Rudder layup schedule

Discussion in 'Fiberglass and Composite Boat Building' started by Ugo, Sep 8, 2014.

  1. Ugo
    Joined: Dec 2012
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    Ugo Junior Member

    Dear All,
    I'm an amateur boatbuilder - my last completed project was a 26' radius chine sailboat.

    Now I'm replacing the old rudder with a new one:
    spade, semi-compensated, no skeg.
    span 1,27m, root chord 0,35m, max thickness about 52mm.

    The core has been CNC machined from corecell 80kg/m3 foam.
    The design skin thickness is 3,5mm.
    I'm going for epoxy resin + E-glass reinforcement, hand layup, vacuum-bagging.

    I need some advice on skin lamination layup: MAT, rowing, UNI, bi-directional, or a combination of two or more of these?
    sheets orientation?
    Any hint on outer layer (I've read it's better to use a ligth mat as a final layer)?
    I know that wrapping the plyes around the core is not a good idea, then which is the right layup strategy?

    many thanks in advance for any advice!
    all the best,
    Ugo
     
  2. SukiSolo
    Joined: Dec 2012
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    SukiSolo Senior Member

    One good reason for a light surface cloth is so the fabric does not imprint too heavily later on. On this size project, something like a 200 gsm weight glass would be a good surface material woven/biax vacuum bagged. It will also allow a light fairing compound or filler to be squeegeed over it then faired and painted.

    The 52m thickness should ensure enough stiffness as long as the tiller or steering arm is well attached. You can wrap glass around a foil if you drape it over the leading edge. Best done with the foil held horizontally with the trailing edge pointing down. Hard to vacuum in this orientation (not impossible if you trim the trailing edge) but I have done quite a few like this mostly hand layup with a roller. Partly dependent on leading edge shape, and much easier if this is straight or only gently curved, when the cloth will accomodate the distortion easily. A severely curved or segmented form may make this option difficult. I suspect this is where the 'Do not wrap around core' advice comes from, it can depend. There is another thread on building a larger rudder with vacuum on this Forum, look for that too. Vacuum can 'pull' the cloth into wrinkles if you are not careful.

    Personally I don't use any CSM (mat) on foils at all. Always use rovings and cloths, much more consistent and even. In effect you are aiming at skinning the foam to mimic a timber cored blank, so the skin needs to replicate the strength it would have if it were a longditudinal laminated timber blade. This should give you good clues as to a layup guide ie unidirectional in a vertical (top to bottom) plane, but with crossing layups, plain or biax either side. The skin is taking the stress not the core so engineer with this in mind.

    Perhaps other can input on the exact layup, although I believe it will be more than sufficient in skin thickness. On the upper end of what I tend to work on, so forgive my limited input. You should pay attention to where the rudder fittings go. If you are bolting through, which seems the most likely, ensure there is a non compressible core for each bolt ie solid epoxy microfibre mix that will not allow water ingress and also spread the load correctly. Check the buoyancy too with a quick calculation, it may be a floater, neutral, or sinker, which may have some bearing on how to attach the rudder.

    A sketch would be useful for others to comment.
     
    1 person likes this.
  3. PAR
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    PAR Yacht Designer/Builder

    I'd agree you really don't want much CSM in this laminate. I do use a very thin layer (20 to 40 GSM) as the outer skin for two reasons; it helps prevent print through and offers a resin rich surface for impact and abrasion resistance. Other then this, you don't need or want it, unless using polyester resin.

    As to wrapping the edges, I do, but it's not entirely necessary and depends on the way you're laying it up. If doing it by hand (horizontal foil blank), just cut the fabric several cm over size, so the weight of the resin on this excess fabric, will bend the edges around by themselves (gravity).
     
  4. Ugo
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    Ugo Junior Member

    many thanks for the suggestions.
    I've attached a sketch of rudder blade for futher discussion:
    View attachment rudder_sketch.pdf
    looking forward for any further comment!
    all the best,
    Ugo
     
  5. wet feet
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    wet feet Senior Member

    Is it correct to understand that the glass will be laid up over the foam core and no mould will be involved?
     
  6. Ugo
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    Ugo Junior Member

    yes, correct.
    the foam core comes in two halves, both cnc machined.
    rudder shaft slot is also cnc machined.
    the two halves will be glued with thickened epoxy glue with rudder shaft inside.

    Then the glass will be just laid up over the foam blade.
     
  7. SukiSolo
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    SukiSolo Senior Member

    Drawing looks good Ugo. I might be tempted to put a thin layer of glass over the inner tabbing (each side), maybe as part of the 'thick' epoxy bonding the two halves. Just to aid transfer of load from tube to outer skin, via core without crushing it. Tabbing looks substantial as does the rudder tube, so I doubt you'd get any problems at all. Well thought out and considered.

    Nice job. Is the concept behind the slightly drooping bottom edge to keep laminar flow arond the bottom? Only reservation is it makes the trailing edge slightly more prone to damage if grounding. More usual endings are eliptical and square (ie part bulb) partly to take load on the structurally thicker leading edge. As for best hydro shape, at this lower edge, I have no particular preference, most likely very very little difference in the real world. The overall shape of the blade looks very good IMHO.
     
  8. Ugo
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    Ugo Junior Member

    SukiSolo, many thanks for your suggestions and for your appreciation.

    Once installed, this rudder has a 4° aft sweep (this is needed because of canoe body profile at stern, and it should also reduce drag marginally as a side effect - B. Eck, "5o5 fins - a definitive work", Tanktalk, march 1976).
    Since I wanted to have a flat and "horizontal" blade tip, I inclined it to compensate for rudder aft sweep.
    see attachment for the sake of clarity.
     

    Attached Files:

  9. SukiSolo
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    SukiSolo Senior Member

    Thanks Ugo, it makes sense now..;) Probably got so used to vertical leading edges most of the time!. Slight sweep is I understand, not detrimental at all, certainly the elliptical or leading edge swept from vertical has not affected any blade I have ever used/made/modified. The only ones that I've ever found bad were ones with dramatic sweep from the waterline,ie 12-15 degrees plus aft and those really weight the helm badly with only light heel.

    I refer to that 505 stuff periodically, it is good work and a great reference to have. Fortunately a lot of it confirms my own empirical testing and thinking. Still we have other computational tools available to help us!.

    Good work, hopefully you will be able to post a part built or built, but not painted shot? Looking forward to it.
     

  10. Ugo
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    Ugo Junior Member

    For sure!
    I'll post new pics as soon as Il'll take them

    at the moment only shaft and supports photos available (see attachment).

    From my side I hope to read some further advice on the main topic (glass layup/schedule, cloths orientation, building tricks...)
    ;)
    bye!
    Ugo
     

    Attached Files:

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