Thursday, July 13, 2017

Why Build a Drogue

Around this time one year ago, I was, what felt like 500 months pregnant (in reality, 35-36 weeks) and decided to build a drogue. We had actually decided to build a series drogue a couple of years earlier and had been carrying around the materials to make it since then. I was determined that if it was not made before the baby came, then we were getting rid of all the materials and never having a drogue

Irrational? Maybe! But the bits and pieces for the drogue were taking up precious space on this boat!  And besides, I was pregnant! While I did the actual building of the drogue, Bryce was the brains behind the research and design so I'm going to hand it over to him to share all his drogue-y knowledge! This is part 1 of 2. Part 1 will cover why we decided to build a drogue and Part 2 will cover how we actually built it!

With modern weather monitoring services, it is extremely unlikely that we would ever be in the situation where we would need to use our drogue. BUT, if a nasty weather system were to pop out of no where and surprise us, we want to be prepared! So just keep that in mind as you read what Bryce has to say! :)


This is Bryce now!

As some background, the series drogue is a "series" of cones that are streamed out from the back of the boat to slow the boat down as our last line of defence in a "survival storm" scenario (i.e. a storm so severe that the only objective is survival).  The main features are: Once deployed it requires no tending in hideous conditions, it prevents the boat from being capsized or rolled, and allows all crew to go below-decks and hunker down until the nasty weather passes.  In many ways, it is like an airbag in a car - you hope you never need it, but it might just save your life if it all goes very wrong.  A lot of my thoughts on this topic have been heavily influenced by (see there for a thorough analysis).  A very brief analysis of the options follows.

There are other techniques that people use for dealing with a survival storm, mainly consisting of "drag devices" .  They all work by slowing the boat down, and ideally stopping the boat from being thrown off the face of a large breaking wave (the boat landing in the trough of the wave, possibly tens of feet below, is the main issue, as is the rolling of the boat that will likely follow this scenario).  If a single drag device is deployed and fails at a critical moment, it fails spectacularly and there is no backup.  Further, any single unit could feasibly pull out of the face of the wave that it is dragging through, again potentially resulting in the associated free-fall and smash at the bottom.  The solution to these issues seems to be using multiple drag devices, but streaming several independent drag devices in these type of conditions is a recipe for them to tangle and become ineffective.  A "series" drogue has many independent cones performing light duty; the failure of several cones will be of little consequence.  Of course, there remains the issue of a single point of failure in the line and attachment points, so these need to be of high quality.

Assuming then, that a "series" drag device is the answer, the question of where to deploy it from is next.  Deploying from the sides of the boat is out of the question, as that would present the largest area to the wind as well as leaving the boat sitting broadside to breaking waves, inviting a capsize.  That only leaves us with the option of deploying from the the bow (a sea-anchor) or the stern (a drogue).  For us, a bow deployment was inferior.  I know from anchoring that when we get a little bit of the bow off the wind, that will expose the side of the boat to the wind which pushes us further in that direction until the anchor line comes tight and we get whipped around so the other side of the boat is now presented to the wind.  This process repeats!  At anchor, where we are mostly protected from any significant wave activity, this is only an annoyance.  In a survival storm, however, these loads will be ferocious and present an unacceptable risk of either a broadside wave-strike after the bow gets pushed around, or else the sea-anchor line chafing through from the constant yawing back and forth.  Contrast that with the behavior of the boat with the stern into the wind.  In this position, the boat is remarkably stable, and the opposite behavior occurs (i.e. if the bow finds its way out of position, the wind quickly snaps it back into place).  Additionally, the wider transom provides a platform to use a bridle rather than a single line, further accentuating the tendency to sit still and stable whilst stern-to in a strong wind.  

So now that we've decided on a series drogue, we need to look at some of the specific risks.  One risk that has been identified is the potential for a pooping wave to flood the cockpit and risk flooding the cabin through the companionway hatch.  From first-hand reports, this concern isn't especially significant provided a couple of factors are addressed:  
a) The cockpit drains quickly (ours has VERY large drains), and 
b) the stern has lots of buoyancy (ours does), and 
c) the companionway has a good hatch cover that is in place when the drogue is deployed (ours does and will be).  

Also, as alluded to above, the attachment points need to be very strong and chafe-free.  For our boat, each attachment point must be able to withstand forces of about 9,000kg.  The attachment points are still a work-in-progress for us, but we will upgrade this before we do any serious offshore sailing.  

That's all for now! Stay tuned for Part 2 in this series where we will share exactly how we built the drogue!

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