Monday, March 14, 2016

Playing with Drogues

A bit over a week ago I blogged on emergency steering. Since then I've plunged headlong into research for several articles on drogues for steering and storm management. I've got sample from the major players which I've been dragging around the Chesapeake, collecting some baseline data and learning their basic traits. Not under storm conditions, mind you--writing doesn't pay that well!

I've been able to learn about full-size drogues at higher load with 2 tricks. First, pulling them at short scope (actually the best method for emergency steering) gives insight relating to pulling out of a wave face, since a following wave also changes the rode angle. Second, getting a running start at a drogue, hitting it at as much as 7.7 knots (with extended transoms and a clean bottom, that is my best speed) with a non-stretch rode gives some high forces, but I wanted something a little easier to manage and some thing that I could pull continuously at storm-like loading. Like the tandem anchoring trials (coming in 2017 in Practical Sailor), I went to models.

3 times smaller, but faithful in performance characteristics.

From left to right, 1/3rd scale models of Seabrake 24, Delta Drogue 72, and Jimmy Green Marine 60cm Drogue. Sewn from scrap Sunbrella and 1500-pound bull tape. I saw as much as 200 pounds drag out of the mini Seabrake/Delta combination at 7 knots! I later added a mini-Galerider.
(note. These are NOT official copies and any statements should not be taken as such.)

For anchors, I was able to get roughly 8 inches across, I was able to find 2-pound surrogates in the form of a 2-pound Guardian, 2-pound Claw, and a 2-pound Mantus Dingy Anchor (very nice--get one!). But for drogues I was out of luck. So I sat down with a pile of old canvas, a roll of pulling tape, a palm, scissors and tape measure and made my own. Roughly 8 inches in diameter, if towed by a small diameter rode with mating scale chain, they perform just like their big brothers!
  • Drag is about 9 times less (expected to vary as the square of diameter)
  • speed/drag relationship is consistent (varies with V^2), like the larger versions, but now proven through a larger speed range
  • Pulsation behavior is similar (load varies at about the same range and frequency as larger drogues)
  • Behavior--yawing and surfacing--is just about identical
 Because they perform to scale, I can accurately predict how larger models will behave, without testing models beyond the capabilities of my boat.

And so now I have a set of tiny scale drogues I can test at scale in small craft advisory conditions, as through it were a gale.


The really weird part is that I have already learned some things I that either the manufacturers don't know or are in denial about. Failure patterns--both mechanical and performance--seem to come from pretty basic errors. Not the better manufacturers, though; they seem to know and communicate better.


The early take-aways?
  • Anyone can need emergency steering when they hit a log. It is not a black art, but you need the right gear. I tried warps and chain, and it aint' going to work for you. No way.
  • Towing warps is not much of a storm strategy. The drag is perhaps 10 times lower than recommended drogues.
  • You get what you pay for. So far, the more money, the more stable and rugged the drogue.
  • There is a speed limit. All drogues have stability problems over ~ 5 knots in big waves, and my gut at this point is there is no engineering solution. You either carry enough drogue to stay below 5 knots or the risk of the drogue pulling out of a wave face goes up when the waves get steep.
Fun stuff!

I have one article on drogues in Practical Sailor, with more on the way.

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