SailNet Community - View Single Post - series drogue or para anchor?
View Single Post
  #11  
Old 09-06-2013
JonEisberg JonEisberg is offline
Senior Member
 
Join Date: Dec 2010
Posts: 3,397
Thanks: 0
Thanked 113 Times in 101 Posts
Rep Power: 4
JonEisberg will become famous soon enough
Re: series drogue or para anchor?

Quote:
Originally Posted by smackdaddy View Post
It took me a LONG time to become a believer in the JSD for that very reason (stern-to waves seems wrong). I'd read the Pardy book as well and saw the "common sense" in the bow-to approach.

However, after lots of research, and especially after reading Hal Roth's "Handling Storms at Sea", a more recent heavy weather book than the Pardy's, with its very meticulous comparison of the various tactics...I became a believer.

I also agree with Jack above on the issue of how your boat is set up to handle a boarding wave. Ours is pretty well set up to deal with such an occurrence (relatively small and shallow cockpit, top-down companionway entry - not vertical with large hatchboards, pretty good drains, etc.) - to a degree of course.

Anyway, all this is certainly hypothetical for me. And I hope it stays that way. But I definitely trust Hal Roth's take. So the JSD for me.
Let me be clear, I have the utmost respect for the Pardeys, what they've accomplished, and what they've learned and passed onto other sailors... However, I feel their recommendation regarding sea anchor tactics are very specific to the sort of full-keeler they sail, and have become outmoded when applied to the more moderate or split underbody designs most of us sail today...

If anyone has managed to get their bridle system using snatch blocks to work during a storm at sea, I'd love to hear about it... Hell, I've never managed to get their setup using a pennant riding on the anchor line with a snatch block to work at anchor, much less in large, confused seas offshore :-)

I've laid numerous times to a bridle in open roadstead anchorages, or harbors with a surge, in an effort to reduce rolling... Baracoa, Cuba, is a classic example of a place where the ocean swell wraps around into the anchorage at a deep angle to the prevailing wind, without a bridle the rolling would be very uncomfortable...





But in my experience, the only thing that works is to have the pennant FIXED to the main rode, and adjusted at your primary winches. Trying to use a snatch block as the Pardeys claim they do, it's just a matter of time before it moves up or down the rode, and I'd invite anyone who has had success using their setup in heavy weather to explain how they managed it... I suspect many who endorse this techinque have perhaps read Lin & Larry's book, or watched their video, but have never actually tried this approach in heavy weather offshore :-)

Evans Starzinger does a pretty thorough job of de-constucting the 'myth' of the viability of the use of a parachute in this fashion, although the bolded emphasis is mine :-)

Quote:

9a. How about the bridled para-anchor technique?

As mentioned above, we generally think the para-anchor tactic the least useful because it prevents you from sailing away from the worst weather and is difficult to change/adapt as conditions change. The bridled para-anchor deployment approach is the most complex possible way to deploy the para-anchor, with the most points of potential failure and human error. Those are bad traits when fatigued in severe storm conditions. Virtually every crew we know who has tried the approach in real storm conditions has considered it a failure. So, we like this 'bridled' approach even less than the 'over-the bow' approach.

We believe the bridle approach violates three fundamental principles regarding breaking wave tactics.

(1) An essential guiding principle for surviving breaking waves is to keep either the bow or stern pointed into the waves. The bridle technique violates this principle by setting the boat up at about 50 degrees to the waves.

(2) An essential principle of para-anchors is that big ones are needed to keep the boat in place through big breaking waves. The bridle technique again violates this principle by (at least in the Pardey's technique) using a much smaller para-anchor. It is a simple fact of physics that a boat with its head 50 degrees off the waves hanging on a smaller para-anchor is more likely to roll than one with head directly into the waves on a bigger para-anchor - head off the waves means greater surface area exposed to the wave impact, smaller para-anchor does reduce immediate shock loads but it does so by allowing the bow to move backwards through the water increasing the likelihood of being rolled; the bridle solution spreads some of the para-anchor effort to the aft bridle leg reducing the force keeping the bow up. It can be argued that the extra risk is not a significant factor compared to the extra motion comfort and somewhat reduced shock loads, but in an absolute sense there is some amount of increased risk.

(3) An essential guiding principle of all heavy weather tactics is that they should be simple and easy to execute, difficult to mess up even when it's pitch black and raining, with waves washing over the deck and an extremely fatigued crew, and relatively easy to adapt to changing storm conditions. The bridled para-anchor approach is none of these things.

A core assumption of the 'bridled small para-achor' approach is that creating a 'slick' (e.g. turbulence in the water upwind of the boat caused by the hull and keel drifting to leeward) is a significant factor in stopping waves from breaking on the boat. We question this assumption. First it is essentially impossible to hold a vessel with exactly zero forward motion and thus actually directly behind any potential 'slick'. If the boat has even 1/2kt of forward drift, it will be moving 50'/minute. In which case the slick will angle aft and the boat will always be right at the forward edge of the slick. Thus a wave coming from directly downwind or forward of that will be able to reach the boat with minimal interference from the slick. Second, it is equally clear that the effect dissipates very quickly as it moves away from the hull. The effect is impossible to discern more than about 10' from the hull. It will certainly not have much effect on the really large breaking waves that are the true danger. John Neil, who likely has more miles and heavy weather experience on a 'modern' cruising boat (a Halberg Rassey 46) than anyone, agrees: "My experience (and it's a fair amount now) is that the entire 'slick' reducing breaking waves concept is not at all realistic".

Here is a great video of a real breaking wave (not just the more common breaking crest) that a hull slick would not have any significant effect on. In contrast we would consider most of the action in this video to be breaking crests, still dangerous to a sailboat but not the worst case breaking waves. The hull slick is just too near the boat and too narrow to affect a massive wave like that in the first video clip, which will already be breaking in a huge 30kt landslide of water by the time they hit the hull slick and often come sliding in at an angle different than the hull slick angle. Further the 'slick' will have zero affect on some of the most dangerous waves - those where two different wave trains become synchronized and form extremely steep pyramid-shaped waves and/or create 'bottomless troughs' - because these dangerous wave shapes are created by wave energy that runs deep underwater and is not affected by any surface 'slick'. These were the sort of waves identified as most dangerous in the Queen's birthday storm.

If the slick was as valuable a factor as suggested then lying a-hull, which creates an even wider slick than lying hove-to, would be an effective tactic rather than being the most dangerous possible technique. The para-anchor may potentially create turbulence at the right distance from the boat, but only with a shortish rode (e.g. 100' is about right but 300' is too far from the boat) and obviously a bigger para-anchor will create wider more effective turbulence than a smaller one. Interestingly, the slow speed steady state drift rate of the big and small para-anchor are in the same range (e.g. .5 - 1.5kts) in heavy weather. The extra drag of the bigger para-anchor comes into effect when a breaking wave actually hits, and prevents the boat from accelerating more effectively than the smaller one.

As a final practical point with the bridle approach - we have never found a chafe-free way to lead the bridle legs in over our toe rails. We also have concerns about engineering the bridle so it is strong enough for Hawk's expected loads (about 20,000lbs loading in the worst case breaking wave). The strongest available snatch block is not strong enough, so the bridle legs would have to be spliced and then tied or shackled to the end of the main rode. That would be strong enough but would make recovery even more difficult because we would have to work the splice/knot/shackle around the recovery winch.

While much controversy exists as to what actual parachute loads are in storm conditions, there is absolutely no question that worst case breaking wave loads are a high fraction of displacement. There are four ways to estimate these loads, all of which arrive at the same ballpark figure. The only reason there is any debate about how big the loads are, is that big breaking waves are, fortunately, extremely rare.

(1) The physics of a boat falling down a 45-foot near vertical breaking wave are very clear and very impressive. If you are skeptical about the existence of such waves take a look at the video footage of the 1998 Sydney to Hobart (the link is to a report on the storm; we have yet to find video footage on the web). Don Jordan’s web site is one of many that provide a good analysis of these loads and also has another great video of 60' ketch getting knocked down.

(2) Commercial vessel design and experience confirms this order of magnitude loading with big waves - in an article on new findings about big waves: "Most modern merchant vessels are designed to withstand about fifteen tons of pressure per square meter, but these unusual waves exert a pressure of about one hundred tons per square meter" and other testing has found common wave loading of about 100,000-170,000 lbs/sq meter for 7 seconds on static structures*. To return to our own practical experience. There are many many example of actual high wave loading but just to cite one - the 1,000-foot cruise ship Norwegian Dawn encountered a 70-foot wave that broke windows designed for 5 ton/sq meter loads up to the 10th floor.

(3) We have measured the steady-state loads on our smallest Galerider drogue at 3,500lbs (10% of displacement) in a static, non-breaking wave situation. Para-anchor loads will certainly be much higher and worst case breaking wave loads will additionally be much higher. Scaling this load by frontal surface area produces peak load estimates for our series drogue (150 cones) of approximately 10,700lbs and for a 12' dia. para-anchor (the bridle approach recommendation) of 41,000lbs (e.g. 100% of Hawk's actual cruising displacement). The physics are a bit more complex than a simple scaling, so this needs to be taken with a grain of salt, but these are useful ballpark estimates for sizing equipment and attachment points (remember that you need to add a 2x-3x safety factor).

(4) Looking at the wave energy in a storm serious enough to require a drogue or para-anchor is another way to estimate the likely loads. To ballpark the absolute minimum peak load on the para-anchor, in a storm bad enough to make the para-anchor needed, take your 1 degree righting moment (2700ft-lbs for Hawk) x 60 degrees (generally a boat's angle of peak righting moment) / half beam (7' for Hawk) x .5 (nylon rode shock absorbing) = 12,000lbs for Hawk (40% of displacement). This is the minimum loading that you can expect in a storm bad enough it could roll your boat. It assumes that the boat is caught perfectly beam on to the waves, that the wave delivers only just exactly enough energy to capsize the boat and no more, and that the nylon rode acts as a perfect spring and averages out the entire wave load. If the waves don't have at least this much energy then they can't capsize the boat, even if she is lying ahull, and you really don't need to set the para-anchor. In a bad storm the peak energy will be much higher. An upper limit ballpark can be calculated from the case of a vessel falling off a 60' near vertical wave with a 400' nylon rode (properly sized for 15% stretch at a working load of 15% of breaking strength). In this case the peak load will be equal to 100% of the vessel's displacement. These are both rough calculations but they do bound the likely peak para-anchor loads between 40%-100% of vessel displacement, and we use halfway between them (e.g. 70%) in our calculations/equipment sizing.

We would only consider using a para-anchor in one situation: if too close to a lee shore to use our drogue and dismasted so we couldn't forereach and with a broken engine so we couldn't use the trawler tactic of powering into the waves. In that situation we would first try our series drogue and see if the drift rate is slow enough to keep us off the shore, if not we would put out the largest possible para-anchor off the bow. This would minimize leeway and be better and easier (in our opinion) than the smaller para-anchor/bridle technique. Honestly, if you are caught way too close to a lee shore, while dismasted and engineless, you have probably lost your boat no matter what technique you use. Top of Page

*(Note: Recent scientific work on big waves is at ICMS Workshop on Rogue Waves Programme page).

Seamanship FAQ.
John Harries doesn't like parachutes, either:

Large Sea Anchors Not Recommended For Offshore Sailboats

While I do carry a Para-Anchor on my own boat, it would require a very specialized set of circumstances of the sort Evans describes, for it to be my first choice as a storm tactic... If I'm gonna put anything over the side, it will be a Series Drogue first... And, I sail a boat that's "easily pooped", compared to most :-)

And, for the OP, I'd suggest a good light-air sail inventory will be far more important, and see far more use for the trip you have in mind, than either sea anchors or drogues... :-)

Last edited by JonEisberg; 09-06-2013 at 07:56 PM.
Reply With Quote Share with Facebook