What load on a sea anchor?

[goeasy123]

On a recent incident a yacht (40ft...ish) lost it's rudder. The crew tried to use a sea anchor on two spinacker sheets to steer the boat. The load on the swivel between the anchor and sheets overhualed the swivel which siezed causing the sheets to twist as the anchor rotated. The sheets wound the anchor into the back of the boat, fouling the prop.

The consequent recommendation is to use a well maintain ball bearing swivel. What loading should the swivel be rated to? And can anyone recommend a suitable device?

 

[Neeves]

It will depend on the size of the yacht - there is probably no one size fits all.

There have been a number of threads on drogues and there are a number of suppliers of same or similar. Many of the suppliers and debates have had details of loads and what to plan for, in terms of the securement, chain plates etc - have a search.

Swivels with ball bearings are not cheap. K&W the Cromox (Duplex anchor chain) people in Germany make a neat stainless swivel.

There are a number of articles on same

Emergency Steering? You Can Jury-Rig a Drogue For That - Practical Sailor

Again a Google search would be valuable.

Add 'Jordan Series Drogue' to your search requirements

Jonathan

 

[thinwater]

The problem was using the wrong drogue (not a sea anchor). Quality drogues do not rotate, even on a long single line. That is a basic design criteria. Even an anchor under a fender as a drogue (I have used this with the rudder removed, as did Osprey--Goggle it). You should not need a swivel on a drogue at all.

The load varies with the size of the boat, but as a first approximation it is the same (gale conditions) as the working load on the genoa winch, normally much, much less. Based on testing with several boats. For a 40-foot boat at ordinary speeds, about 300-600 pounds. More in a gale, up to about 2000 pounds.

Try ...

• Delta Drogue
• Gale Rider
• Small Shark
• Sea Brake

How Much Drag is in a Drogue? - Practical Sailor
Emergency Steering? You Can Jury-Rig a Drogue For That - Practical Sailor

---

As for swivels failing to rotate, it usually is NOT about strength rating, it is about friction to turn (ratio of load to spin). Is the friction under load more than the shallow bridle angle? For example, mooring swivels go at the bottom, not the top, to minimized the friction load on the swivel (they untwist when there is no load on the chain and the swivel is on the bottom). This is also why a Boomerang (anchor turner) works better than a swivel. Curiously, if they had kept the drogue closer, the bridle would have been wider and the swivel might have worked. But for the sea anchor to wind up that far, it must have been a terrible design. I've tested many, and never had that happen. Good drogues don't spin at all. Perhaps adding a small weight to the lower lip would help.

The other mistake they made was NOT testing the rig beforehand, just as you practice MOB recovery. You have to practice to work out the kinks and to learn about the kinks ahead of time. Practice.

 

[Neeves]

The problem was using the wrong drogue (not a sea anchor). Quality drogues do not rotate, even on a long single line. That is a basic design criteria. Even an anchor under a fender as a drogue (I have used this with the rudder removed, as did Osprey--Goggle it). You should not need a swivel on a drogue at all.

The load varies with the size of the boat, but as a first approximation it is the same (gale conditions) as the working load on the genoa winch, normally much, much less. Based on testing with several boats. For a 40-foot boat at ordinary speeds, about 300-600 pounds. More in a gale, up to about 2000 pounds.

Try ...

• Delta Drogue
• Gale Rider
• Small Shark
• Sea Brake

How Much Drag is in a Drogue? - Practical Sailor
Emergency Steering? You Can Jury-Rig a Drogue For That - Practical Sailor

---

As for swivels failing to rotate, it usually is NOT about strength rating, it is about friction to turn (ratio of load to spin). Is the friction under load more than the shallow bridle angle? For example, mooring swivels go at the bottom, not the top, to minimized the friction load on the swivel (they untwist when there is no load on the chain and the swivel is on the bottom). This is also why a Boomerang (anchor turner) works better than a swivel. Curiously, if they had kept the drogue closer, the bridle would have been wider and the swivel might have worked. But for the sea anchor to wind up that far, it must have been a terrible design. I've tested many, and never had that happen. Good drogues don't spin at all. Perhaps adding a small weight to the lower lip would help.

The other mistake they made was NOT testing the rig beforehand, just as you practice MOB recovery. You have to practice to work out the kinks and to learn about the kinks ahead of time. Practice.

Also amazing that despite their predicament in losing the rudder they did not maintain an occasional watch on their drogue and did not notice it was rotating and that as it rotated the twists got closer and closer to the yacht. To have actually fouled the prop suggests the drogue was attached quite far forward (or the prop was behind the transom). However they might have had other issues.

Jonathan

 

[thinwater]

In fact, for good steering on a mono, the turning blocks will normally be near the widest beam, typically not too far behind the mast. On a multi we put them on the transoms.

Yes, fouling the prop or rudder is a consideration ... which is why you practice. To learn how to deploy and to rig in the safety of home waters. Things will go wrong the first few sets, like recovering an MOB. They did for me. It probably took me 10 tries, in light to near gale conditions, to become proficient at this new skill in all conditions. Maybe I'm slow.

 

[Neeves]

In fact, for good steering on a mono, the turning blocks will normally be near the widest beam, typically not too far behind the mast. On a multi we put them on the transoms.

Yes, fouling the prop or rudder is a consideration ... which is why you practice. To learn how to deploy and to rig in the safety of home waters. Things will go wrong the first few sets, like recovering an MOB. They did for me. It probably took me 10 tries, in light to near gale conditions, to become proficient at this new skill in all conditions. Maybe I'm slow.

There is nothing wrong with slow, I recall a story about a tortoise and hare and of 'softly, softly catch the monkey' (my MacBook did not like my original quote )

 

[RunAgroundHard]

Neeeves, you didn't search for "softly, softly, spank the monkey" did you?

 

[sarabande]

The problem was using the wrong drogue (not a sea anchor). Quality drogues do not rotate, even on a long single line. That is a basic design criteria. Even an anchor under a fender as a drogue (I have used this with the rudder removed, as did Osprey--Goggle it). You should not need a swivel on a drogue at all.

The load varies with the size of the boat, but as a first approximation it is the same (gale conditions) as the working load on the genoa winch, normally much, much less. Based on testing with several boats. For a 40-foot boat at ordinary speeds, about 300-600 pounds. More in a gale, up to about 2000 pounds.

TW Have you got a source for those loads please ? Based just on a little bit of towing experience in calm conditions , I would expect those figures to be higher, and also to need to have snatch load approximations in big waves. ( yes, a load cell in a tow line would be a very useful adjunct !)

 

[Neeves]

Neeeves, you didn't search for "softly, softly, spank the monkey" did you?

I think the phrase is or was, in colonial times,

Softly, softly, catchee monkey​

Oxford Reference
https://www.oxfordreference.com › display › authority....


Proverbial saying, early 20th century, advocating caution or guile as the best way to achieve an end. From: softly, softly, catchee monkey in The Oxford ...


Why you might think anyone would want to spank a monkey - beggars belief.

Jonathan

 

[B27]

I wouldn't be surprised if any drogue rotated, if you towed it across a wave system at an angle.....

 

[LittleSister]

Why you might think anyone would want to spank a monkey - beggars belief.

Jonathan

'Spanking the monkey' was the forceful but necessarily skilful blows applied to the Monkey, a key but somewhat troublesome ratchet valve mechanism used in early steam-driven cloth mills. This was a somewhat hazardous operation done while the machinery was still running, in order to avoid interrupting the operation of the looms, etc.

 

[thinwater]

TW Have you got a source for those loads please ? Based just on a little bit of towing experience in calm conditions , I would expect those figures to be higher, and also to need to have snatch load approximations in big waves. ( yes, a load cell in a tow line would be a very useful adjunct !)

The data is in the links. Yes, a load cell was used. Many tests, many speeds, many drogues.

The load will be pretty high in a storm. As I said, up to the genoa sheet load, as an approximation. As the weather gets stronger, even for a steering drogue, the rode gets longer, mostly to prevent pulling out of wave faces. Polyester makes the best drogue for most storm and steering drogue applications, with some stretch, but not too much (you don't want the yacht to surf or broach while the nylon stretches).

No, there is no snatch loading, since there is no slack. It's not like a ground anchor. The drogue pulls through the water faster when a wave strikes, and steering drogues are intentionally not that big, about 3 feet for the boat in question. Yes, the force varies with the speed through water. It's in the link.

Drogue sizing to prevent surfing and storm management is slightly larger. However, there is still no snatch loading per se, for the same reasons. Depending on the size of the drogues and the number of drogues, there are a continuum of behaviors, leading to the multi-drogue JSD. But note that a Dyneema Rode is often used with a JSD. No snatch loading. There are surges, of course.

 

[Neeves]

'Spanking the monkey' was the forceful but necessarily skilful blows applied to the Monkey, a key but somewhat troublesome ratchet valve mechanism used in early steam-driven cloth mills. This was a somewhat hazardous operation done while the machinery was still running, in order to avoid interrupting the operation of the looms, etc.

Another part of my education in the use of the English language.

Jonathan