5.5m keeldesign

Discussion in 'Boat Design' started by MYD, Nov 25, 2022.

  1. MYD
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    MYD Junior Member

    Hi Is there anybody out there able to educate me on the design of the keels for the 5.5m racing yachts.
    The class rule says minimum 150 mm thickness but how long is the corda and what wingsection do they use. I have also seen that some of them has an extended leading edge at the root. Would be interesting to know more about the design of these.
     

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  2. jehardiman
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    jehardiman Senior Member

    I took a quick look at the rules, and as typical with most meter classes the shape and section of the keel is totally up to the designer beyond the limits of the rules.
    So really anything goes within those limits and a total vessel displacement between 1742.5 and 2050 kg. What is actually used is what the Naval Architect is for. Actually this is more open than most classes because there is no midships girth requirement.

    Edit to add: A deeper read of the rules shows that boats in the "Classic" division (i.e. pre-1970) are not allowed to alter their original keel. Boats in the "Evolution" (1970 through 1993) and "Modern" (post 1993) are able to.

    Second Edit: Perhaps I didn't answer your question well. Are you asking about what people have done historically? Why a particular keel shape and section was selected? Or what keel shape and section you should select? Because this is a calculated rule with some box elements as well as scantling rules, keels will be as individual as the boats designers. However, most keel forms will all be hovering around some era-dependent accepted best design practice (like the flapped keels of the 1970's and 80's).
     
    Last edited: Nov 25, 2022
  3. MYD
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    MYD Junior Member

    Hi and thanks for your reply. Thats about how far I got myself.
    I'm presently restoring a 2.4 mR and the keel is pretty similar in size so it would be interesting to compare.
    What I am looking for is some data from present modern 5.5's. The minimum athwartships measurement of 150 mm does affect both choice of length and wingsection so what cord length and wing section they use is of interest. As designers most probably have done CFD analyze I assume there is some consensus.
     
  4. jehardiman
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    jehardiman Senior Member

    Absolutely not; there is, and should not be, any consensus except as it compares to local conditions. Based upon the simplistic 2x2 matrix of local conditions (heavy air, light air, smooth water, chop) there are at least 4 different optimum keel designs. Remember CFD (Colourful Fanciful Drawings...or more exactly Completely Fictitious Drawings) is very subject to GIGO. Based on the condition matrix I would design a different keel (and boat) for each condition. There is a whole history of underhanded dealing and politics in the meter classes about this.
     
  5. MYD
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    MYD Junior Member

    Still there might be someone out there who knows something about cord length used for 5.5 keels.
     
  6. jehardiman
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    jehardiman Senior Member

    I think you'll find that it is generally more than 1250mm (12% thickness) and less than 3000mm (5% thickness). Just look at all the photos of the keels....lots of variation given the sweep and taper.
     
  7. wet feet
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    wet feet Senior Member

    I've never set eyes on a 5.5 Metre but I would suspect the simplest explanation for the reverse slope of the leading edge of the keel is that it moves the bulk of the mass tot he lowest point of the boat,where it can produce the greatest righting moment when the boat heels.Might I recommend seeking out the papers from Peter van Oossanen regarding the research on keels for 12 Metres in the early eighties?The class may not be the same but the water they sail in is identical and the general principles apply.
     
  8. The Q
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    The Q Senior Member

    The other reason for the reverse sweep of the keel. Is it causes the water to flow up the keel, so the hull becomes a endplate, reducing losses of water over the bottom of the keel. So increasing performance.

    I'd looked at it for my keel design...
    For 2.5seconds...
    Too much weed here for a reverse keel, or wings or anything that will catch the lilies.
     

  9. jehardiman
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    jehardiman Senior Member

    While both of these statements have an element of truth, they are not the main reason for the choice of a "hatchet" or "whale flipper" type keel's narrow root. Considering that the interference drag between the appendage and the hull can be as much as 25% of the lift of the appendage, it pays to make the root as thin and as short as practical. This is part of the overall appendage trade-off between root drag, tip drag, and effective span.

    Van Oossanen's 12m work needs to be taken in context. At that time they were trying to make the most effective hydrodynamic keel given that the main focus of the keel was to subvert the 12m rule. It was not that those types of keels where hydrodynamically superior to other types, but rather they improved the vessel in the context of the racing conditions. AUSTRALIA II was significantly shallower, wider, and carried more sail area in the light conditions off Newport. It did not have a more hydrodynamically effective keel. Like a carbon fibre mast, it is not that the item is superior to another type, but what it brings to the design table. Since the 5.5m rule has box elements, I doubt going to a hatchet type keel will offer the significant advantage gained by AUSTRALIA II.
     
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