Wave motion

To understand why a series drogue is necessary and a conventional sea anchor (parachute or otherwise) is not always suited, and in fact can be downright dangerous, under certain sea conditions – such as breaking waves, one has to understand a bit about wave motion.

The water particles making up a wave at sea don’t actually travel across the ocean with the waves. They just go round and round in a wave, rotating in the direction that the wave is “moving” and passing their energy on to the particles in the next wave, which in turn are also just going round and round. The bigger the wave the further these particles travel from the top to the bottom of the wave but they then travel back up to the top of the wave to repeat the whole process all over again. The wind is responsible for the direction in which the waves “move” and anything on the surface of the wave will be driven along the surface by the wind. Of course there is also current to consider which can result from tides or the rotation of the earth – as is the case with the big ocean currents that travel up and down the sides of continents, always in the same direction since the earth always rotates in the same direction of course. The swell at sea is caused primarily by the wind but whereas the wind systems will typically rotate clockwise or anti-clockwise (depending on whether they are high or low pressure systems and also whether they are in the northern or southern hemisphere – where such systems act in opposite directions to each other depending on the pressure) the swell tends to carry on moving in more or less a straight line because of the denser medium of water.

Consider then a vessel sitting in big seas with waves over 6m high and a sea anchor deployed. Depending on where the sea anchor is in relation to the boat at any one time it could be in one of several places: it could either be sitting in the same wave that the boat is currently on, if the line to the sea anchor is very short – i.e. less than 12m or so long; the sea anchor could be sitting in the trough area between waves, or ; it could be sitting in one of the next waves approaching if it is deployed at a length that so happens t be a multiple of the wavelength of the waves concerned.

If it is in the same wave as the vessel the sea anchor could be moving up the one side of the wave whilst the vessel is moving down the other side of the wave and as such the sea anchor would effectively be doing very little to arrest the motion of the boat. Likewise, if the anchor is in another wave one or more wavelengths away from the wave that the vessel is in then exactly the same scenario could once again occur. You would need to know exactly what the wavelength of the waves is in order to deploy the sea anchor at the right distance from the vessel. But then not all waves are created equal of course and it is impractical if not impossible to keep adjusting the length of the line to the sea anchor to keep it in the right part of a particular approaching wave. Similarly, the sea anchor could be in the relatively motionless dead zone between two waves and apart from the risk of the sea anchor then breaching the surface, rendering it useless as a brake, the vessel could also be caught in the trough of a wave and with little or no directional force on the vessel from the sea anchor the boat could swing broadside into another advancing wave.

Add to these scenarios the one major risk of mountainous seas and the thing that all skippers dread – breaking waves – and you can have the makings of a calamity on your hands if you can’t prevent your vessel from being picked up by such a wave and tumbled into the trough with the white water.
It is in circumstances like this that a series drogue really comes into its own. The series drogue for a typical 40’ yacht, for example, consists of about 100 mini windsock-type cones attached to about 50m of line with an additional leader of about 25m of line before the first cone. A weight is attached to the end of the drogue. This means that a good number of the cones are always submersed in the water regardless of the wave size or length with very little chance that the bulk of the cones will either be in the same wave as the boat or in the same part of an advancing wave – see diagram on this page. The leader ensures that the drogue is predominantly in following waves and the weight on the end causes the drogue to adopt a “J” shape lying on its side so that the cones are held deep in the bottom part of any waves. By having so many cones there is a greater likelihood that most of the cones are exerting a drag on the vessel and if any were to get damaged for any reason, there are a lot more to continue doing the work, unlike a sea anchor which usually relies on one parachute that when broken is rendered useless. The weight on the end of the series drogue also allows the drogue to act as a type of resistive shock absorber, similar to a coiled spring, as it straightens out under load and then reforms its “J” shape when the load is released. As the tension on the drogue increases, the weight lifts and provides further resistance in doing so, only to sink again as the tension reduces. The weight also causes the cones on the drogue to semi-collapse when the line goes slack, as tends to happen when the vessel is in the trough of a large wave, so that the weight sinks more easily and when the cones fill again and the tension is taken up there is far less lack in the line, resulting in less snatching.

The series drogue is always attached to the stern of the vessel and the stern cleat backing plates typically need to be beefed up to accommodate the extra forces that the drogue exerts on these cleats. The bow cleats on most vessels are usually designed to cater for large forces exerted by its anchor and chain but the stern cleats don’t typically get subjected to such forces. It is also possible to attach the series drogue to the cockpit winches after running the line through the stern fairleads and in fact this is how one can position the series drogue to steer the boat under sail or motor in the event of a steering failure. Deploying the drogue is relatively easy and ideally needs to be done before the boat is racing down a wave face. Retrieving the drogue is also quite straightforward using the floating retrieval line which brings the drogue in backwards with the cones presenting their smaller openings to the direction in which the line is being pulled. Parachute-type sea anchors, by comparison, are extremely difficult to retrieve in other than moderate sea and wind conditions.

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