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Old 21-03-2010, 17:25   #31
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It is a very good question without a very good answer.

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Originally Posted by barnakiel View Post
Dynamic? Static?

Pull = pounds (per ft)?

Meaning - what do we understand by X pounds of pull? I think the wave is moving, so the static PRESSURE (e.g. pounds per sq ft) could be in pounds, but as the water travels at a specific speed ... so - how do we interpret the quoted number?

I am sorry to ask here as I might probably find on the net, just trying for a shortcut.

THX
barnie
The short answer is that for my size boat, it would be the total force of the swell and moving water on the boat, for a duration of 5-20 seconds. For a better answer, read the JSD/Coast Guard report in the reference section a the end of the post. You will find a better, if guessed at, answer. It's not a simple thing at all, because every breaking wave will be different. The water in the wave is moving very little (although there is a slope) but the crest will be moving at wave speed.
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Old 21-03-2010, 18:07   #32
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I believe the extrapolations are reasonable, but I fairly certain the data below 10 knots (except for the chutes) is accurate.
When I looked at the chart, the portion that is really relevant to me was very compacted together and hard to read. The portion that applies to high speed stands out in the chart, but isn't very relevant in my mind. While boats do surf at 20kts, you aren't going to be surfing at 20kts with a drogue deployed. That's the point of the drogue. I'm more concerned with two cases:
1. the drogue fails, e.g. comes out the water, lines chafe, etc. and then off the boat goes
2. the drogue slows the boat down too much and does allow for adequate steerage.

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Old 21-03-2010, 19:51   #33
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1) if the drogue slows us too much to keep steerage (as per Paul) it also keeps the stern pointed towards the waves, so: good&bad, depending on what comes behind, and we have to be aware they do come in series (my own experience - two bad ones and one that follows),

2) thinwater - THX again! so - we want to avoid the drogue coming up too much and obviously, since the water in the wave moves 'little' - why care about the wave-length deployment???

So, I would see the drogue - far, far behind the boat - should the wave mess our drogue we have, in the very least, a looooong line towed ... correct?

Yes, I remember the stretch - Spectra?

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Old 21-03-2010, 20:04   #34
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I think it comes down to whether you believe stearing will help much...

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When I looked at the chart, the portion that is really relevant to me was very compacted together and hard to read. The portion that applies to high speed stands out in the chart, but isn't very relevant in my mind. While boats do surf at 20kts, you aren't going to be surfing at 20kts with a drogue deployed. That's the point of the drogue. I'm more concerned with two cases:
1. the drogue fails, e.g. comes out the water, lines chafe, etc. and then off the boat goes
2. the drogue slows the boat down too much and does allow for adequate steerage.

Paul L
... and whether a drogue that will prevent surfing in the event of a wave strike will still allow steerage. The conventional wisdom - not a new idea from me - is that you can't get both. A low-drag unit will allow steering (warps, Gale Rider) while a high-drag unit (JSD, Seabrake) will hold the stern to the wind and withstand steep waves and breakers to some extent. You can have one or the other, but not both.

One of the points of the chart is that with some of the lower drag options - warps for example - can be towed at surfing speed if the wave is breaking and providing great force. Therefor, warps cannot do much to prevent pitchpoling or broaching. The same may be true of the Gale Rider, Small Shark, and Delta Drogue. Their literature (Delta Drogue at least, and the other 2 have similar drag) says as much.
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Old 21-03-2010, 20:11   #35
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Hey, I only posted flat water drag data!

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Originally Posted by barnakiel View Post
1) if the drogue slows us too much to keep steerage (as per Paul) it also keeps the stern pointed towards the waves, so: good&bad, depending on what comes behind, and we have to be aware they do come in series (my own experience - two bad ones and one that follows),

2) thinwater - THX again! so - we want to avoid the drogue coming up too much and obviously, since the water in the wave moves 'little' - why care about the wave-length deployment???

So, I would see the drogue - far, far behind the boat - should the wave mess our drogue we have, in the very least, a looooong line towed ... correct?

Yes, I remember the stretch - Spectra?

b.
You're asking real world questions and I'm not inclined to make up nonsense answers.

Regarding wavelength deployment, in my limited expereince, if there are regular waves, they aren't breaking, if they are breaking, they are irregular as hell and theory becomes shaky.

Regarding Spectra, with a 15-foot parachute, that would be fatal. The impact force would break something before the storm ended, if only the crew. Regarding drogues, JSDs are sometimes made with Spectra, to save space an weight. It probably wouldn't hurt the other either, but I don't know. Seabrake took a middle road and suggested polyester.
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Old 23-03-2010, 08:02   #36
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Quote:
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Therefor, warps cannot do much to prevent pitchpoling or broaching. .
We have a galerider (along with a JSD and a delta drogue). Our galerider creates a maximum force of 3500 lbs (we have measured that) when accelerating down a wave face.

We have spectra line of our JSD. It makes the package much lighter and easier to handle. The JSD design avoids snatching/shock loads (unlike a para-anchor) so the rode does not need to be elastic.

On our first boat we used straight warps (without a drogue) several times, and they did help prevent broaching, because they helped keep the boat stern aligned with the waves. There was obviously not as much correcting force as with a full drogue, but enough to stop mots of the tendency for the stern to get out of alignment.

The plain fact is that in the very worst case, NOTHING will prevent a broach/pitchpole. There are unfortunately waves out there big and steep enough to overwhelm any system. We know a 50' cat that was reverse pitchpoled while lying to a para-anchor. We know boats that have broached with single element drogues. We know a boat that has rolled 360 degrees while on a JSD.
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Old 23-03-2010, 08:13   #37
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Well... I think Evans' post pretty much trumps all others...

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Old 23-03-2010, 08:28   #38
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I think the storms I envision are not the storms that they have known!

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Well... I think Evans' post pretty much trumps all others...

Bill
Everything they said rings true. I've sailed a beach cat and a small monohull dingy in violent conditions and there is no question that at some point things can be overwhelming. I like the fact that I fact that I have been completely beaten in a small boat; it keeps me humble and realistic. I remain VERY careful with weather windows off-shore.

As I said, this was only a summary of flat water data; a good scientist always limits his findings and admits when he is extrapolating or guessing. I said "that the lower drag units could not prevent a pitchpole or broach in the case of a breaking strike." I did not say that other units would prevent a broach; careful use of words, perhaps, but my attempt to indictate that certain drogues provide a critical level of braking, while others probably do not.

I would love to know the size and speed of the Gale Rider that pulled 3500 pounds, so that I can see how it fits. I can see that the largest size at ~ 12 knots would generate about 3500 pounds of drag. I hope all readers understand that the data and my statements were all scaled to one specific boat and were focused on coastal and limited off-shore passages. I know my limitations and I'm no authority on big storms - and don't want to be!
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Old 23-03-2010, 17:27   #39
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I would love to know the size and speed of the Gale Rider that pulled 3500 pounds, so that I can see how it fits.
Our galerider is 42" (mouth diameter) x 48" (depth of basket). As you probably know, its made of webbing. I don't know how the load would scale/correspond to other drogue designs.

If we get a chance, this summer I will load test all three of our drogues and report back.

We have used a 300' long x 3/4" dia warp without a drogue. We have not measured its load but I would guess it's on the order of 1/10 (350lbs) the drogue loading when accelerating down a wave (At about 8kts). I know we cannot pull it by hand (Which makes it more than about 150lbs) but it does not stretch the line noticably (so less than 1500lbs)

The forces on all the drogue systems (and a strength rode) is quite cyclic, higher when going down the face of a wave and much lower (sometimes completely slack) when going up the back of a wave.
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Old 23-03-2010, 18:51   #40
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I calculated a curve for the larger Galerider and posted it on the same blog site...

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Originally Posted by estarzinger View Post
Our galerider is 42" (mouth diameter) x 48" (depth of basket). As you probably know, its made of webbing. I don't know how the load would scale/correspond to other drogue designs.

If we get a chance, this summer I will load test all three of our drogues and report back.

We have used a 300' long x 3/4" dia warp without a drogue. We have not measured its load but I would guess it's on the order of 1/10 (350lbs) the drogue loading when accelerating down a wave (At about 8kts). I know we cannot pull it by hand (Which makes it more than about 150lbs) but it does not stretch the line noticably (so less than 1500lbs)

The forces on all the drogue systems (and a strength rode) is quite cyclic, higher when going down the face of a wave and much lower (sometimes completely slack) when going up the back of a wave.
... Along with this text:
The data for the 42" x 48" Galerider was extrapolated from the smaller unit and corrected for size; both the frontal area ratio and the cylinder area ratio were very close and the average value was used. However, we have 2 data points (Sail Magazine and measured storm expereince) and they do not match. The storm drag figure on the larger size reported 3,500 pounds at 8-9 knots, though the correlation suggests that the bridle. rode and drogue would have produced only 1,500 pounds drag at that speed. Given the 30% error bar I believe to be probable in this comparison, a speed of 10.5 knots would have been predicted to generate 3,500 pounds of combined drag.

Sail Delmarva: Drogue and Parachute Sea Anchor Testing: A Summary for Small to Medium Cruising Catamarans

If the speed during the surf was a little higher, 10+ knots, the agreement is good (a generous addition must be made for the rode and submerged bridle). If not, well, there is more to learn. I assume the stress was measured with a gauge of some sort?

The 350-pound guess for a 300-foot x 3/4" drogue matches up well with the 287 pounds I got with 200 feet x 3/4" at 8 knots. Ball park.

Another thing to consider; manufacturers each have an idea of how much drag is best for a certain size boat in certain conditions. Just because one devise generates greater drag, does not make it "better." A different size could be chosen, for example. Many choices.

Flat water tests are easy; storm tests are not. I look forward to seeing what you come up with in the summer, particular the JSD, for which I have the best data.
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Old 23-03-2010, 18:53   #41
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JSD on Spectra

Cool. I like the idea.

BTW Anybody tried towing two smaller SeaBrakes in place of a single, bigger one?

I would think if the are spaced enough, we could get a bit less drag but in a more consistent way. Asking because there is no way I can keep ours at anything close to constant drag.

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Old 24-03-2010, 14:46   #42
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Clearly, that was much of the intent of the JSD...

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Originally Posted by barnakiel View Post
Cool. I like the idea.

BTW Anybody tried towing two smaller SeaBrakes in place of a single, bigger one?

I would think if the are spaced enough, we could get a bit less drag but in a more consistent way. Asking because there is no way I can keep ours at anything close to constant drag.

b.
Could you expand on why you are not getting consistent drag?
  • Drogue coming out of water? When?
  • Too much/little rode stretch?
  • Chaotic wave action (not one wave length)?
  • Motion of water in waves (normal surging)?
  • Do you think the problem would be the same with any single-element drogue.
I also suggested in some post that lengthening the bridle and perhaps using over size line so that it would function as a towed warp seems like a potential improvement. Thoughts?

My concern with 2 drogues would be chafe. Also, one of the advantages of the JSD is that the small elements are not subject to fatigue from opening and closing - they are too small.

But I like your idea. Simpler to retrieve and it could be deployed in phases, 150-feet apart. A compromise.

Thanks.
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Old 24-03-2010, 15:55   #43
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Here's what the people at SeaBrake had to say in response to my query about stretch or non stretch line :

When Seabrake is used as a drogue underway (ie off the stern) a low stretch rope is recommended to ensure continuous adjustment to drag levels in relation to boat speed. A stretchy rope will reduce the effectiveness of the unit.
Shock loading is not an issue as neither vessel or Seabrake will be acting independently from each other, the drag levels the Seabrake unit generates always relate directly to boat speed due to the variable pressure/drag wave Seabrake creates.

When a vessel is attached to a fixed unit (ie one not moving through the water) such as at anchor, attached to a dock, or to a lesser extent attached to sea or parachute anchor which generate relatively high but constant drag rates, shock loading of fittings can cause problems this is one reason why stretchy warps are traditionally utilized in these situations.

Also the Seabrakes warp should not be a buoyant material as weight of warp and chain combine to ensure the Seabrake unit stays below the surface.
Braided polyester is usually recommend as it is low stretch, does not float and is relatively cost effective. Earlier models were actually supplied with even lower stretch Kevlar core ropes.

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Old 24-03-2010, 16:29   #44
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greed. Makes sense, but...

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Here's what the people at SeaBrake had to say in response to my query about stretch or non stretch line :

When Seabrake is used as a drogue underway (ie off the stern) a low stretch rope is recommended to ensure continuous adjustment to drag levels in relation to boat speed. A stretchy rope will reduce the effectiveness of the unit.
Shock loading is not an issue as neither vessel or Seabrake will be acting independently from each other, the drag levels the Seabrake unit generates always relate directly to boat speed due to the variable pressure/drag wave Seabrake creates.

When a vessel is attached to a fixed unit (ie one not moving through the water) such as at anchor, attached to a dock, or to a lesser extent attached to sea or parachute anchor which generate relatively high but constant drag rates, shock loading of fittings can cause problems this is one reason why stretchy warps are traditionally utilized in these situations.

Also the Seabrakes warp should not be a buoyant material as weight of warp and chain combine to ensure the Seabrake unit stays below the surface.
Braided polyester is usually recommend as it is low stretch, does not float and is relatively cost effective. Earlier models were actually supplied with even lower stretch Kevlar core ropes.
... why does EVERY other drogue maker (except JSD which has different issues) recommend nylon? I understand their logic, but I don't understand the apparent conflict. These are all experienced companies.
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Old 24-03-2010, 20:30   #45
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Agreed, I meant to say, not "greed."

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... why does EVERY other drogue maker (except JSD which has different issues) recommend nylon? I understand their logic, but I don't understand the apparent conflict. These are all experienced companies.
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