series drogue-toe rail attachment?

I would like to know if the aluminum plates welded to the port and starboard stern ends of the toe rail on our Swan 41 are a strong enough attachment point for our series drogue.   I'm planning to shackle the drogue bridle through the holes in the plates.  The drogue's designer, Jordan, recommends that the attachment point be able to withstand a one-time force of approximately 14,000 lbs/6,364 kg from a breaking rogue wave.  I've emailed Nautor this question but did not get a reply (I actually did not know whom to email at Nautor, so just sent off a query to the warranty folks).  Any insight/suggestion much appreciated.

Mark S/V Anthea (Swan 41/59)

PS I've found the threads about galvanic corrosion (mast, boom, spreaders, fittings, toerail, etc.) and packing around the rudder post MOST helpful and illuminating. 

Mark

The lug on the toerail is quite high, and if the load happens to come from the side the lug bending strength in that direction needs to be checked. If you can give the lug plate thickness and length as well as the hole height above the toe rail an estimate can be made. A picture of the lug would also be helpful.

Other possibilities are the mooring cleats, depending on the size of their fasteners, or the genoa sheet turning blocks. A special fitting could also be built engaging in the toerail holes.

Kind regards

Lars

Dear Lars,

Your comments are most appreciated.  I will take down the measurements you requested and post on this thread.  Unfortunately, we live 250 miles from where we berth Anthea and it will be about a month before we're next on board - so it will be a while before I can get the measurements.

Yes, the stern cleat and genoa block are options, though their holding power would have to be checked and the fact that they're both forward of the transom raises chafe issues. 

I'll post the dimensions and a photo or two and shall look forward to your response.   Thank you. 

Best regards,

-Mark

Dear Lars,

Here’s a tardy response to the information you requested in order to calculate if we can secure the Jordan Series Drogue to the aluminum plate on our toerail, should the need ever arise.  Attached is a photo of the thick aluminum plate welded to the aft end of the toe rail on our Swan 41 (hull 59).  The plates measure 22 cm long, 16 cm high, and are 15 mm thick. The center of the hole is approximately 12 cm above the deck.  The plates are welded to the toe rail along its base and 6 cm up, on both the forward and the aft ends.  The hole in the plate (where we would attach the drogue using stainless shackles) is lined with stainless steel.  There is a minimum of 2.5 cm from the edge of the hole to the edge of the aluminum plate.  The note above mentions the one time force that may be anticipated from a large wave.  Thanks so much for your willingness to do the calculation!  Here's hoping you get this post on this rather old thread.

Kim and Mark

Anthea/Swan 41/059

Kim and Mark
Matteo kindly pointed out that you posted a response, a big thanks to him.
The stated drogue load applied in the lug hole exceeds the sideways strength of the lug by a factor of 3.
The suggestion is to fasten a fitting lower down, as low as possible considering the nuts if thrubolted.
For example a Wichard 12 mm U-bolt appears to have a suitable working load, but it needs to be checked in which direction this load is allowed.
If you inform which fitting you intend to use I may be able to give further comments.
Best regards
Lars

Dear Lars,

Yes, a big thanks to Matteo for alerting you to our post.  Thanks very much for your illuminating calculation and reply.  Wow, just imagining that kind of force is quite daunting, let alone experiencing it on board.  Now we’re wondering what is the best way to secure the drogue.  The Wichard 12 mm U-bolt would of course require drilling through the hull (perhaps where a stringer is positioned on the port and starboard sides; as opposed to on the transom) and determining the appropriate washer or back plate size to prevent it pulling out.  I’m a little leery of drilling holes through the hull.  And the direction of pull would have to be checked, as you noted.

What about your earlier suggestion of fashioning a special fitting that would engage (bolt?) through the toerail holes?  Seems like a long piece of stainless steel, appropriately sized and rated, might work.  That option would also have the advantage of being able to be bolted on when undertaking a passage and removing it at other times.  Would you recommend that approach?

Lastly, securing it to the stern mooring cleats or to the genoa turning blocks (perhaps with a woven spectra line loop?) were also options you mentioned.  How might we determine if the mooring cleats can withstand the stated drogue load?

Thanks very much for your insight regarding this matter!

Best regards,

Mark (and Kim)

Mark
Thank you for your reply.
I had a location lower down on the lug in mind. This requires holes drilled through the lug for attaching the fitting, which could be removable. A fitting engaging in the toerail holes would need to be unreasonably long due to the thin vertical toerail flange. Assume the cleats are attached with four bolts, and provided these have 12 mm or bigger diameter this would be sufficient. The footblocks probably have more bolts than that.
Best regards
Lars

Dear Lars,

Silly me, down lower on the lug makes perfect sense; not sure why I was thinking through the hull.  Great suggestion – thanks very much.  We’ll probably go that route as it provides a wider, better angle for the drogue bridle than the stern cleats.  You mentioned the Wichard 12 mm U-bolt would need to be checked to see which direction the load is allowed.  Can you provide insight on that one or should we contact Wichard?  We very much appreciate your guidance and are happy about this approach for securing the drogue. 

Best regards,

Mark

Mark
Suggest you contact Wichard, but you could also have a look at the Harken catalog Padeyes. Their working load is dependent on the load angle, and it can be assumed that similar principles are valid for Wichard. The smallest working load refers if the angle is not known.
Best regards
Lars

Hope you don't mind the sidestep question ... my experience with drogues is only theoretical. Why the stern cleats instead of the bow?
Of course it may become advisable to slow the boat, e.g. by towing warps. However, I would have thought that the stopping power of a drogue is so large as to not only slow but almost "hold" the boat. Doesn't that put a lot of stress on the rudder if the drogue is on the stern? Even with the stern shape of the S&S Swans, I'd have gone for the bow cleats to get the bow into wind and waves. Or would that mean an even greater risk to the rudder in case the drogue lines break, or the boat is slammed backwards by a wave?
Always eager to learn,
Martin

Hi Martin,

Our knowledge of drogues is also only theoretical and we hope it stays that way.  The designer of the Jordan Series Drogue argues that the stern of a sailboat is less likely to "yaw" back and both than the bow; it will hence be better able to hold a stable position into the waves than the bow.  I have not heard of problems with the rudder.  With the Jordan Drogue, one anticipates forward movment of around 1.5 knots; not sure if that addresses the rudder issue you raised.  Descriptions by people who have deployed the drogue liken it to being attached to the ocean by a flexible "bungi" cord that allows the boat to ride stern to the waves.  We have found internet discussions of drogues quite helpful.  This site has much information about the Jordan drogue: http://www.jordanseriesdrogue.com/.

Hope that helps.

Mark

Hi Mark,

Can you share the attachement point that you decided on / installed ? 

Have you tried out the JDS in a Blow ?

Cheers

Chris

Saga

47-015

Hi Chris,

In the end, we decided to attach the ends of the drogue to the stern cleats.  We sacrificed a little stability by having the attachment points closer together, but it's much more convenient than installing U-bolts on the toe rail lug.  

As of yet, we have not deployed it.  When making passages, we attach it to the stern cleats, flake it carefully on the deck aft of the cockpit combing, and cover it an easy-to-remove canvas cover, for easy deployment should we need it.  

Good luck!  

Best,
Mark (Anthea, 41/59)