Wednesday, January 18, 2017

Drogues in Action--Small Scale Testing Duplicates Gale Conditions

Why the interest in drogue if I'm not going to cross an ocean? "As a coastal sailor, why should I care?" Because a little over two years ago I bent a rudder on a submerged log. Only because I had a catamaran did this avoid becoming a serious problem (I have a spare rudder). Other wise, take your hands off the wheel and tell me how you plan on getting back home. It's not that easy.

However, this post is not about emergency steering. I'll get to that later. This is about drogues for speed limitation is serious storms. It turns out that they have not been telling us the who story.

___________________

Parachute sea anchor, speed limiting drogue, or JSD (Jordan Series Drogue).  Sailors that have never used them will argue, and even those that have used them generally remember what they saw but don't understand it. My nature is to expereiment until I understand.

 How many people have a full collection of 1/3 scale drogues? [Cone, Small Shark, Delta Drogue, Gale Rider, Sea Brake]

Small Scale Testing

The trouble with testing full scale drogues is that you need a full-scale storm to test them in, and mistakes have full scale consequences. Not a forgiving classroom in which to demonstrate things things that fail, and only by knowing what fails can we understand what works. It is also a mistake to build a scale model and believe everything you see, without understanding the theory and how scale up works for engineers. But that is how I have spent the last 30 years, so I know a few tricks.

These drogues are 1/3 scale, run at 80% speed, in smaller waves, and at reduced scope. Combined, these scale factors give a realistic picture of how a speed limiting drogue functions in gale conditions. In the first example I show a Seabrake 8 standing in for the real-world Seabrake 24. Like all drogues, it pulls out of the water only when a following wave catches it; at that moment,  the angle of the rode to the water suddenly becomes steep, like an anchor on short scope, and out she comes. As soon as the wave passes, she settles back into the wave face. The conditions were mild.

video

In the real world the danger here is three fold:
  1. A wave may strike the boat while the drogue is out of the water. Not very likely in a moderate storm.
  2. In a serious storm, the drogue may spring out of the wave and be thrown far forward. Sailors often claim the wave threw it forward but they are only 1/3 right. The greatest factor is actually the contraction of the nylon rode (think rubber band) as the tension comes off, and that of the wind. This effect was minor in the video because the rode is oversized compared to common practice. This is also part of the case for using a polyester rode with a drogue and not the more common nylon. Several manufacturers specify non-stretch rodes.
  3. As the storm builds, right when you most need the drogue to work, it will fail. That is when the load is peak (required to pull the drogue out) and that is when the waves get steeper.
I then added a second drogue in tandem, 30 feet farther back (Delta Drogue 24, standing in for a Delta Drogue 72). The conditions are also now near gale force (sustained over 30 knots with heavy rain). As you can see, the first drogue runs closer to the surface, the result of the secondary rode tension. On the other hand, the secondary rode stabilizes the primary drogue when it does pull out of a wave, pulling it back in quickly. The first drogue may take slightly more of a beating on the surface, but the pull, measured by load cells, is far more stable (15% fluctuation vs. 60% fluctuation). A fair trade-off.

These waves are only about 5 feet, but considering the scale factor, these are 15-foot breaking waves. Pretty tough.


It is worth remembering that both of these trial were at the same speed (4 knots). In reality, adding a second drogue will slow the boat significantly, also making the tandem drogue far more stable. Another way of looking at it is that a tandem drogue can handle 40% more wind (70 knots vs 50 knots sustained) before the first drogue begins pulling out--a pretty big difference--and far more if we consider overall stability of the combined system.



_________________


 Gale Rider 30. A little small as a speed limiting brake (they make larger sizes), but my favorite for emergency steering. Very smooth and easy to handle.

The other things to consider are these:
  • Redundancy. You have two.
  • You can adjust to the strength of the storm.\
  •  Smaller drogues can be carried. This is no joke on a larger boat, since large drogues get very physical to handle in a blow. I later tested full scale models in gale conditions, and it was a workout.
  • A smaller drogue is more useful for emergency steering. This gives you the ability to transition between emergency steering and speed limiting.
However, full-scale speed limiting drogues and emergency steering are for future posts.

-----

A detailed treatment and more data are included in "Faster Cruising," which I should complete in a month or so.

No comments:

Post a Comment