Vortex at the tip of a classic long keel

Discussion in 'Boat Design' started by Guillermo, Feb 6, 2016.

  1. Guillermo
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    Guillermo Ingeniero Naval

    Hello everybody,
    There is some time I am not active in these forums, altough I still follow some of the discussions. Too many things to be done and too little time to do them, I'm afraid...

    Well, I a week ago racing aboard a friend's long keeled classic sailboat, I realized something I had never previously seen in my life...

    When beating to winward against a short chopped sea in our home waters of the Pontevedra Ría (NW Spain), the vortex created by the bottom of the keel was notoriously visible because of the many bubbles it carried. You may see it as a long and witish patch at the center of the attached image. It was not continuous, but forming every few seconds, several meters long, as the boat pitched, yawed and swayed with waves.

    The boat is the 12 tonner (Thames) Hillyard in the second image (taken the same day)

    Discussion arose among crew about what was the cause of the bubbles. Some said it was cavitation but I defended it was just air bubbles in suspension (ventilation) because of the choppy sea, but never cavitation because of the slow speed (+/- 3.5 knots).

    As discussion follows and I have found nothing in texts about ventilation/cavitation in long keeled heavy displacement boats, I ask from the sapience and kindness of this Forum's members to guide me to where I can find something on the matter and also if someone has also experienced the same phenomenon.

    Thanks in advance.
     

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  2. daiquiri
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    daiquiri Engineering and Design

    Great picture, Guillermo! It should be printed in textbooks of fluid flow, as a practical example of a vortex flow visualization around the keel. :)

    My two cents worth on what's going on down there:

    The sea water close to the surface is never a pure water. It is a mix of water and air, and the percentage of dissolved air rises when the sea is choppy. That's how nature provides oxygen for the sea life.

    Now, a vortex detaching from the keel has a low-pressure core, which pulls the fluid particles towards the axis of rotation, and thus equilibrates the centrifugal force which comes from the fluid rotation.
    Since the pressure in the vortex core is low, the diameter of dissolved air bubbles increases and air bubbles become visible. It is a consequence of the Laplace pressure law: https://en.wikipedia.org/wiki/Laplace_pressure, which says that a decrease in the ambient pressure causes an increase of the gas bubbles.

    So what you see there is IMO the air-rich vortex core.

    Cheers
     
    Last edited: Feb 6, 2016
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  3. Doug Halsey
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    Doug Halsey Senior Member

    That is a great picture. It reminded me of something I had seen before, and of course it turned out to be from Marchaj's Sailing Theory & Practice.

    It's a photo of a model Dragon. Not much description beyond the basics in the text though.
     

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  4. viking north
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    viking north VINLAND

    In the far reaches of the past on the forum I described this phenomenon in a more humorous tale. My son at time being somewhere around 5 or 6 yrs. old and I were out sailing among a pod of whales. The vessel being one of my early conversions of ships life's boats into long keeled Motor Sailors. I was occupied looking forward for a clear path thru the pod when my son suddenly started yelling and pointing aft over the stern "Dad he's trying to bite the rope". Rope ? What Rope? Tying the tiller I looked down over the stern to see a young whale snapping at what looked like a piece of pure white samson braid trailing about 30ft. aft and breaking up into pockets of bubbles. It was these the critter were interested in. The samson braid being nothing more than the vortex trailing off the bottom of the keel and winding out in a long curve aft. The vortex of course created as described in most detail by Daiquiri in post 5,037 (wow) above. Im my simple layman's terms i,ve always reasoned out the process as trapped air caught between the wave action at the bow on the leeward side of the hull being carried down along the high pressure side of the keel. Seeking pressure equalization it rotates under the keel to the low pressure side. This rotation action along with the forward motion of the vessel forming the corkscrew vortex of samson braid :D
     
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  5. Guillermo
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    Guillermo Ingeniero Naval

    daiquiri: I fully agree with your explanation (but I think it should be "centrifugal" instead of "centripetal" in your text)

    Doug: the same image is at Marchaj's "Seaworthiness..." book, but not explanation either about the forming of bubbles.

    viking north: lovely story that of your son and the whale trying to bite the vortex.

    Thanks a lot to you three. :)
     
  6. daiquiri
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    daiquiri Engineering and Design

    Oops, sorry. Centrifugal force, of course. The acceleration is centripetal. :)
     
  7. Guillermo
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    Guillermo Ingeniero Naval

    A colleague in Spain tells me this about the vortex in another long keeled boat:

    "I own a 7 m long, 2.5 m beam, 0.9 m draught long keeled boat, 30 sqm sail area and 2200 kg displacement. The keel vortex was very strong and pretty conspicuous when going to winward. I eliminated it by installing winglets at the rear end of the keel. Now the boat beats better to winward with an smaller angle, but needs more sail when in little wind because of the increased drag. They are trapezoidal, 60 cm long at root and 40 cm span. Section is flat-convex with flat side upwards. GRP laminated to hull forming a 20 deg angle with the horizontal."

    BTW my friend's boat has an around 1.40 m draught.
     

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  8. tom28571
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    tom28571 Senior Member

    Great photo Guillermo, and explanation by Daiquiri,

    I knew it could not be cavitation but seeing that at less than 4 knots was new to me. Next time I'm sailing a Rhodes Reliant long keel is those conditions, I will look for it. Would the boat need to be making quite a bit of leeway for that to happen?
     
  9. pogo
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    pogo ingenious dilletante

    I think that positioning your winglets at the rear only stops the already existing vortex.
    Winglets normally begin at about 40% , that's much more effective.
    Those winglets don't need much span, delta shape with a max. span at the rear of two times keel thickness ( tip) should be sufficient.
    See also tiptanks ( F 104) or bulbs ( t-keels).

    I saw the "rope" pretty often , always in a flat sea, no waves. the rope has nothing to do with ventilation.
    Best views i got in the night thru bioluminiszens ---great !!!!

    pogo
     
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  10. Guillermo
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    Guillermo Ingeniero Naval

    Tom: yes leeway is needed for the vortex to form. The more the leeway, the bigger the vortex.

    pogo: The word "rope" suggest to me a not that strong vortex, perhaps the same as the one in viking north's story. What kind of hull and keel was the one producing it?

    All: As we can see in the image I posted initially, the vortex in my friend's boat was not a "rope" like one, but a +/- 10 --> 20 cm diameter irregular "elongated cone" where the rotating bubbles were easily visible.

    The fact that it was not a continuous vortex but intermittent and less than one boat length long, formed just at a specific combination of the pitching, swinging and yawing of the hull, I am inclined to think that on top of the increasing of the diameter of the microbubbles in suspension in the mass of the water close to surface daiquiri has explained, there were also direct bubbles coming from the waves breaking against hull. But maybe I'm wrong.

    I attach another images were the intermittent and irregular form of the vortex may be seen.
     

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  11. philSweet
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    philSweet Senior Member

    You can sometimes get a sink drain to burp. I've one that goes GLOOMP GLOOMP, GLOOMP in certain seaways if the drain isn't closed. Was the head forwards and to leewards?
     
  12. keith66
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    keith66 Senior Member

    Any appendage might cause similar, a big anode might well do it especially as it comes near the surface.
    On a related note years ago a friend built an electric launch this was a 19th century fantail counter stern design shortened somewhat. Her buttock lines & run were very steep & to allow removal from the mould the builder put a flat sternpost on about 3" wide. The prop was about 14" dia & did a max of about 1000rpm. At slow speed from a boat running alongside you could see the vortex forming on the surface at the top of the sternpost & extending downwards towards the prop shaft whereupon it would burp & cavitate badly every 10 seconds or so. As speed increased it got worse.
     
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  13. Manfred.pech
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    Manfred.pech Senior Member

  14. Guillermo
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    Guillermo Ingeniero Naval

    Phil: that could be an explanation as the loo is on the port side, thanks. To be investigated.

    keith66 and Manfred.pech: Thanks for your comments. In a hurry now I will come back to review your posts this evening.

    Cheers.
     

  15. gonzo
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    gonzo Senior Member

    Herreshoff mentions those vortices in The Common Sense of Yacht Design. He said to have first noticed them racing a 12 meter of his design. The vortices would surface up to several hundred feet behind.
    By the way, good to hear from you. I may be in Pontevedra this summer
     
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