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Thread: Beginner keelfin placement questions...

  1. #1
    Join Date
    Nov 2010
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    Johannesburg
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    Default Beginner keelfin placement questions...

    Hello gents, I'd appreciate a bit of a lesson on this please. I can't seem to find a simple explanation, but if there is a link please pass it on.

    My daughter decided she wanted a yacht (I couldnt possibly let her down ) so I'm building her the hard chined Aspara from here:

    http://rg65.free.fr/Apsara/apsara%20...2008%20Eng.pdf
    The hard chine lets me build it quickly for her and I have the hull skinned in 1mm balsa ready for a light cloth/epoxy layup.

    So I moved onto the keelfin and realized that in my 2 previous builds I didn't have to bother about placement/keelfin angles because my first attempt at a boat was a JIF - and keelfin placement was the least of my worries - and then a Round Ranger that was a kit that had the keel box etc. cut and angled so I just had to get the hole cut in the correct place.

    My issue is that on the plan the keelfin angles backwards. Is that on purpose or merely because its aimed at newer builders and that is the angle of the bottom panel on the hull and you adjust the bulb to balance out the boat?
    Is it better for performance If I install the keelbox so the keelfin is vertical when the boat is at rest?
    And the plans quote top and lower widths for the fin. Does that imply a straight, vertical leading edge with just the trailing edge angled or do both sides of the fin angle in to create a sort of "V" shape?

    Thanks in advance

  2. #2

    Default Re: Beginner keelfin placement questions...

    The purpose of the backswept keel is to enable the mast to be placed as a far aft as possible without upsetting the relationship between the sail and the fin. This helps reduce the tendency to dive when going downwind. If you change the keel angle you'll also have to refigure the mast position, which as a beginner you may not want to do. The designer has worked all this stuff out, so I'd stick to the plans.

    Cheers,

    Earl

  3. #3
    Join Date
    Apr 2005
    Location
    Toulouse (Fra)
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    3,289

    Default Re: Beginner keelfin placement questions...

    Hi, this is the drawing picked up from your file. The side view is very explicit. If a line is strait it shall be kept strait. The fin dimensions and 'form' are outlined. The designer decided for a tilted Fin configuration, therefore if you whish to maintain the design performances, it shall be tilted as from drawing . I assume, as often made so, that the Sail Plan CE is vertically positioned in the first quarter of the Fin Chord. Only tuning will say where it should stay. Drawing call for a Swing Rig Sail Plan, but indicate also the mast step position for a JIF-2 Sail plan. Regards ClaudioD
    Attached Thumbnails Attached Thumbnails Click image for larger version

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  4. #4
    Join Date
    Nov 2010
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    Johannesburg
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    Default Re: Beginner keelfin placement questions...

    Thanks so much.
    So follow the plans implicitly because the designer has thought of more things than I am aware of

    And thanks so much for the updated picture Claudio.
    I didn't notice that the keel fin was occupying that 5cm space, so now I can get the angle correct.
    I'll post pics of the finished thing

  5. #5
    Join Date
    Apr 2005
    Location
    Toulouse (Fra)
    Posts
    3,289

    Default Re: Beginner keelfin placement questions...

    The designer, in principle, has developed the Curve of Area from which he can derive the Displacement, the Longitudinal Centre of Buoyancy, essential for the Fin position and the Prismatic Coefficient and the Centre of Lateral Resistance. These design data are often hidden ! ClaudioD

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