Fin keel & bulb construction, quick connect cassettes

Discussion in 'Fiberglass and Composite Boat Building' started by nickrj, Jan 3, 2022.

  1. nickrj
    Joined: Nov 2021
    Posts: 19
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    Location: AU

    nickrj Junior Member

    Hi all,

    Could someone school me or point to any relevant existing documentation on how modern bulb fin keels are constructed? Are the internals steel with welded plates top and bottom (for bolting to the lead bulb and bolting to the hull), which is then shaped & faired with composites? Or is the shaft section 100% composite material?

    Lastly, I was reading about the Farr X2 and how it was designed to fit into a shipping container. A paragraph reads:

    Her milled steel fin with a T bulb allows for best in-class stability, whilst the quick connect socket simplifies keel installation, by inserting the fin into its own female cassette, which is industry best practice.
    This is so the keel can be removed for shipping - does anyone have any pictures or know what these quick connect sockets & female cassettes look like? Or are they custom pieces of hardware made & designed for the X2?
  2. Olav
    Joined: Dec 2003
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    Location: Filia pulchra Lubecæ

    Olav naval architect


    the construction of the keel can be anything from solid cast steel/iron, milled steel or aluminium, welded hollow metal construction, a structural steel beam with fairings around to fully composite, depending on budget, fabrication capability, class rules or whatever.

    ISO 12215-9 not only is on how to calculate sailboat appendages in terms of strength and stiffness, but also (to a much lesser extent) provides some information on different ways of fabrication and design. Here you can browse that standard for a modest fee of € 2.40 if you don't want to buy it (although at € 24.89 it's not very expensive). An excellent article on appendages engineering and construction was written by Eric Sponberg; available from his website.

    The female cassette/socket and its male counterpart on top of the keel fin will be custom made as it is the case for most sailboat keels no matter the design. Basically it's a simple recess in the bottom of the hull into which a plug on the keel fin will fit snugly. All longitudinal and transverse forces and moments will be transferred via the form-fit of that connection to the internal hull structure (floors, bulkheads, etc.) while there are bolts on top to take the weight of the keel and keep everything together. Please see attached photo of a Class 40 keel attachment:
    Photo source: Screenshot from YouTube video

    The same principle has been used on (R/C) model yachts for ages.

    Hope this helps.
  3. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    It appears that ISO 12215-9: 2012 is still in force.

  4. Olav
    Joined: Dec 2003
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    Location: Filia pulchra Lubecæ

    Olav naval architect

    You are right, Ignacio, I have the 12215-9:2018 which states in its foreword that "This document supersedes EN ISO 12215-9:2012" but at the bottom of the page it reads "The text of ISO 12215-9:2012 has been approved by CEN as EN ISO 12215-9:2018 without any modification."

    So it seems there is a only legal difference between the "international" and the "European" view on this standard, while technically everything remains the same.

    Just out of curiosity I checked at Beuth Verlag (the standard publishing house in Germany) and of course there is the national German issue DIN EN ISO 12215-9:2019 that is the same as EN ISO 12215-9:2018 which as per above is the same as ISO 12215-9:2012. Oh, and it's the same for the Spanish UNE EN ISO... while the Estonian issue EVS EN ISO is from 2018 as the EN. Just crazy. o_O
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