Don't pull your boat's drogue into thin air
The caption for the illustration on page 58 of my article on streaming a drogue from a catamaran offshore ("Learn by doing," Issue No. 92, Sept./Oct. 1998) was incorrect. It stated that a drogue needs to be in the same part of the wave train as the boat (e.g., the drogue should be in the crest of the wave if your boat is in the crest).
According to the Drag Device Database by Victor Shane, "Try to position the drogue behind a wave so it will not pull out. When the boat is moving down a crest the drogue should be behind a matching crest so that it is being pulled through the meaty part of the wave and not pulled into thin air."
You really want the drogue in the part of the wave train opposite to the boat's (e.g., the drogue should be in the trough when your boat is on the crest). This way the drogue will develop maximum drag.
John Kettlewell is a freelance boating writer, editor, and photographer.
Earl Hinz comments,
We often mistakenly use the term drogue to represent any drag-producing device deployed off the stern of the boat. In my book Understanding Sea Anchors and Drogues, I campaigned hard for the boating community to recognize the difference between a sea anchor and a drogue. Now I think we have at least a 95% acceptance of the difference, but a new problem has arisen. Now the industry has come up with new concepts for drogues, further dividing them into two discrete performance classes: speed-limiting and boat-stopping. The Galerider and Delta drogues are typical of the former, and the series and Shewmon parachute drogues are typical of the latter.
The number-one problem in using any drogue is to deploy it in such a way that it doesn't surface at any time. While it has become dogma to say the tether should be more than one wavelength long to ride in the meaty part of the crest, that bit of advice applies only to the boat-stopping parachute-type drogueand even then it is only part of how you prevent surfacing of the drogue. It clearly doesn't apply to the series drogue, nor does it apply to the Galerider and Delta speed-limiting drogues.
The problem with that one-dimensional view of drogue behavior is that the sea is not the simple cycloidal shape we think it to be. The Fastnet storm of 1979 and the Queen's Birthday Storm off New Zealand in 1994, to name two, attest to that. While the simple theory indicates you can position a drogue on or in a wave at a particular place to yield a constant drag force, you cannot be assured it will stay there without being weighted to also keep it submerged.
An un-weighted parachute-type, boat-stopping drogue is likely to pop out of the water whether it is +/- one wavelength behind the boat. This is because, contrary to idealized sketches, storm waves in higher Beaufort number storms are not very well behaved. I would venture to say in most Beaufort force 9 storms and above, there are multiple trains of waves coming from several directions.
An adjustable-length tether with a chain segment between the rope and the drogue is the only way you can be assured that the parachute drogue will stay underwater doing its job.
It is significant that the boat-stopping Series drogue, which started life un-weighted, soon added a weight to its trailing end. Speed-limiting drogues also use weights, but there is another interesting aspect of their deployment. They are often positioned close behind the boat at much less than one wavelength from the transom. They would (and do) tumble if there is no weight present.
When you are actively running with a speed-limiting drogue, it is good to keep track of how your drogue is behaving. You can do that if it is not too far away and if it is weighted.
When you deploy a boat-stopping parachute drogue, you want to deploy it on a long tether (weighted), maybe as much as two wavelengths away, not only to assure its effectiveness, but also to attain a measure of tether elasticity to ease the shock loads on the stern of the boat.