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DELTA DROGUE TM
Speed-Limiting and Steering Assist Drogue to Small Craft
There is a direct relationship between sea state tolerance (survivability) and vessel alignment. When a boat is properly aligned with the seaway its sea state tolerance increases dramatically. When it falls out of alignment survivability is reduced to the same extent. The problem is this: boats will not remain aligned with the seaway unassisted. (A quick reminder that most of the random/catastrophic events that characterized Fastnet '79 were caused by a lack of proper alignment with the seaway). Boats must be forcefully kept in alignment by some sort of drag device, sails or an engine. Making the right choice in different heavy weather situations is what we call "Seamanship".
|DROGUE OR SEA ANCHOR?
Sea anchors are used to align a vessel with the seaway bow-to, that is, facing directly into the wind and the sea. Since all boats are designed to take the seas on the bow a sea anchor should be a sailor's FIRST LINE OF DEFENSE. in other situations a suitable drogue may be towed off the stern to keep the stern aligned with the seaway as she is running or quartering the seas downwind. The two options differ widely. The proper use of a sea anchor off the bow should not be confused with the proper use of a drogue off the stern. The dichotomy between the two concepts along with documented case histories has been combined into a publication entitled the Drag Device Data Base (available from Para-Tech). Sailors planning extensive offshore passages should obtain a copy with a view towards more in depth study of the different applications of both devices.
A speed-limiting drogue is towed off the stern. Its purpose is (A) limit the speed of the boat, (B) reduce her angle of yaw and (C) provide for steering assist in strong following seas. It is a LOW PULL device that allows a fair amount of directional control through the helm. You can, and may have to, steer the boat with the drogue in tow. It follows that there is a "Catch 22" associated with any speed limiting drogue: The same low pull that allows for steering control may also allow the boat to broach, capsize and pitch-pole in the heaviest seas. The forces that pitch-pole yachts are formidable enough to yank the drogue through the water in the course of throwing the boat end-over-end. Sailors should be aware of this "Catch 22" and make allowances for it in the course of their decisions at sea.
WARNING: in all likelihood the insufficient drag of a speed-limiting drogue will not prevent pitch-pole in the heaviest of storms, especially those packing ESW's (Extreme Storm Waves) rogues, and "Three Sisters" type developments.
FOUR USES FOR DROGUES:
1) The above notwithstanding, speed-limiting drogues can be of immense value in strong following seas and have been used to stabilize the attitude of small craft since antiquity. No doubt there are many yachts sailing today that would have gone down had it not been for the grace of God and some sort of drag device that slowed them down.
2) Towed off the stern with a bridle a speed-limiting drogue can be used as an emergency steering device in the event of steering failure.
3) Towed off the windward quarter a speed-limiting drogue can be used to help maintain directional stability while negotiating dangerous harbor entrances.
4) Towed off the stern of a vessel in tow a drogue can all but eliminate the "whiplash" effect.
On a downwind leg in stiff Trades, for example, a speed-limiting drogue will turn an uncomfortable roller-coaster ride into a "Sunday afternoon sail".
Properly deployed a speed-limiting drogue will significantly increase the efficiency of your auto-pilot. Used in conjunction with a bridle it may allow you to go down below and get some rest. Used without a bridle (with the towline brought well forward of the rudder) it may allow you to steer through an arc of 90°, usually enough to help you avoid obstacles, headlands, etc.
Weight and storage volume are important considerations on small craft. Drogues made from textiles provide more drag area per pound of weight & storage volume than solid devices. The drawback has always been "stability". Existing textile cones, for instance, are very unstable under tow. They tend to kite, zig-zag, squid and flutter. The principle cause of thislnstabllltyhas to do with the non-uniform wake that forms at the separation edges of the drogue, producing unstable side-to-side motion. Flow-thru basket like fabrications are much more stable but require some sort of hoop or metal ring to keep the drogue mouth open.
Para-Tech's approach to this age-old stability problem follows a solution already in use by the parachute industry. The parachute industry long ago encountered and solved the problem. One solution was the introduction of a canopy with a "guide surface" to create a strong, uniform airflow separation around the leading edges (similar to using rear spoilers on a car). The angled leading edge of a guide surface chute also creates a massive normalizing force. This normalizing force combines with the stable drag force to create an extraordinary stabilizing moment when the drogue is displaced form its zero angle-of attack position.
NEW' DELTA DROGUETM
Para-Tech's new DELTA-DROGUE TM uses this "guide surface principle". The basic design is a triangular "Delta" shape that provides strong uniform flow separation at all leading edges. Shape retention is achieved by the pressure of the water inside (similar to the "Ballute" drogues used in Aerospace recovery applications). The relatively small mouth at the leading edge of the drogue makes it highly unlikely that one will ever get turned inside out (a problem associated with cones and small parachute drogues). Additionally, the three vent holes of the drogue have the effect of increasing the drogue's drag as speed increases.
Notwithstanding Para-Tech's DELTA-DROGUE is SIMPLE in both design & construction. So simple that it is made from very heavy (14 oz) vinyl coated fabric at an affordable price. The drogue's three bails are made of heavy nylon webbing and terminate with a stainless steel swivel (included in the basic price).
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