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Speed-Limiting and Steering Assist for Small Craft |
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OVERVIEW: 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 radom/catastrophic events that characterized Fastnet '79 were caused by 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 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 in 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. SPEED-LIMITING DROGUE 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) rouges, 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 winds, for example, a speed-limiting drogue will turn an uncomfortable roller-coaster ride into a "Sunday
afternoon sail". DROGUE STABILITY 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 this "instability" has 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 |
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