Difference between a drogue and a sea anchor
Traditionally, the nomenclature used for describing an external in-water braking mechanism for sea-going vessels, particularly yachts, has been that of a “sea anchor”. These devices came in various shapes, sizes and forms, ranging from one or more car tyres attached to the end of a line off the stern or bow to more sophisticated self-opening parachutes attached to a long enough line with an additional weight to allow them to remain submerged. The main aim of such sea anchors was either to halt as much as possible the forward or astern movement of the boat. This would be necessary, for example, in a strong tidal stream or when the vessel was approaching a lee shore in water that was too deep to deploy a traditional anchor (main or kedge). Another situation where a sea anchor might be used is when one wanted to have the vessel to hold its position facing into oncoming waves rather than having to contend with waves impacting broadside onto it with the attendant risk of capsize, or at the very least a very uncomfortable ride for the crew. Either way, the intention of a sea anchor is basically to stop the vessel and as such it makes no sense having any sails up or the motor running as the anchor should hold the boat more or less in one position, depending on where the line has been attached to on the boat, and the vessel will then drift downstream together with the sea anchor in the direction of the current.
A drogue, on the other hand, is a device that is intended to slow the motion of the vessel rather than trying to stop it in its tracks and is typically always attached to the stern of the vessel. A drogue can be as simple as one or more warps trailing behind the vessel to the more modern series drogue which consists of a large number of small parachute-type anchors attached to a line connected to the stern of a vessel. The towing of warps from the stern of a vessel in order to slow its progress while surfing down big waves has been used on ocean going vessels as a storm survival technique since time immemorial and pre-dates the use of parachute-type sea anchors deployed from the bow. Myriad devices have been used for the purpose of slowing a vessel down: lines with bottles attached or indeed a warp with car tyres attached to it could also be considered a drogue, which is possibly one of the reasons why there has been a fair amount of confusion over what constitutes a sea anchor and that of a drogue. The defining difference however, is that a sea anchor is intended to bring a vessel to a halt whereas a drogue is intended to slow it down. Whereas a sea anchor that has been right-sized for a particular vessel typically reduces its speed to less than 2 knots through the water, a drogue that has been right-sized for a particular vessel will reduce the vessel’s speed to around 5 knots and can be used in conjunction with some sail up – most often reefed substantially in the conditions one would typically encounter when needing to use a drogue.
Although a sea anchor can be used to point a boat in a desired direction in the event of the steering being compromised, depending on where it has been attached to the boat, a drogue that has been properly deployed for the purpose can also be used to steer a boat that has lost its steering in order to allow the crew to carry on sailing. Indeed there are many examples of skippers who have successfully used a drogue in this manner, some of whom have sailed their boat several thousand miles to safety after losing their rudders.
Numerous skippers, particularly those of catamarans, have reported that lying-to a sea anchor in big seas and strong winds can result in the vessel tending to swing wildly in a yawing motion that leaves the boat abeam of the waves on occasion, which is exactly what one wants to avoid. Such a yawing phenomenon, also known as shearing, can be very dangerous because if a wave hits the bow in this shear cycle the boat can be forced backwards, resulting in any one of a number of issues, including; damage to the rudder, capsizing due to being rolled over, the sea anchor rode being broken or water being forced up through the vessel’s exhaust into the engine.
The yawing action mentioned above can be reduced by deploying a sea anchor a sufficient distance from the boat and by using a bridle rather than a single line, but a contributing factor to the yawing is the tendency of the sea anchor to snatch at the line as it collapses and fills in the wave motion. It is usually in the trough of a wave or swell that the parachute will collapse and the vessel will then start to yaw. In the event that the yawing causes the vessel to lie ahull and therefore beam-on to oncoming waves it could very well be knocked flat or rolled right over by a large breaking wave, as has happened to numerous vessels subjected to such waves under these particular circumstances.
A modern yacht will tend to yaw away from the wind when the towline goes slack, even when using a large sea anchor and it is for this reason that the U.S. Coast Guard Report No CG-D-20-87 recommends that parachute sea anchors are not to be used in such sea conditions.
A series drogue, on the other hand, doesn’t suffer nearly as much collapsing of the cones as firstly, they are much smaller than a sea anchor at about half a foot in diameter compared to 10-12’ for the average-sized parachute on a sea anchor and secondly, a series drogue’s cones lie distributed over a distance of at least 50m; whatever wave action is acting on one section of the drogue is highly unlikely to be acting on the other parts of the drogue. In fact there is a good chance that another section of the drogue is experiencing wave action in the opposite direction to that of other sections. This is partly what makes the series drogue so stable under these conditions. Another problem reported with sea anchors is that they can twist and end up collapsing as a result. A good quality swivel and sufficient number of support lines attaching the parachute to the swivel will help prevent this but should one of the lines get entangled in the swivel then it is highly likely that the swivel will get jammed. A serried drogue, however, is a much simpler construction consisting of only one line for all the cones with no mechanical devices to get jammed. A sea anchor’s parachute that repeatedly collapses and fills also puts a great deal of strain on the fabric used to construct the parachute, with the result that it can tear apart more easily and with only one parachute to rely on the sea anchor is then effectively rendered useless. A collapsing parachute also puts a huge amount of strain on the deck fittings it is attached to – usually the cleats, as it snatches at them when refilling which also causes chafe on the lines. It can be very disconcerting sliding astern in a big wave whilst waiting for the parachute on a bow-deployed sea anchor to refill and hoping that it hasn’t torn.
One technique successfully used with a sea anchor is to heave-to while it is deployed, however conditions don’t necessarily always suit heaving-to, especially in really big seas, and conversely one doesn’t always want to heave-to and sit out the bad weather in milder sea conditions, preferring instead to make some way towards one’s destination. Heaving-to can by all accounts also be very uncomfortable for the crew in very confused seas and sailing with the direction of the waves, albeit at a reduced speed by using a series drogue deployed off the stern, can be a lot more comfortable and safer for everyone on board. Furthermore, not all yachts heave-to successfully and in really strong winds it is sometimes necessary to take down all the sails (sometimes even taking off the boom in order to reduce windage, especially if it has a stack pack for the mainsail) and resorting to bare poles, in which case heaving-to is no longer an option without headsail of some sort to provide steerage for the vessel.