extruded rudders and centreboards

Discussion in 'Boat Design' started by grob, Jun 10, 2009.

  1. grob
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    grob www.windknife.com

    I am having a die made to extrude aluminium foil shapes, the idea is that a common nose extrusion can be made to produce a variety of sizes of foils between 15mm-30mm thick see attached picture.

    This is a similar concept to the extruded aluminium rudders used on the 29er dinghy.

    My question is would people want to have the rudder filled with foam and if so can anyone recommend a foam to fill the rudder with.

    Gareth
    [​IMG]
     
  2. daiquiri
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    daiquiri Engineering and Design

    Aluminum is very sensible to pitting and crevice corrosion, so it is of vital importance that you impede the water penetration under the foam surface, where it can stagnate in the form of moisture.
    Therefore, a closed-cell foam is a must, and it has to adhere perfectly to the walls.

    A good choice might be an elastomeric nitrile foam. It is used for Armaflex insulation sheets, has 100% closed-cell structure which makes it very resistant to water and moisture penetration. Another one might be a neoprene, another elastomeric foam similar to Armaflex.
    They both have the same disadvantage, though... Their formation requires a thermal curing, so if you don't have tools for that, you'll have to buy them in form of sheets. Then you would need to cut them into a correct form and fill the rudder, sealing carefuly any gap between the foam and the aluminum walls, before welding (or glueing?) the two aluminum pieces together.

    An alternative could be a polyurethane foam, which is also closed-cell, though 90% only. In contact with other surfaces (like aluminum walls) it forms a silicone-like layer which stops the spreading of the moisture along the outer walls.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Concur with daiquiri above.
    Moisture is a killer in these situations. Also you need to be careful what grade of alloy you are using too, since some are not very marine friendly!
     
  4. grob
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    grob www.windknife.com

    Thanks for the replies, this is one of the reasons I wonder if it is better to have no foam at all. The aluminium will be 6082-t6 anodised.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    6082-T6 is ideal...no need to anodise either.

    One question..where are you putting the stock and how?
     
  6. grob
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    grob www.windknife.com

    I am still undecided as to whether to use a propietary rudder stock or make my own.

    Gareth
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    well, best thing would be to ascertain the max torque for the rudder, then design the stock to suit, this would then be applicable across the board. Ok, heaver and 'over engineered' for some applications..but mass produced!

    But where to put it within the section and how to attach it...have you decided that yet too?
     
  8. grob
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    grob www.windknife.com

    From your description we may mean different things when talking about rudder stock, what do you mean by rudder stock?

    Gareth
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    the solid bar, or hollow (depending upon the calculated stress values), that extends from the tillar/yoke inside the boat, through the hull, via a bearing, to the rudder and down the inside the rudder.
    As the rudder turns, the force on the rudder is trying to rotate it and it is resisted from rotating by the stock, the solid bar.
     
  10. grob
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    grob www.windknife.com

    This is designed mostly for dinghies, rudder stocks as you described them are not usually used on these boats, the rudders are usually sandwiched between cheeks in a rudder block.

    Gareth
     
  11. HJS
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    HJS Member

    Keep the trailing edge sharp or flat
    absolutely not rounded
    otherwise you can get vibrations at some speeds
    attachment from 1979

    good luck

    js
     

    Attached Files:

  12. ut038
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    ut038 New Member

    Gareth,

    What became of this good idea of yours? I need to find foil extrusions to design to.

    Simon
     
  13. Steve W
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    Steve W Senior Member

    I would also like to know if this idea transpired.

    Steve.
     
  14. grob
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    grob www.windknife.com

    Hi Guys,

    Sorry about not replying to this thread I am not getting any subscription notifications, and thankyou to Simon for the prompt.

    Yes this idea did make it into production I have a website called www.windknife.com with some details of the product.

    I would be happy to answer any questions either privately or via this thread.

    Regards

    Gareth
    gareth@www.windknife.com
    [​IMG]
     

  15. grob
    Joined: Oct 2002
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    Location: Cotswolds Waterpark, UK

    grob www.windknife.com

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