2016, rev. 7/28/2024
Just a bunch of data I developed by dragging a bunch of drogues through the water behind my PDQ 32. Data for higher speeds, particularly for larger drogues, were extrapolated using a simple square rule, which turns out to be very accurate for drogues over about 1 knot.
This is steady-state drag. It does not include the effects of waves or fluctuating speed. For example, the JSD is quite stable with wide fluctuations in speed and load because the elements are small and the tail weight keeps the slack out. Large parachutes are quite unstable and can tangle if the a minimum load is not maintained (hence the need for a long elastic rode and a riding sail).
The data also assumes you are able to keep the drogue underwater and away from wave faces by using a very long (perhaps much longer than makes sense because of excessive rope stretch in some cases) rode and a lot of tail weight. All drogues become unstable above about 5 knots and will pull out of waves faces at even lower speeds.
All of the commercial drogues (Gale Rider, Sea Brake, Delta Drogues, Small Shark) had similar stability in waves and all of them pull out of wave faces when overloaded. The Gale Rider is more of a strainer and is a little more stable, but has to be larger.
More drag is not "better." It depends on what you are trying to achieve. For emergency steering and stabilization in a breeze less drag is good. For parking the boat bow-to-wind maximum drag is best. Running before a storm, something in the middle. There is no "one size" even for a given boat.
I didn't drag a 90-cone JSD. I used Jordan's data.
So what can we learn just from the data?
- Drag goes up with the square of speed for all designs. Special features do not increase drag at higher speeds. No such magic.
- All drogues have closed to the same drag vs. area ratio, something like the drag coefficient of a plate the size of the inflated drogue, which is not always the same as the name or spec. For example, a parachute drogue is measured as flat diameter of the cloth, the Delta drogue is closer to the diameter plus the length of the shrouds, and the Sea Brake model is the actual inflated diameter in inches.
- Towing warps (in a loop, weighted) doesn't do much. It will provide a little directional stabilization will help the autopilot. That's worth something.
- The recommended sizes for storm drogues generally give more drag than the boat itself and seem about right.
- The recommended sizes for the JSD and parachutes (not shown, but several times greater than JSD) give a LOT more drag because they stop the boat.
Do you need a drogue? If you are crossing oceans this is a topic you probably want to learn more about. If you are a coastal sailor, you probably only have need for a steering drogue, in case your rudder fails, such as hitting a log. Posts on this topic are coming, and have also been published in PS. Having jammed a rudder, I can think of some good reasons to have a functional rig even on the Chesapeake Bay. Going in circles is a drag, and unlike a car, you can't bet on getting a tow truck (cheaply) within an hour.
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More articles to follow. There were longer articles in PS in 2016.
Given that we just acquired the same drogue, and ordered the 9/16 line to go with it, I am of course very interested in this! I will freely admit though that your math is over my head. If the end result is that it functions as it claims, that is good enough for me. I look forward to doing our own experimentation this season. Thanks for the post!
ReplyDeleteMike
I'm still puzzled that Sea Brake specifies polyester line, when every other maker suggests nylon line. I would think it had something to do with stretch, but they leave the length open ended, and the longer the line the more stretch.
ReplyDeleteFor example, they say "15. Can a normal anchor line be used to tow the Seabrake?
Yes, however, minimal stretch in the line is essential. See Note 1 (9/16-inch polyester)", when clearly normal anchor line is nylon.
Is a puzzlement.
Yes, you're right. The guy at the chandlery thought I was a bit strange ordering that size and type of line for it, but who am I to argue with the manufacturer?
ReplyDeleteNow I need to splice some eyes in the line.
very interesting reading, have to let this simmer a while
ReplyDeleteIt seems all of the drogue (not parachute) makers (small shark, JSD, Delta, Sea Brake) now recommend line with controlled stretch; polyester double braid, for example. With this sort of product, it is important that load transfer to the drogue as soon as possible following a wave strike. The drogue itself will provide the required shock absorption.
ReplyDeleteDrew, I had a JSD, but in the end got rid of it. It was a large bag and the line had no other use. I opted for 300ft 3/4 double braid nylon and a 1 inch 12 braid nylon bridle. Two 20ft opening drogues in series, for a 44ft boat; 33K lbs. The rope is "s" folded into a custom "roll open" bag by which (the open bag) attaches to my aft winches. This allows an orderly deployment of the line in a similar vein to parachute lines. Never tried it, even in our big storm off NZ. My concern is this, when the boat is moving 6-9kts under bare poles and it is time to deploy, will the rope come to a sudden stop and take out my two aft cleats? Even with the stretch! Is there an orderly way to let out the line slowly so that the drogue doesn't exert a sudden and dramatic pull as would happen if the rope was let to free flow out?
ReplyDeleteCould the impact rip out the cleats during deployment? I understand the fear, but no, for 2 reasons:
ReplyDelete* The JSD does not fill all at once. During deployment the weighted end pulls the apex down some distance, so the JSD is not vertical but forms an arc. Straightening this arc absorbs energy, like the catenary of an anchor chain, and takes some time.
* The lines are designed to absorb a breaking wave strike, which is FAR greater in energy than arresting the boat from 9 knots to 3-4 knots. I did this many times during testing, and the strain is not nearly so great as you might imagine, even with parachutes. The rope really stretches.