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Old 23-04-2013, 19:53   #16
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Re: Storm Sailing Advice

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Originally Posted by estarzinger View Post
I am just writing an article on this . . . see survival sailing

It's a working draft - content is all there but it has not had its clean up line editing yet.
Great contribution Estarzinger… thanks for sharing your draft.

One additional factor that is often overlooked in teaching survival tactics, is an awareness of the differences in prevailing Tidal Dynamics as it effects trends and behavior upon breaking seas

Offshore: In the open ocean, tidal currents are manifested as rotary currents, which are continually changing direction in a consistent manner. Predicting when these tidal currents will act against prevailing wind and sea, will help to alert yacht crew of increased danger from larger breaking cross-seas.

When approaching Near Shore: Coastlines obstruct tidal movements, and thus rotary currents are not observed. Instead, in estuaries and rivers emptying to the sea, reversing tidal currents occur in and out in association with high and low tides. This is when dramatic changes in sea state occur in combination with shoaling waters. So prudence as when to make a landfall is advised.
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Old 23-04-2013, 20:37   #17
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In terms of current best practices, I think that Evans' article, posted in this thread, is a better option for larger, fin-keeled boats than the Pardey's approach, which gears toward smaller full-keeled boats.
True any tactic must be adapted to the boat and conditions.

But to do this you must know both the boat and the tactics hands-on...during a storm conditions is not the ideal time to learn.

For example, Ive hove-to under sail only in storms and squalls many times but I have never used the Pardeys sea anchor assisted method. I plan to drag the old sea anchor out and try it sometime soon in benign conditions.

I think their tactic would actually work well for fin keeled boats which dont tend to heave-to very well under sail alone.
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Old 23-04-2013, 20:37   #18
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Re: Storm Sailing Advice

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Great contribution Estarzingerů thanks for sharing your draft.

One additional factor that is often overlooked in teaching survival tactics, is an awareness of the differences in prevailing Tidal Dynamics as it effects trends and behavior upon breaking seas

Offshore: In the open ocean, tidal currents are manifested as rotary currents, which are continually changing direction in a consistent manner. Predicting when these tidal currents will act against prevailing wind and sea, will help to alert yacht crew of increased danger from larger breaking cross-seas.

When approaching Near Shore: Coastlines obstruct tidal movements, and thus rotary currents are not observed. Instead, in estuaries and rivers emptying to the sea, reversing tidal currents occur in and out in association with high and low tides. This is when dramatic changes in sea state occur in combination with shoaling waters. So prudence as when to make a landfall is advised.

Yeah he covered the second one. It's not just circular currents, though. The Gulf Stream flows in one direction, N/NE, for a very long time. If the wind is also from the north, it can be a rough sail. But it's not that wide, and if you can get out of it (to the deep water side; it runs very close to the coast in some places) -- things might settle down considerably. But surely anyone in this situation would know what major current they were in or near ...
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Old 23-04-2013, 21:27   #19
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Re: Storm Sailing Advice

Yes I agree, prevailing offshore currents like the GS, counter -equatorial or the Kuroshio are well known and not circular. You should be aware of them and in heavy weather you can either catch a ride on, or avoid if you have opposing wind and sea.

Technically, they are not defined as tidal currents

My point is realizing when Offshore tidal currents will change sea states is a lesson every offshore tug boat captain learns when monitoring his tow lengths and stresses. I believe this is also a helpful point for a yachtsman to be aware of.
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Old 24-04-2013, 00:08   #20
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Re: Storm Sailing Advice

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My point is realizing when Offshore tidal currents will change sea states is a lesson every offshore tug boat captain learns when monitoring his tow lengths and stresses. I believe this is also a helpful point for a yachtsman to be aware of.
The deep sea fishermen I have talked to have mentioned that they like to get well off a seamount if it looks like blowing hard.

Apparently even the deep ones (800+ meters or so) can cause some pretty wild currents at sea level making the sea state much worse. Sounds like these surface currents may be caused by fast upwelling currents from much deeper.

Since hearing this I like to avoid any steep sided subsea features if it's looking nasty, and I have heard of a few boats getting into trouble near these sorts of features.
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Old 24-04-2013, 00:14   #21
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Re: Storm Sailing Advice

Yes.. you will often see notations on charts of reported rough water in certain sectors far offshore
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Old 24-04-2013, 01:27   #22
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Re: Storm Sailing Advice

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Yes.. you will often see notations on charts of reported rough water in certain sectors far offshore
I've seen plenty of notations for shallower banks, typically 30 meters depth or so warning of "breakers rep" or "breaks in heavy swell" But not for the deeper features such as 500 meter seamounts. I feel lucky if there are a few contours and soundings.
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Old 24-04-2013, 02:36   #23
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Re: Storm Sailing Advice

Interesting, SnowP, I'd independently come to the same conclusion about vertical currents, not just adjacent to deeply submerged seamounts, but (as you point out) to steep undersea features with major vertical extent, even if their ridge lies in deep waters... but you're the first other person I've heard talk about this.

It seems to me from untutored observation that a vertical component to a current eddy is a lot more provocative in humping up swell trains than a similar horizontal component, and I suppose that makes intuitive sense.

While I've got your attention: Have you (or anyone else reading this) ever encountered a high swell train (say 7-10m) arriving fully formed within a very small period, from a direction with virtually unlimited fetch?

I'm thinking of less than an hour to go from 0 to 7m (in this case, 'stood up' by a counterflowing tidal current - but not created by the current).
The wind rose at about the same time, but only to a decent summer's afternoon breeze (certainly not >25kn). The swell endured for at least as long as it took us to get to shelter, maybe three hours, showing no signs of abating.

The venue was the outer approaches to French Pass on passage from Kaiteri, and the train pulled in from the NNW.

It happened to me when I was a young'un, and it's taken me until about six months ago to suddenly come up - in the early hours of the morning, as it happens ... with what seems to me a plausible mechanism for something which had previously seemed (to me) impossible to explain.

Given that even phenomenally strong winds have to blow for many hours to couple enough energy into the water to initiate such a high swell, what happened to the lower precursor waves?
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Old 24-04-2013, 02:40   #24
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Re: Storm Sailing Advice

Thanks, Evans.
Very comprehensive and to the point.
I hope never to be in a survival storm, but wishful thinking is always a bad idea, I reckon, and never more so than for a sailor.
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Old 24-04-2013, 03:23   #25
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Re: Storm Sailing Advice

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Given that even phenomenally strong winds have to blow for many hours to couple enough energy into the water to initiate such a high swell, what happened to the lower precursor waves?
I don't think this applies to the situation you've described, but I've seen waves kick up significantly under a tight tradewinds squall. We're sailing downwind, watching this big black squall cloud approaching for about an hour. It finally catches up to us, and along with the wind-shift, and the windspeed instantly jumping from 15 to 30 kts, the waves also kick up instantly and significantly.

I figure that the squall is traveling about the same speed as the waves, so that one small patch of water underneath has seen high wind for a long time.
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Old 24-04-2013, 03:35   #26
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Re: Storm Sailing Advice

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...
I figure that the squall is traveling about the same speed as the waves, so that one small patch of water underneath has seen high wind for a long time....
That's an interesting possibility, Paul, and nicely reasoned.

As you say, probably not applicable in the instance I raised, but very plausible for mid-latitude squalls, I reckon.

I haven't done a lot of miles in the trades (well, I suppose thousands, but not tens of thousands) so I have to admit to being the last person to have an informed opinion.

The behaviour you describe does seem to tally with at least one rapid-onset squall I recall from those waters, though ...
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Old 24-04-2013, 03:37   #27
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Re: Storm Sailing Advice

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It seems to me from untutored observation that a vertical component to a current eddy is a lot more provocative in humping up swell trains than a similar horizontal component, and I suppose that makes intuitive sense.
You also see this in the good old Karori rip. Sometimes it heaps up pretty bad even at slack without any real flow. The best reason I can think of is a subsea current being thrown up at this point, however I have read something about internal waves that might also explain it somehow.

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While I've got your attention: Have you (or anyone else reading this) ever encountered a high swell train (say 7-10m) arriving fully formed within a very small period, from a direction with virtually unlimited fetch?

I'm thinking of less than an hour to go from 0 to 7m (in this case, 'stood up' by a counterflowing tidal current - but not created by the current).
The wind rose at about the same time, but only to a decent summer's afternoon breeze (certainly not >25kn). The swell endured for at least as long as it took us to get to shelter, maybe three hours, showing no signs of abating.
There is a phenomena called a wave front? where the waves are moving about the same speed as the weather system or front. It can cause the waves to rapidly increase in size much faster than the normal wind/fetch/time graphs show. Swells can also hit quickly the same way in some cases, without much warning, instead of gradually building. It would be interesting to hear an oceanographers take on it. It is pretty rare from my experience to get something so dramatic.

Though I can remember once sailing to Bluff from Hobart when the sea state rapidly changed from a big fun swell to a very big scary breaking sea over a few hours. I remember a rumble like distant thunder as that big underlying swell steepened up and started to break. I imagine this is the greybeards the old sailors talked about.

The wind had been increasing slowly all day from 30 to 50 knots. but it was quite a sudden and drastic change in the sea state, as if it had suddenly reached a trigger point. It was not so much that waves slowly increased in size rather it was the existing swell getting steeper and steeper and then suddenly reaching the critical slope angle to start breaking.

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Given that even phenomenally strong winds have to blow for many hours to couple enough energy into the water to initiate such a high swell, what happened to the lower precursor waves?
I think the lower precursor waves move slower, and dissipate faster, almost being left behind if the system is moving real fast. At other times like a slow moving front or squall the wind is almost stationary and the small precursor waves are a good indication that you need to reef and do it fast.
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Old 24-04-2013, 04:00   #28
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Re: Storm Sailing Advice

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I think the lower precursor waves move slower, and dissipate faster, almost being left behind if the system is moving real fast. At other times like a slow moving front or squall the wind is almost stationary and the small precursor waves are a good indication that you need to reef and do it fast.
That's pretty much the explanation I eventually came up with, except it took me decades, and you minutes ! (Bastard ! )

I figured that the system generating the swells was a long way away, because it was a very well-organised train: all waves exactly the same height and shape.

So it seems to me that the waves had done that 'sorting' algorithm thing that turns them from wind waves to swells, where all the waves of a given wavelength are travelling together due to sharing the same speed of propagation, and the system had moved far enough and the highest waves were so high that the biggest, fastest waves had all reached the head of the train by the time they reached us.*

I'm not sure why the wind arrived at the same time, because it was clearly not connected with the wind which created them in the first place. But it occurred to me to wonder: can a swell train with such steep, high swells, travelling at the best part of thirty knots, create a breeze of its own?
(It arrived out of a flat calm, the pressure distribution gave no prospect of a gradient wind developing, and there was no cumulus development over the shore to indicate unstable rising air which would support a sea breeze)

- - -

* It's perhaps a bit like how all the grains of sand deposited a given distance inland from a surf beach tend to be the same size, except there, they get smaller as the distance is increased. (Not a great analogy, but kind of faintly relevant)
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Old 24-04-2013, 04:00   #29
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Re: Storm Sailing Advice

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...windspeed instantly jumping from 15 to 30 kts, the waves also kick up instantly and significantly.

I figure that the squall is traveling about the same speed as the waves, so that one small patch of water underneath has seen high wind for a long time.
You must have gotten this post in while I was still typing (i'm a slow typer). exactly what I was trying to describe in a much more concise way. Cheers.
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Old 24-04-2013, 04:04   #30
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Re: Storm Sailing Advice

That's an interesting story, SnowP, about the Bluff to Hobart gig.

Do you think there might have been a counter-flowing ocean current to cause the swell to steepen so seriously, off soundings? It's great being able to get satellite data on this stuff these days, no?

(Or, thinking of our recent musings.... any sudden changes in seafloor topology?)
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