Sea Anchor Drogue line diameter?

FullCircle

FullCircle

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I have a large size (for 35ft boat) Lalizas Sea Anchor Drogue which tests seem to show produces a drag of around 120kg at 7 knots, but of course that is a linear measurement and does not includes snatch loads.

I have read that a short length of chain helps keep it submerged, and I am going to add a metre of 8mm that I have lying around.

The line needs to be 5 x boat length (according to the instructions, but gives no clue what diameter.line to use.

My guess is 200ft of 3 Strand polyprop 14mm....... any advance on that?
 
I would have thought that the snatch loads could be several multiples of the normal drag. As you would, presumably, be using it in a survival situation, I would go for 14 or 16mm nylon. Polyprop is weaker than nylon and, to the best of my knowledge, more prone to deterioration over time.
 
how about soft polysteel, J ?


The rope floats, is easily visible (light blue/green) and is v strong

10mm Polysteel is 2,070Kg.
12mm 3,150Kg
14mm 3970kg

(I know it's kg breaking strain, and not Kilo Newts or something madly foreign :)

The upward buoyancy on 200 ft of drogue line will not be huge, but enough to stop the drogue heading for the bottom when you are stopped and in the recovery mode. It will also keep the rope visible and out of the way of the prop.

https://www.ropesdirect.co.uk/10mm-polysteel-rope-sold-by-the-metre.html


I've used polysteel for the mooring ropes this year, and like it.
 
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Just found this: http://www.safety-marine.co.uk/spages/parachute-sea-anchors-and-drogues-whats-the-difference.htm

which recommends 100 metres of 18mm nylon.

I also seem to recall from somewhere that 3 strand isn't recommended for extreme conditions because the continuous stretching & relaxing can generate enough heat in the rope to melt it. So maybe braided nylon?

I'm not usually keen on over-engineering things but if one invests in a drogue, then there's absolutely no point in skimping on the rope.
 
I suspect in a storm that all parts of a nylon rope would be subject to lots of cooling water, even on the bitts, but it is a valid point.

18mm braided rope is bulky, and much increased drag over 12mm poly.
 
I thought one of the characteristics you needed on a drogue (and mooring) line was elasticity, and polysteel might be strong but I thought it had low elasticity (I stand to be corrected). Nylon has the best elasticity and seems a better bet.

Thinwater completed some work for Practical Sailor in this area (and has done some work on mooring lines) and hopefully he will come on, give you chapter and verse and reference the specific article for you. FullCircle - if he does not come on - send him a PM - he is both accommodating and knowledgeable.

Jonathan
 
Neeves said:
I thought one of the characteristics you needed on a drogue (and mooring) line was elasticity, and polysteel might be strong but I thought it had low elasticity (I stand to be corrected). Nylon has the best elasticity and seems a better bet.

Thinwater completed some work for Practical Sailor in this area (and has done some work on mooring lines) and hopefully he will come on, give you chapter and verse and reference the specific article for you. FullCircle - if he does not come on - send him a PM - he is both accommodating and knowledgeable.

Jonathan
Click to expand...
Thanks, will do.
 
I am slightly at a loss over the figure of 179kg drag at 7 kts, for a drogue with a dia of 1.9m.

Is the figure from the manufacturer please ? It doesn't seem very much.



EDIT


real fag packet stuff:

Drag coefficient (flat plate) 2.0
Area 3m2
Speed 3.6 m/sec
Density of fluid 1000 kg/m3

gives drag of 38k Newtons. Very roughly.




Someone like to check the figures please ?
 
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breaking strain is important, but I suspect chafe resistance might matter more (within limits, of course)

I have a Jordan drogue ( not sea anchor). Chafe prevention on that took some working out.
 
I just finished a long series of instrumented drogue tests, up in to near gale conditions. There is a lot of infomation in this thread that is off target.

http://www.practical-sailor.com/issues/37_69/features/How-Much-Drag-is-in-a-Drogue_12074-1.html

1. Design. My impression, given the limited reputation of this manufacturer, combined with the lack of instructions, is that they are not serious about the drogue business. I also know from testing the cone style drogues such as this, are unstable at high speed and are unstable and steep waves (they tend to pull out and then skip along). My first impression is that this drogue is designed for moderate weather and slowing down to fish.

2. Rigging instructions. A balanced explanation of how to rig drogues could be found that websites for other manufacturers. Google Sea Brake, Paratech, and Small Shark. However in general, you will need at least 5 m of chain and at least 200 m of rope of approximately the same strength as your anchor rode. Advice varies as to whether this should be polyester or nylon. It should NOT be a floating line.

Bridles are also very important. Look it up.

3. Snatch loads. If this is a speed limiting drogue with drag in the 175-500kg range, there really aren't snatch loads. This is not a ground anchor or parachute anchor were the boat is thrown against it. Instead, it is designed to rip through the water at the maximum drag figure with the boat is struck by a wave. The maximum load will be at most 150% of the steady-state drag figure. On the other hand, if this is truly a 1.9 m diameter drogue and you intend to deploy it off the bow, the snatch loads can be significant and you need to follow the advice of parachute see anchor manufacturers. As much as 150 m of large nylon line maybe required. 15mm is probably not too large.

And this gets to the question of nylon versus polyester. Some vendors (Sea Brake) specify polyester so that the Speed limiting drogue can build drag more quickly when the boat begins to surf off a wave. Others believe nylon provides more even strain. Nylon is always recommended for parachute sea anchors. My experiences that 150m polyester provides enough stretch for speed limiting drogues, and nylon is required for parachute sea anchors.

As for minimum rode length, it can be pretty darn long. It's not so much about synchronizing the drogue with a certain wavelength as some would have you believe. In a storm with confused waves (the dangerous sort) there is no such thing as a constant wavelength. However, a long length of rope dragging through the waves really helps keep the drogue down in the waves and reduces the tendency of it to pull out of wave faces, the primary failing of speed limiting drogues. If you think about it, steep wave face is a lot like anchoring short scope; if the rope is not long enough, it is drawn into a straight line from drogue to transom, and pulled up and out of the wave. Chain helps, but a long the rode helps more. Sea anchors on the other hand (devices that basically stop the boat) are not prone to pulling out because they set lower in the water. It is the high loading per square foot of speed limiting drogues that allows a them to pull out. All speed limiting drogue designs become unstable above 4-5 knots in big waves.

4. I have tested a very similar design (Jimmy Green Marine) and got 250 kg of Drag at 7 kn with a 0.6 m diameter drogue. The figure of 175 kg at 7 kn with a 1.9m drogue cannot be correct. I suggest you reconfirm the data. I suspect the correct figure is closer to 2500 kg at 7 kn, if that is the correct diameter. I also suspect this drogue is very unstable at 7 kn and that the static pull probably cycles from 2000-4000 kg every few seconds, Based upon testing. This is a big drogue.

5. This is way too small to be deployed off the bow of the boat. The boat will drift backwards at several knots with waves and actually get sideways to the waves. Very bad.


The best bet is to return this drogue to the manufacturer and get something suitable. Either something smaller from a reputable manufacturer intended to be towed off the transom (about 0.6M, but a better design), or sufficiently large parachute drogue for the bow (about 4M is needed). This is neither and will probably only give a false sense of security.
 
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Sea anchor drogue is a mix of parachute anchor used by long keel and skeg hung rudder boats off the bow. Drogues are small cones with many in line used by spade rudder boats off the stern. The size of rope and length of each is decided by the boat weight. They are usually used by long distance sailors who have to take any weather that comes along, if you are not intending to do that I wouldn't waste money on buying one of them. When you buy one the diameter and length of the rope will be set by what you have bought, there is no one size/length fits all. We had a steel long keel ketch which weighed 15 tons fully loaded and we had an 18 foot diameter parachute bought here: http://www.paraanchor.com.au/para-anchors.html
 
I am currently in discussion with Marlow Ropes on a technical matter.


Their Tech Manager has sent me an email with some useful information, and a very informative diagram of load vs extension.

Shared for general info.

"Attached is a graph that shows the elongation of some typical yachting products. In this case the Marstron is a polypropylene rope and so floating. The only materials available for rope production that have a density less than 1 (floating) are polypropylene or polyethylene. A rough rule of thumb is that nylon will stretch almost double the amount a floating rope will at any given load. "

ropes_zpsusl7mlsq.jpg
[/IMG]

I am ferreting away at a challenge for the need for large dia (>15mm) ropes, when modern materials can provide ropes which are much lighter and less bulky for the same strength. In particular, the likely extension ("stretch") of nylon and polyprop ropes for a given load. It looks as if a 200ft nylon rope at 50% max load will stretch 28 ft, and a polysteel rope some 14 ft.

Now 50% BS is a huge load (for 14mm polyester braid it's 2700k) and given the nature of seas when anchors/drogues are likely to be deployed, I wonder how quickly the full stretch is achieved; a gut feeling is that shockloads at sea take place over something like 1 to 3 seconds ( ? ) during which time the 'slip' of the sea anchor through the water may be in excess of the stretch in the line. Many variables and design params, but fascinating figures.
 
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This is thread drift but much depends on where your compromises are but Dyneema floats, is strong but has no, or virtually no, extension. I think the OP needs something with stretch - and as you point out nylon meets that need.

Jonathan

Jonathan
 
1. Stretch is important (depending on the drogue type--for a JSD it is not, for example, but sea anchors do), but it is no substitute for long length. The long rodes are NOT just for stretch.

2. Worry more about fatigue strength than breaking strength. For example, polyester can be worked at 25% BS for extended periods, but nylon only at about 10%.

The terms drogue and sea anchor are being used alternately here, which is confusing. In the US at least, a sea anchor is a stopping drogue (typically 12-24 foot diameter) intended to reduce drift to < 1knot. A speed limiting drogue (2-3 feet dia) only slows the boat a bit and prevents surfing. JSD is somewhere in the middle, in its own class. I think the OP is thinking sea anchor, but the size is much too small.
 
Thanks for all this information, but I am now wondering why Lalizas would market something that is so dangerous/useless? It seems similar in concept and design to the Jimmy Green item used in the PBO tests here.....

http://www.pbo.co.uk/gear/which-drogue-should-you-buy-25543

Lalizas also show this item requiring a 5x boat length piece of line, but the opinion here is getting on for 3 times that amount - say 60m (Lalizas) vs 150m (Forum) and 18mm or more, not to mention a huge para anchor to go with it, To be honest, I have nowhere to store such a device, and the budget for that would be North of £2000, and I don't have that either.

So, assuming that I am not going to be rounding Cape Horn (that's an analogy for those without humour), and I am no Moitessier, but may need to see out a good size mid atlantic blow, then are there any other recommendations?

Sorry to pee on my own bonfire, but should I just bin this idea?

PS I do have 5 litres of oil aboard.....
 
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