Mr. Jordan's series drogue -- possible design improvements?

Discussion in 'Stability' started by BillyDoc, Sep 1, 2008.

  1. BillyDoc
    Joined: May 2005
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    Location: Pensacola, Florida

    BillyDoc Senior Member

    I've been thinking a lot lately about Mr. Jordan's series drogue, and the more I do the more I admire his design. I also greatly admire the man for not attempting to squeeze dollars out of this important safety device with patents and the like. There is an aspect to it, though, that I think has not been sufficiently examined -- and that is the reported ability of a series drogue to play Moses and part the waves approaching a boat using it. I have a theory as to why this parting of the waves may happen, and it has implications for designing and deploying serial drogues generally, so I want to put it here and solicit your comments.

    As I understand it, Mr. Jordan's drogue is typically deployed as a line of small, 5” diameter cone-like drogues, with the support line fed through the open cone-bottom and with cones spaced every 20” or so for several hundred feet behind the boat. At the distal end of the drogue line is a weight, usually about 20 pounds of chain. As the boat streaming a drogue line is pushed along in the general direction of the waves by wind and waves the drogue line is pulled through the water in the direction of the open end of the drogue cones, and therefore resists the motion. As the boat attempts to surf down the face of a wave the drogue line also resists the motion, but as the boat backs down the rear face of a wave that has just passed under the boat the weight of the chain pulls the drogue line back through the water against the small end of the drogue cones, thus collapsing them, and keeps the line taut and straight. The end result of all this is a line of drogue cones streaming more or less horizontally behind the boat, but with the weighted distal end drooping downward significantly.

    Meanwhile, the waves marching along to encounter the tethered stern of the boat have a strong cyclical component that is rotating generally in the same direction a wheel on a hard surface would when rolling in the same direction. This cyclical motion is energy intensive enough to both raise the water above the mean surface in one area, and depress it in another – all from the centrifugal forces produced by internal rotation. As most of us have observed at one time or another, this centrifugal force can be strong enough to cause the water at the top of the wave to defy both surface tension and gravity and break away, thus forming a very energy intensive jet of water that may be very dangerous to any boat in it's path.

    When a boat is lying to a serial drogue and a wave approaches, the topmost part of the wave will be rotating toward the boat, but the bottom of the same wave will be rotating away from the boat and directly into the open tops of the trailing drogue cones below the surface. This rotating water will thus exert a force upon the boat that acts to arrest any forward motion of the boat and pull it back into the wave. This is an extremely useful effect all by itself, as we want to arrest the forward motion of the boat as much as possible without involving other collateral effects. But I believe there is another effect that is also important: when the rotating water within a wave acts on the cones of a serial drogue to pull the boat backward it expends energy. This expended energy comes directly from the rotational inertia of the water within the wave body, and because the wave no longer has the same energy content – it falls back upon itself to the extent of that expended energy. A breaking wave rolling along toward a boat could thus appear to collapse into itself in that local area, simply because it no longer contained the kinetic energy to sustain itself. That energy would have been used to pull the boat backward. Thus the extremely useful Moses Effect.

    If this theory is correct I think it has implications for serial drogue design, but I'm not at all sure what they would be. I think a means of positioning the drogue line so that it “grabs” more of the bottom most rotating water would be good, and I also think that the weight of the chain at the distal end of the drogue should be kept as low as is practical to keep the line actually submerged, which is probably a tricky balancing act.

    What do you think?

    BillyDoc
     
  2. safewalrus
    Joined: Feb 2005
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    safewalrus Ancient Marriner

    BillyDoc nice to see you again! totally agree with what you're saying but regards the weight on the end in the conditions that you would deploy the drogue are not of the best and variable to the extreme the weight required would vary constantly? In this case surely it is better to slap a largish weight on the end to ensure that at the worse time it works at optimal efficency, whereas at the best it may be a bit heavy, but you need the safety when it's rough. Just my take on it is all!
     
  3. eponodyne
    Joined: Aug 2007
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    Location: Upper Midwest

    eponodyne Senior Member

    Perhaps a volleyball-sized buoy every twenty feet or so with a small weight evenly spaced between them might work as to the situation described above.
     

  4. BillyDoc
    Joined: May 2005
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    Location: Pensacola, Florida

    BillyDoc Senior Member

    Hi Safe,

    You're right, design for the worst and the best takes care of itself.

    Hi Eponodyne,

    I thought of something like that, but if you use floats of any kind you will probably defeat the "pull back" action that you get from the chain-weight. Which may be OK, actually. I'm hoping someone on here will have some way of analyzing this --- so we'll see! Another approach would be to use polypropylene lines (which float). It could be that the supposed "pull back" from the bottom of each wave would be sufficient to pull the line tight enough so it only actually comes to the surface in the troughs of the waves. If this is so, then a careful balance between chain weight and floats (as you suggest) or floating lines might be the answer.

    BillyDoc
     
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