Replacing a heavy solid AL plate OB mounting plate with something lighter?

Discussion in 'Materials' started by Judy Blumhorst, Jan 7, 2017.

  1. Judy Blumhorst
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    Judy Blumhorst Junior Member

    Hello,

    I'm a sailmaker, not a boat builder, so I'm coming here for advice.

    I have a an early 1992 Corsair F24 Mk1 with a heavy out board motor mounting plate. I want to replace it. The plate is mounted horizontally, with one end bolted to the port side on transom, with the outboard end extending unsupported and unconstrained. The outboard is clamped onto the unsupported area.

    Here'a a drawing of the existing plate. It's aluminum, but I don't which alloy it is. It is 20 mm thick. The new panel can be thinner. Fortunately, the plate has not buckled over many years of service so I think it's fair to say it's strong for the applicaton. Unfortunately, it weighs about 5.5 kg. And it's ugly and clunky looking.

    I'd like to replace it with something lighter. The motor mount will be used with an outboard weighing under 60 pounds and up to 6 hp(US). I can fabricate a foam or wood cored panel of glass myself. But I don't know what the laminating schedule should be.

    Thank you very much for any tech support on this question. I am happy to pay a small consulting fee in return for the advice.

    Click here for a drawing: http://www.blumhorst.com/hydesails/F0241-motor-mount-plate-small.jpg

    Click here for picture of a sister ship with a similar mounting plate: http://blumhorst.com/hydesails/F-241outboard-example.jpg

    Judy Blumhorst
    http://www.hydesailsdirect.copm
    judy@hydesailsdirect.com


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    Last edited: Jan 8, 2017
  2. mydauphin
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    mydauphin Senior Member

    I don't see the picture/drawing. Are you sure you mean 20 mm? That is pretty thick. But I would replace it with a thinner aluminum plate, actual size depends on the boat and motor, and conditions.
     
  3. Judy Blumhorst
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    Judy Blumhorst Junior Member

    Last edited: Jan 7, 2017
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I can't see those images for some reason, all I can think of as a substitute would be a hollow extrusion, you might peruse a suppliers catalogue of available extrusions to see if they have a stock one to suit. Still, 5.5 kg is pretty light.
     
  5. Judy Blumhorst
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    Judy Blumhorst Junior Member

    I think I've finally fixed the image links.
    Please let me know if you still can't see them.

    Judy
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    Not working here. No matter.
     
  7. Judy Blumhorst
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    Judy Blumhorst Junior Member


    Yes, it really is 20 mm thick! At some point in the past, a prior owner replaced the original mount with a massive aluminum plate.

    The plate has been strong enough to resist the stress and strain of high winds and confused sea states on San Francisco Bay.

    I'm wondering what I can replace it with that's lighter weight and have adequate resistance to torsion, flex and buckling.
     
    Last edited: Jan 9, 2017
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    An extrusion seems your obvious answer, something like an alloy painter's/scaffold plank in design. a rectangular hollow extrusions with internal webbing. You might be lucky and get an off-cut.
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    5.5kg is nowt when compared to the longevity and safety the item has thus given you so far. And you wish to shave off what 1-2kgs?..what will you do with the extra 1-2kgs...order more burgers/fries to be delivered?....seriously. If it aint broke...don't fix it.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    If the surface area on 0.078 sq m is right and the thickness 20mm, it weighs about 4.2 kg. Very little.
     
  11. Judy Blumhorst
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    Judy Blumhorst Junior Member

    It ain't broke..... IT'S UGLY!!!

    It is old, decrepit, and pitted. It's an ugly shape that Looks ungainly compared with the lines of the hull. i want to replace it with something that looks better and weighs less.

    I have the skill to fabricate a laminated panel, all the materials, but not the engineering knowledge.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    No-one is going to advise you to make a laminated panel to replace it, imo, I get the impression you are just hanging out for someone to agree with your idea.
     
  13. Judy Blumhorst
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    Judy Blumhorst Junior Member

    Mr Efficiency,

    Would you please explain why you think a composite motor mount is such a bad idea that you advise against it?

    Respectfully
    Judy
     
  14. PAR
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    PAR Yacht Designer/Builder

    A replacement for a 12 pound mount, really? How about some elbow grease, cleaning and polish. Too bad to clean up, some filler and paint? Shaving a few pounds off this isn't going to change anything, so you're addressing the aesthetics.
     

  15. Judy Blumhorst
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    Judy Blumhorst Junior Member

    Yes, I do realize that this is about aesthetics. Aesthetics matter to me. I don't want a slab of painted Al that will chip and corrode.

    I am willing to pay someone with with appropriate credentials to specify a composite lamination schedule Please email me at judy@hydesailsdirect.com i am willing to pay a reasonable fee.
     
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