Sunday, March 7, 2010

Drogue and Parachute Sea Anchor Testing: A Summary for Small to Medium Cruising Catamarans

Rev. 12-20-2013
Rev 4-10-2010
After my initial post on the Seabrake 24, I completed several additional tests: 5-foot parachute sea anchor, 200-foot x 3/4-inch warp. I also located additional data (Jordon Series Drogue [JSD] from 2 sources, 30x36 Galerider, Small Shark S, 48-inch Delta Drogue, and 12-foot sea anchor from 2 sources). It all correlated rather nicely, so I have posted it here. Note that I have extrapolated the table by using a general square rule for velocity vs. drag; where I had several speed data points, the fit was very good, and so it seemed  enough to justify its broad use. I revised some of the data based on field notes, older tests, and simple statistics. I've made no attempt to go beyond flat water and take readings or make calculations regarding waves; what comments I have on waves are based loosely on the Jordon Coast Guard report. I have not considered applicability to boats other than the PDQ 32. Note that the drag for the towed warp is dependent on the beam of the boat, in this case, 16 feet.

Only once before have I played with a sea anchor or drogue; I sat out a thunderstorm with a 60-inch parachute sea anchor on my Stiletto 27. It worked perfectly through some pretty intense wind; we drifted straight back at less than1-knot, seemingly anchored while I read a book below. However, for a number of reasons, I've decided to acquire a stern deployed drogue for use with this boat. I posted my reasoning here: However, I'll add a chute to my storm quiver if I come across one for a reasonable sum!

Seabrake 24 at 4.2 knots with no chain weight. Note that the lip is above the water. Click to enlarge image.
Only the last 25 feet of line is in the water.
In waves more would bury, including the bridle,
because of the bow-down attitude of the boat.
Test day. The goal was to collect some engineering numbers without looking for a storm, and to gain some practical experience in deployment, rigging and retrieving. In order to generate real-world forces, I repeated my procedure for testing the sea anchor on the Stiletto some years ago; get a running start and let the stored kinetic energy and rope stretch combine to generate moderate storm-level forces. I hitched the drogue to 150 feet of 11mm climbing rope and my standard anchoring bridle (I would not use this in a storm, I would use 2 separate lines with extra length cleated-off to allow for angle adjustments, but the anchor bridle was right at hand). I let the drogue out with light hand tension (only enough to keep it filled and off the bottom) and powered up to full throttle, which takes us to 7.4 knots. Three people on board and no cruising load.

Kinetic energy. 7.4 knots and 8,000 pounds of boat combine to give ~ 19,500 ft-pounds of energy. The drogue slowed the boat to 4.2 knots, or about 6,300 ft-pounds of energy, or a difference of 13,200 ft-pounds.

Braking distance and forces. It took about 4.5 seconds to go from 7.4 knots to 4.2 knots, during which we covered about 40 feet. Making some crazy assumptions about the stretch of 150 feet of 11mm climbing rope at this load (about 30 feet at peak load, based upon later testing of the line) and adding the on-going thrust from the engines, we saw a peak drogue load--a pretty broad peak--of about 950 pounds. The snatch load when the slack came out was not very noticeable, as the drogue stretched the line and the Seabrake expanded to its full girth. Judging from the way the knot was cinched down and having caught many long climber falls, this seems about right. It wasn't overly difficult to untie.

The running thrust of 19.8 HP (twin Yamaha high-thrust 9.9s) wide open is about 500 pounds (based upon bollard pull testing by several owners), less what ever it takes to move the boat, perhaps 170 pounds a 4.3 knots, so the engine thrust contribution is zero at 7.4 knots and is about 330 pounds at 4.3 knots.

For comparison, the 60-inch parachute allowed my Stiletto (18 HP vs 19.8 HP with the PDQ) to reach only 2.4 knots with a slightly lower thrust engine combination (the PDQ engines are high thrust, the Stiletto engine had high prop pitch to match the boat's higher speed). It's too small for storm use on the PDQ; in storm conditions it would be pulled through the water at greater speeds, and parachutes are known to be unstable at speeds beyond about 3 knots. They tend to become erratic and tear themselves apart with repeated opening and collapsing, rather like an over-trimmed spinnaker. Although I didn't see that behavior, it did oscillate from side to side a tiny bit, unlike the Seabrake which tracks very straight. I have kept the parachute for back-up and for use on the dingy.

From "Tuning an Anchor Rode" in reference section at bottom of post:

Boat Type: Catamaran
Length 32.0 ft

Wind speed 40 kt
Static mode tension 671 lb
Dynamic over tension with mixed rode 1476 lb 

Wind Speed 60 kt
Static mode tension 1510 lb

Dynamic over tension with mixed rode 3322 lb

The AYBC Anchor Guidelines:
Suggests about 2400 pounds for the PDQ 32 (the above calculator showed that a 40-foot monohull was equivalent to a "typical" 32-foot catamaran, whatever that means) for a storm anchor system.

Actual expereince? In a 20-knot breeze I do about 3.6 knots under bare poles, or about 125 pounds of hull drag, using my figures. The "Tuning Anchor Rode" reference gives 115 pounds for a 32-foot cat in the same wind, which is very good agreement. Waves contribute in both cases, which is difficult to model.

Rough theoretical calculations can also be made: Drag = 0.5(drag coefficient)V^2(area)(density) with units in kg and meters. Remember that parachutes are smaller than their nominal diameter when inflated, and other devices may expand when pulled (Seabrake/Small Shark) or have differing drag coefficients. These calculations were not used to create the table, but were used to reasonableness-check parachute test figures, which in turn were used to cross check other test data. No significant conflicts were found.

Below is the expanded data set, collected either from references (JSD, Galerider, Delta Drogue, Small Shark) or trial (5-foot parachute, warp, Seabrake) in a manner similar to that described above. I consider the data to be +/- 30% accuracy, while the comparisons between units should be more accurate. The data for the 42" x 48" Galerider was extrapolated from the smaller unit and corrected for size; both the frontal area ratio and the cylinder area ratio were very close and the average value was used. However, we have 2 data points (Sail Magazine and measured storm expereince) and they do not match. The storm drag figure on the larger size reported 3,500 pounds at 8-9 knots, though the correlation suggests that the bridle. rode and drogue would have produced only 1,500 pounds drag at that speed. Given the 30% error bar I believe to be probable in this comparison, a speed of 10.5 knots would have been predicted to generate 3,500 pounds of combined drag.

So what have we learned about the Seabrake 24? You can see in the photo that the lip of the drogue is above the water a bit; it needs some chain to help keep it in the water, though at these loads no practical amount of weight is going to take it down. I think the main purpose of the chain is to keep it under the crest of a breaking wave. Our speed reduction (3.2 knots vs. 1.85 knots) was greater than achieved in Sail Magazine's May 2008 tests, in part because we had a higher speed boat (7.4 knots vs. 6.0 knots), while limiting speed was the same (4.3 knots). Retrieving it seemed simple to me - it came in easily hand-over-hand with the vessel stopped - but that could be my perception because my experience with the sea anchor (when retrieved without using a trip line) was far worse and because the steps on the PDQ make a nice working platform (I didn't stand on them, but I did drag the Sea Brake up onto them, to allow it to drain). I won't use a trip line on the Seabrake. It also made it clear the that amount of drag increases very greatly with speed. At 1.5 feet per second (a fast hand-over-hand recovery rate - a bit slower is more comfortable), the drag was about 18 pounds. Extrapolated to greater speeds, we should see 1,800 pounds at 10 knots and 7,500 pounds at 20 knots.  That should keep her tail in the water and prevent surfing even during a pretty good wave strike, while minimizing the impact by giving a little bit. Storm waves typically travel at 20-25 knots; the PDQ will sail at 12 knots on flat water without being significantly bow-down; faster than that, I'm told, but that lies beyond my personal expereince.

What is the actual strength of construction? I took a good close look a the nylon tape and stitching, and I would guess each of the 4 legs would be rated at ~ 2,000 pounds breaking strength, or about 8,000 pounds total. The manufacturer recommends towing it with 9/16-inch polyester line which should be a reasonable match.

Can we just chuck a Seabrake off the stern, like the JSD, while the boat is being blown down wind at hull speed? The instructions state you should "lower off the side at very slow speed", but we know that isn't practical in a full-blown storm. I'm sure the advice depends on the type (sail vs power) and size (4 tons vs 10 tons) of vessel. How would you slow down? By placing your boat broadside to breaking waves? Well, it seems with the PDQ 32 you can just dump it over at 7 knots, and with a generous safety margin. Of course, the bridle must be well arranged, the line flaked to run smoothly, the line must be at least 150 feet long and have some stretch, and light hand (gloved) pressure should be kept on the line as it streams out so that it neither snarls nor yanks the Seabrake at an unfair angle when the slack runs out. However, the 1,100 pound load we saw was tame and the equipment can easily handle triple that. Of course, should you try some very low stretch line like Amsteel, then all bets are off. I am speaking ONLY to nylon and moderate stretch polyester double braid (which would give 2-3 times the impact load - watch your speed a bit with that stuff). Another trick that reduces the impact load during deployment significantly is to slowly turn about 30 degrees while playing out line, so that the drogue has to turn and the line has to straighten when the strain comes on.

What does the expanded data table tell us? Without a long discussion of sea state, which I am unqualified to lead, it gives only a few clues as to what we might find in a 50 knots breeze:
  •   Since wind and non-breaking wind forces will not exceed 1,000 pounds (10-15% of the displacement--the result of wind pressure, the slope of the wave, and particle rotation) for this boat, when boat drag plus device drag equal 1,000 pounds, that is the speed limit. Breaking wave forces raise that limit to perhaps 5,000 pounds (Jordan CG study), then that speed might be reached in a strike, though acceleration takes some time an the result would be less in most cases.
  • Any devise that will allow surfing (speeds approaching wave speed of 20 knots) before providing 5,000 pounds of resistance will not prevent pitch-poling or capsize in a severe wave strike. This qualifies only the Seabrake, JSD, and parachutes, in my opinion. The other devises could serve this purpose, if sized above builder recommendations in order to give similar drag. Experience shows that cats moving faster than this can dig into a bad set of waves and have the stern lift by this force, unless an offsetting force is holding the stern down and back. There is certainly a range of answers to this question, but there is also a large gap in the capabilities of the drag devises. 
  • A 12-foot chute will hold the PDQ at 1-knot drift.
  • A JSD will hold the PDQ at 6-knot drift. I've heard much less, about 2.5-3.5 knots, but the published drag data don't support this assertion (Sail Mag study and Jordon CG report data below in reference section). Sustained winds near the water surface of 40 knots would produce a 4.5-knot drift.
  • A Seabrake will hold the PDQ at a 7-knot drift at 50 knots sustained, and a 5-knot drift at 40 knots sustained.
  • A 5-foot chute holds well, but I'm not certain it would withstand the shock loads. It could be pulled through the water a bit to fast for the design (3 knots and faster in a wave strike) according to this calculation, but it is probably slower also, so it should be useful for parking in moderately bad weather, better than the Seabrake.
  • Towing a warp is probably a waste of time in really severe conditions. It is only to slow down in a bit in moderately strong conditions to reduce surfing, and to give the auto pilot some help. It is likely that towing warps is more effective on catamarans because the beam spreads the loop and because they are MUCH lighter per unit of beam. Still, they're not likely to help in the event of a large wave strike. You would need to trail far more warps than you will likely have line on board (3 x 200 feet in staggered lengths). See reference below.
  • I extrapolated the Delta Drogue, Gale Rider, and Small Shark data from Sail Magazine (May 2008) tests--I don't know much about them, beyond what I have read. All are well-proven. All are manufacturer sized for this boat, though for a different purpose--slowing and steering assistance rather than protection from breaking waves. They are lower drag devises than the chutes, Seabrake, or JSD when sized as recommended.
  • Boat speed must be reduced to 1-knot to retrieve the high-resistance drogues easily. 
  • Chain. I tested the drogues without chain, for convenience and because I was in shallow water (7 feet). In practice, weight is needed to keep the drogue under the particle rotation of a wave, or at least 12 feet. Given the average load for the PDQ 32 in 60 knot winds (50 knots at the surface, gusts and waves don't count because the drogue surfaces slowly and they will average out, the drag of the hull must be subtracted) will be about 400 pounds, that 240 feet of rode are deployed, and that chain weighs about 65% as much underwater, about 400x12/(240x0.65)=30 pounds of chain will keep it down. That would be about 40 feet of 1/4-inch G4 or 20 feet of 3/8-inch proof coil. Seabrake recommends 8 feet of 3/8-inch chain, but user experience seems to suggest that may be light for extreme conditions. I think a little more sounds better. Adding weights or chain in the center of the rode has been tried with sea anchors--however, the center of the rode jumps around a lot, the weigh can cause damage, and center weight is no longer recommended. The Small Shark recommends adding weight to the tail and has a fitting for that purpose; that seems to make a lot of sense.
  • Where you are in the storm should be considered. If I am running east in a west-to-east storm, slower is better because the storm will pass me more quickly. I will rig the highest drag devise I have. If I am running west in a west-to-east storm, the quickly I move the more quickly the storm passes. I might be satisfied with towing warps, if they are enough. I should also be sailing toward a less dangerous portion of the storm. Lots to think about.
Note:  There are many other differences--drag is only a part of the picture. The JSD functions differently because it has many elements. Parachutes work best off the bow, while the others are stern deployed. Real world drift rates are much lower than flat water predictions due to lower wind speeds at deck level because of wave effects, time averaging, and conservative assumptions about windage. Read-up.

What will I investigate next? I'm curious about the potential for using a very long bridle (about 60-80 feet for each leg, instead of 20 feet) made of over-size line, perhaps 3/4-inch. While on flat water or when passing over a swell, a conventional bridle and most of the rode are suspended above the water by the tension on the drogue. However, when a wave begins to overtake the boat, a longer bridle is pulled solidly down into the body of the wave and acts as a heavily weighted towed warp, providing some limited resistance close behind the boat, where the effect is more immediate. Ordinarily warps must be weighted and towed far behind to insure that they will not be caught in the same breaking wave and thrown forward; in this situation the Seabrake will hold the rode down and back.  It will remain planted in a portion of the wave where the particle rotation (down and back) will help maintain the bridle in green water and will add about 2 knots of apparent through-water speed, increasing the drag above measured flat water values. If the boat surfs to 10 knots this force rises to about 500 pounds, which when added to the 1860 pound drag of the Seabrake brings the total drag (Seabrake + oversize bridle) very close to that of the JSD, a very interesting result. Surfing will be prevented, even under the influence of a good size wave strike. The down side? At some point a long bridle can tangle about itself, but this is less of a problem on multihulls, due to significant beam.
    I'm planning to use a 5-foot parachute for moderate-weather parking (because I already had one), the Abbott Drogue for a little speed control and steering assist, as needed, and the Seabrake 24 when the waves get scary.

    While I generated this information for my own use, I thought you would like to know.

    Coast Guard Study of Jordon Series Drogue. Definitely the ultimate drogue design. However, even if you are interested in a different drogue design, much of the simulation and engineering work is applicable.
    The below was an on-water test with a 40-foot motor lifeboat.

    An eariler Jordan report:

    Abbott Drogue. A good discussion of using warps in moderate storms.

    Drag Devise Database. A book of experiences on sea anchors and drogues, 20 years in the making. The author has a financial relationship with Paratech; I do not believe it has distorted his reporting of facts, but he does seem to report more about Paratech products and have some positive bias towards parachute anchors..

    Sail Magazine test, May 2008

    Testing several drogue types

    A long thread on a Cruising Forum, including some manufacturer feedback

    Paratech Frequently Asked Questions

    Tuning an Anchor Rode
    (discontinued... but posted here

    Small Shark Drogue

    Delta Drogue

    Paratech Parachute Sea Anchor




    1. 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!


    2. 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.

      For 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.

    3. 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?

      Now I need to splice some eyes in the line.

    4. very interesting reading, have to let this simmer a while

    5. It 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.

    6. Drew, 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?

    7. Could the impact rip out the cleats during deployment? I understand the fear, but no, for 2 reasons:
      * 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.