Friday, September 28, 2018

Edit: Why a Jordan Series Drogue is NOT a Very Good Steering or Speed Limiting Drogue

Usually there is a specific event, often personal, that motivates me to research something. In this case, ~ some years ago I jammed a rudder on a submerged tree. I remembered a the loss of Too Good to Be (Alpha 42 Catamaran) due to a bent rudder, and several friends who have been towed in after striking logs. A boat was towed hundreds of miles across the Gulf of Mexico last year, after loosing a rudder in nice weather--with a steering drogue I would have been underway again in 10 minutes, no need to make a call, other than perhaps a Pan-Pan to the US Coast Guard as a condition report. It occurred to me that emergency steering is a least as important to the coastal sailor, because there is so much more debris to hit. So I got interested in emergency steering and drogues in general. 


A JSD is unarguably the correct tool for most serious storm conditions. You can Google "JSD Coast Guard Report" for the details, so I will only summarize. It was an innovative and clever leap forward that sidestepped many of the problems associated with conventional drogues:

  • No single point of failure.
  • Can't tangle.
  • Can't be thrown forward by a wave, because the load is distributed.
  • Can't pull out of a wave face, because the load is distributed. 
  • Very smooth drag.
  • No single heavy piece to handle.


However, it has serious short comings when used at higher speeds as a speed limiting drogue, or more importantly to me, for emergency steering:


  • The cones will shred. Unlike a JSD, which spends most of it's time at 2-3 knots, a steering drogue sees a sustained 5-8 knot speeds, resulting an average load per cone that is 2-10 times higher. Although reversal in direction is less common than for a JSD in a serious storm, they will be working near the surface, breaking out of waves regularly and getting pounded.
  • Longer endurance is required. It will be used for a week of more, not overnight. On a long passage, it might be deploy several times, just for comfort.
  • JSD cones are too close together for proper flow at the higher speeds. Testing of JSDs and plain cone drogues suggest drag is reduced about 30-40% by close spacing. Increase from 2 feet to 3.5-4 feet.
     
And yet it has  potential advantages:

  • No single large element to handle. Pulling a Delta drogue 72 over the transom in rough weather is no picnic. That is an understatement. Single drogues much larger than 2 feet (Seabrake 24, Delta Drogue 72) are simply not manageable by normal or aging sailor in boisterous conditions. Standard JSDs have a reputation for difficult recovery, but this would be 1/3 the length; I'm confident it is easier to man handle than a conventional drogue.
  • Less chance of tangling during deployment or recovery. The worst time with a standard drogue is recovery it; I've had a few get in the rudders.
  • Will not pull out of a wave. This is a considerable advantage, because every conventional drogue becomes unstable above about 5 knots in the presence of steep waves. The wave increases the angle of the rode to the water and the drogue pulls out as the wave passes it. The best you can do is try to put this out of phase with the peak load, which in confused seas, is impossible. This is the primary reason conventional drogues can fail catastrophically as the storm rises; everything seems good, until the speed + load reach a critical level, the wave steepness builds, and the drogue pops out. If it is on a nylon rode, the rode contracts, the drogue snaps forward, and maybe the wind and breaking wave help it along. There is no magic design that can make a single drogue run deep enough to avoid this cycle. I observed this hundreds of times in testing.
  • Drag will be more adjustable. Sometimes you need a lot of drag to make the boat track, and sometimes less would increase both speed and pointing (yes, you can sail upwind with a drogue, just not very high).



Recovering a Delta Drogue 72 (about 24-30 inches across). Even in clam weather it is extremely heavy and must be dumped first--impossible unless you are standing at water's edge, which won't always be practical. Other drogues are awaiting their turn in the dinghy, including a Galerider, Paradrogue, Seabrake, cone, and JSD  from Ace Sail Makers.

In use the drogue is often pulled in very close to the transom, to reduce drag by lifting the drogue. This is how a Seabrake 24 deploys in 10-25 knots for emergency steering; in light winds it is in close, in stronger winds I you add an extension between the chain and the bridle.


I decided to test a modified JSD (call it a Series Steering Drogue) as a speed limiting and emergency steering drogue:

  • Fewer cones. Only about 1/3 the number specified for storm service.
  • Strong construction. Upgrade to 6.5 ounce sail cloth and hem the leaving edge (Ace Sailmakers is building JSDs this way now).
  • Increase cone spacing to 4 feet for better flow at higher speed.
  • Less weight. 10 feet of 5/16-inch chain should be enough, or an 8-pound mushroom anchor. The rogue will be more naturally resistant to surfacing at high load or high speed (same thing).
    The most obvious difference is the that the series steering drogue is much longer, about 75-100 feet. But this is part of its strength--only by distributing the load can the drag remain steady is something as fragile and turbulent as water. 



    The standby rode is to allow us to seamlessly add the rest of the JSD if conditions deteriorate; just attach the rest of the JSD, along with a dedicated bridle attached to the JSD chain plates, and cast the spinnaker bridle free (you will recover it when you recover the JSD).



    Alternatively, A conventional drogue, such as the Seabrake can be used as the new primary drogue. Again, the stand-by rode allows for seamless deployment. The Steering Drogue will still provide stabilization for the new primary drogue (the Seabrake), much like a JSD, but with more adjustable deployment.




    And it was a complete failure. 

    First, the trade mark JDS droop, which allows the tail to run deeper under water does not occur. At higher speeds (5-7 knots) the JSD is pulled right up to the surface by water drag. The droop is not unique to the JSD design, but is only an artifact of Jordan specifying enough cones so that enough drag has been added to reduce the speed to 2 knots or so, which allows the tail to sink. Any drogue design will do this in an equivalent manner, as long as there are at least 2 units and the speed and weight are equal. Although it may retain some resistance to damage if the motion is reversed on the back slope, this is not he case with steering and speed limiting drogues--motion is always forward.  

    Second, smaller drogues (cones in this case) are inherently less stable when running near the surface, and are more easily disturbed by the movement of waves. A large drogue can run at shorter scope because it pushes a lump of water above it, whereas small cones push only froth. The small cones simply splash on the surface, completely unstable, unless the scope is at least 75:1, which is impractical with a 4-foot transom (I had to test them by anchoring near the waterline). Cones also zig-zag and flutter. This is very hard on their construction, fatiguing the cloth.

    Efficient shapes. A cone produces only 55-70% as much drag as a Seabrake or Delta Drogue of the same size. The design is easy to sew but not advanced.



    Finally, drogues are inefficient if they follow too closely behind one another, much like a racing cyclist enjoys the draft by following another closely, and the higher the speed the more significant  the effect. Thus, a steering drogue based on JSD cones must have much greater than normal cone spacing and becomes very long, about 50-60% of storm drogue length length even though it contains only 20% of the number of cones. This is quite cumbersome compared to a single unit, with no apparent advantages. The combined effects of inefficient shape (essential, because so many must be built), close draft, and small size (unstable near surface) are limiting to the function of a truncated JSD.

    It seems it is a matter of horses for courses. The JSD principle has considerable advantages for severe storm management, but only if designed for sufficient drag to slow the boat to less than 2 knots. If truncated to allow for better movement, the design has no advantages and more failings.

    Darn. 




    2 comments:

    1. Dave , Ace SailmakersApril 7, 2019 at 6:00 AM

      Customer with a Gran Soleil 50 has aft section of Drogue in two sections. Aft section has 50 cones and tail weight in two pieces ,so less weight might be used as steering device.
      Bridle legs attach midships.

      He reported it worked great and satisfied Pacific Cup requirement.

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    2. I did not intend to imply that a JSD section could not be used as a steering drogue. Practically anything can work, as Osprey demonstrated by sailing half way across the Pacific with a collection of fenders and anchors rigged as a steering drogue. A JSD section would be better that that.

      What I did observe is that a JSD does not deliver superior performance as a steering drogue, as it does as a severe storm drogue. When rigged for higher speeds the design functions entirely differently, skimming near the surface. Using just the tail does eliminate one piece of gear (a Seabrake Shark, or Galerider), but it will be subject to extreme wear and did not perform as consistently in my testing. I was actually disappointed. But perhaps it does work well enough.

      Yes, for steering, bridle legs attach mid-ships.

      ReplyDelete