lot of outboards. It helps keep the water from sucking out when shallow, particularly in turns when jacked. they're also effective without the jack, permitting the top of the wheel arc to bite harder water at speed. If shaped properly, they can also help the boat get up on plane faster too.

    The laminate doesn't need to be very thick, if good sections are used. A few layers of 2x2 twill will do, though the bends seem pretty harsh, maybe consider a curve instead for better flow properties. Also infusion is a lot of bother for a single piece like this. I'd just bag it with room temperature epoxy.
     
  10. jorgepease
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    jorgepease Senior Member

    Yes I think that is the effect that is going on here.

    I shot videos with and without the plate while running. When your planed out the plate is about 5" above the tunnel and doesn't look like it's doing anything but I guess it causes the water that bounces off lower unit and prop to bounce back toward prop. This must cause back pressure in the flow and give the prop more water to bite into.

    The props these micro drafting boats use are heavy cupped, the plate just needs to help them a bit.

    I took mine (by accident lol) over a shelf of rock only 2" deep. It polished the tips of my strakes but didn't put a scratch on the skeg. The boat just seemed to flatten out and lift, was like riding on air. I was able to run that way for a hundred yards or so before I could maneuver off the shelf and take another breath ))

    I usually avoid any water that looks less than 6 inches in case of a protruding obstacle but I do stick to the shallows to help with the pounding on rough days.

    Without the plate, the boat performs noticeably different - prop ventilates more, not able to slow plane under 10 mph, steering is not as good etc ...

    Found some uni carbon that I am thinking would work well for this simple part. I am thinking I could just orient it in various layers 0-90, 45-45 to get my thickness up but not finding any info comparing stiffness based on section size.

    If glass is going to work just as well and only be 1/8 or so thicker, Im thinking I might just go that route. The aluminum option is not shelved, I just need to find a decent metal shop and that's not happening here in Miami.
     
  11. jorgepease
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    jorgepease Senior Member

    oops, I posted same time. So just a couple layers of 2 x 2 twill, what weight?
     
  12. PAR
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    PAR Yacht Designer/Builder

    It sounds like you just need something, not especially a well shaped or foil section type of thing, so go the cheaper route with conventional 'glass. Use biax (12 or 18 ounce), with a light mat outer layer, so it's easy to fair and has a tough, resin rich outer surface.

    I ran some numbers from your previous requirest and a single layer of 2x2 (3k) twill weave is .012" thick. A single layer of 5HS (6k) weave carbon is about .024". So you can see getting a 1/4" will take some layers any likely way over the strength and stiffness you need for such a part. Of course this says nothing about the cost of carbon.

    There are some fabric calculators available online, such as (> http://www.fibreglast.com/fibreglast_materials_calculator <)

    Then there's this:

    [​IMG]
     
  13. jorgepease
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    jorgepease Senior Member

    Yes it's low tech for sure

    I think I'm leaning toward glass for the prototype and if it works like I want then I can start playing with Carbon layups.
     
  14. Deering
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    Deering Senior Member

    This part seems to be a natural for aluminum. Given the bent edges, it should have considerable structural stiffness. If you have some discretion on the dimension for the vertical legs, increasing them would increase overall stiffness, and perhaps increase the pressure zone around the prop.

    I'd start with 1/8" plate - cheap & easy to bend - and see how it works. If too floppy, bump it up to 5/32 or 3/16. If those aren't stiff enough, then go to the composites. Always start with the simplest solution...
     

  15. jorgepease
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    jorgepease Senior Member

    I agree on the aluminum but the easiest solution for me to test or for a one off is composite because I have the tools.

    If I was going to be fabricating these, I think aluminum is the way to go, fast and cheaper and probably better suited.

    Thanks
     
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