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Old 06-05-2013, 03:08   #286
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Re: Storm Sailing Advice

Evans wrote: "if you want a 'standard' set of definitions they have to be about the wave size and shape, which unfortunately 'storm' and 'gale' are not."

I couldn't agree more, but I don't think we're there yet.

The forecasts are not accurate for wave height, and contain nothing about wave shape.

Our instruments do not (yet) measure wave height, unless we're well within soundings, and most sailors overestimate heights by a factor of maybe 2

(in reality I think EVERYONE overestimates them, but the thoughtful among us apply a deflation factor based on careful calibration when objective data is available)

The only way I know to objectively measure steepness is by hindcasting: (a plumb bob and a protractor doesn't work on a small boat!) :

If my log shows that the sun disappeared behind the wave crests at 1450 at my latitude on such and such a summer's day, I can pull out an almanac subsequently and work out the angle from my height of eye in the trough.... provided I'm still around to tell the tale!

Satellite measurements of wave height are a huge leap forward, though, and presumably during the Quikscat era, there was raw data on steepness -- which was used to infer wind strength for output to the general public, kind of the reverse of what you're talking about, and an indication of our historical focus on what is, in some ways, the wrong measure.

In defence of wind strength: it is an important input to what you're pointing out is what matters, in the sense that it helps us estimate the likely development of the wave height and steepness over the duration of that windstrength.

It also gives us a clue of likely duration, in certain circumstances: if we get a remarkable increase in wind strength in advance of the seastate rising to match, we can be pretty sure it will pass quickly and the seastate will never reach proportionate size.

And of course for lots of other, meteorological reasons.
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Old 06-05-2013, 03:33   #287
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pirate Re: Storm Sailing Advice

Quote:
Originally Posted by Rakuflames View Post
1/ I would like to know -- do you take extra protective strategies when you sail by yourself, No...
2/ or do you think you're invincible, or so good that you don't need to wear a PFD, or don't need to think about what's going to happen if the boom wings you in the neck while it's on autopilot. Yes...... never use the main downwind.. also...boom operation is kept manual... so no winged neck from automated gybes..
3/ Do you think you're so good that something like that would never happen to you? I used to walk on water then some nasty buga put nail holes in my feet so now I improvise...


PS... not pickin on ya Raku... just never could resist a giggle...
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Old 06-05-2013, 03:39   #288
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Re: Storm Sailing Advice

I agree with Snowpetrel:

When a masthead anemometer is being subjected to the sort of seastate you get in strong winds, it seems to me the apparent 'wind run' past the mast could easily be 30% higher than it would be in the absence of the rolling, pitching and heaving.

And spurious wind run is not alleviated by averaging, because the instrument does not average the vectors, just the magnitudes of the vectors.

The detailed appearance and behaviour of the surface catspaws, white horses, foam, streaks, spume etc is the only reliable guide, and you can only really calibrate this in sheltered waters with good instruments.


... and interestingly, although I haven't done a lot of lake sailing much of it has been fairly windy, and I reckon the "visual cues calibration factor" differs in fresh water from salt, at least in winds strong enough to pick the water up.

One thing about this aspect of the appearance of the sea surface in stormforce winds: it's pretty rare to see it accurately depicted in a feature movie. So rare, that I can't think of a single instance. (The Perfect Storm was in the top 5%, by having a couple of semi-convincing moments -- out of - what, 90 minutes? A deeply unconvincing movie of a very unconvincing book, I thought)

I felt like walking out of the cinema on the most recent occcasion, which was beyond silly -- not surreal, like "Life of Pi", which I thought was a great exploration of the power of imaginative belief -- but extraordinarily and stupidly unreal in the nature of the seas depicted.

Not even a parody, because a parody relies on manipulating aspects of reality, which were virtually absent in this case. But the most unifying thing about "special effects" waves is that they look so young and fresh.

Big waves in big winds --at least, the ones I've seen -- generally look ancient, verging on pre-historic.

Anyway, I was talking about the movie trailer which had me wanting to puke (not with mal de mer): It was just a few months ago, and I'm proud to report I don't even know the name of the ridiculous movie.

But it worries me that increasingly, kids (and adults) are being force fed a whole bunch of sexed-up representations of a world (not just the sea, but the whole deal: deserts, mountains ... ) they'll probably never get to see with their own eyes, because (like me right now) they're spending increasing proportions of their lives interacting with mirages.

And what troubles me most about that is that they are likely to find the real thing disappointing, understimulating, possibly boring.

A decent storm at sea might fix the last two things on that list, though :-)
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Old 06-05-2013, 04:00   #289
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pirate Re: Storm Sailing Advice

I've rarely owned boats with WS insturments and rely on the Beaufort visuals of the sea state for my windspeeds... they are fairly accurate overall...
above a certain level I could not give a $*^t as I'm reefed, heaved and down below...
My Westerly Longbow had one that recorded WS up to 50knots... that was a pain when the crew saw it.. freaked her right out..
Just another useless toy.. like the one some folk need to tell them where the winds coming from...
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Old 06-05-2013, 04:15   #290
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Re: Storm Sailing Advice

This is a great thread. We have some some very experienced yachtsmen that allow us pick their brains, and gain from their considerable experience.

Lets not let this thread deteriorate into bickering and mindless personal drivel. Posts of this nature have been deleted.
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Old 06-05-2013, 04:33   #291
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pirate Re: Storm Sailing Advice

Quote:
Originally Posted by noelex 77 View Post
This is a great thread. We have some some very experienced yachtsmen that allow us pick their brains, and gain from their considerable experience.

Lets not let this thread deteriorate into bickering and mindless personal drivel. Posts of this nature have been deleted.
Please... name calling is totally unwarranted and offensive... you'll be calling me a 'Politician' next....
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Old 06-05-2013, 04:50   #292
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Re: Storm Sailing Advice

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Originally Posted by Jim Cate View Post
When you think about this, it means that every 15 minutes or so that 2x wave will come along. I guess this means that "merely" being twice the sig height isn't usually a big deal since we've all sailed in biggish seas for much longer than 15 minutes without serious incident.
Another way of looking at it is that those once-every-15 minute waves are the waves we tend to think of when we describe the conditions.

Which either means that (as I think I might have posted in this very thread)

- when we overestimate wave heights by a factor of two, perhaps it's because we actually focus on the high waves.

(It's a pity the 'significant' waves don't come with a handy label, saying 'Pick Me !')

or

- if we tend to overestimate individual waves by a factor of two (which I think is a natural tendency) then we may overestimate the significant wave height by a factor of up to four !
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Old 06-05-2013, 05:02   #293
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Re: Storm Sailing Advice

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Please... name calling is totally unwarranted and offensive... you'll be calling me a 'Politician' next....
Sorry if you don't meet the definition. Is there an operation in your past that you are not mentioning

Anyway you get my vote for prime minister
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Old 06-05-2013, 05:28   #294
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pirate Re: Storm Sailing Advice

yacht

/yɒt/ Show Spelled [yot] Show IPA
noun 1. a vessel used for private cruising, racing, or other noncommercial purposes.


verb (used without object) 2. to sail, voyage, or race in a yacht.



Origin:
1550–60; < early Dutch jaght, short for jaghtschip hunting ship, equivalent to Dutch jacht hunt (derivative of jagen to hunt) + schip ship

Related forms yacht·y, adjective
su·per·yacht, noun

Can be confused: barge, boat, canoe, cruise ship, sailboat, ship, yacht.


However....
The term "seaman" is a general-purpose for a man or a woman who works anywhere on board a modern ship, including in the engine spaces, which is the very opposite of sailing. Furthermore, "seaman" is a short form for the status of an "able-bodied seaman," either in the navies or in the merchant marines. An able-bodied seaman is one who is fully trained and qualified to work on the decks and superstructure of modern ships, even during foul weather,[2] whereas less-qualified sailors are restricted to remaining within the ship during times of foul weather — lest they be swept overboard by the stormy seas or by the high winds.

Its a 'Class' thing...
you wouldn't understand unless you've been one
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Old 06-05-2013, 06:05   #295
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Re: Storm Sailing Advice

Back to storm sailing...

Here's a graph that I've found to be very helpful for estimating wave heights offshore. It's called the Sverdrup-Munk-Bretschneider Nomogram, and it shows the relationship between Wave Height, Wind Speed and Fetch. Wind duration is also a part of the picture, so it's assumed that the waves are "fully developed", which I believe can take two or three days of constant wind.

It doesn't take into account waves produced by other, distant wind sources, but if you're out in a big blow for a few days, the local winds will be the driving factor. I've tested it offshore in a gale that blew 40-45 knots for three days. The highest waves appeared to be in the 25' range, estimated by watching the horizon disappear and by gauging the crests versus the spreaders (consciously trying not to over-estimate), which is pretty close to what's shown on the nomogram.

Click on the image to enlarge it.
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Old 06-05-2013, 06:33   #296
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Re: Storm Sailing Advice

Geek alert . . . only read this if you are interested in statistical details of waves

1. How frequent are waves greater than 2x significant height? Lets assume a Rayleigh distribution for a moment (discussed below). Then waves exceeding 2x height are 1 in 2943. With say a 15 sec period, that's every 12 hrs. However, the 1 in 1000 waves exceed 1.86x sign height and that's in every 4 hrs (statistically). So you can see that this distribution has a very very flat tail and a little increase in height greatly decreases the frequency.

2. One complication to the above is that waves in the real world (because of different interacting wave patterns) come in series or trains (several waves together that are similar height). So in the real world the 2x sig waves are more likely to be 3 close together and then none for a 36hrs, than they are to be one each 12hrs.

3. Another complication is that the Rayleigh distribution is an approximation of the actual real world wave distribution. The wave probability distribution is suppose to be the probability of a given wave at a given location. It is empirically impossible to collect data to accurately create or confirm this model, since in the real world there is only one wave at that one location and that time. A wave buoy can measure waves over time at one location, so it will be mixing multiple wave probability distributions as the storm and waves develop over time during the storm. Radar wave measurement can measure more waves faster, but over a geographic range, and as discussed in posts above, the pressure gradient and waves can vary a lot (more than 50%) over even a 30nm range.

But no theoretical model is ever completely correct, and we can try to correct for these know errors. When that's done, the data I have seen from actual radar wave measurements (hurricane hunter planes and satellites) suggests that the probability of 2x waves is significantly lower (by a factor of 3-5) than the Rayleigh suggests in 'normal' (eg non storm) conditions but significantly higher (also by a factor of 3-5) in hurricane conditions. The buoy data set that was linked to above seems consistent with this, showing one greater than 2x wave over a 48hr period of 'normal' (non-storm) conditions.

4. Regarding the Beaufort scale . . . Yes it does include a 'sea conditions' description, and if we focused on that and essentially ignored the wind scale I think it could be useful.

Breaking waves are the prime factor in storm survival. The Beaufort descriptions mention breaking waves but are really more focused on the 'look and texture' of the surface. So, I would be even happier if we found some set of descriptions even more focused on breaking wave development.

As I have mentioned above, I just find the wind speed to be so poorly correlated to breaking waves as to be really unhelpful. Also, the fact that sailors so poorly report wind speeds may not be a flaw in the Beaufort scale itself but it is definitely a flaw in using wind speeds as the primary metric in discussions about our own experiences and incidents (and also the 'experts'). I know that wind speeds are 'easy shorthand' but I also know they are terribly misleading and when we use them it does not communicate to newbies that it is all about the waves.

Anyway . . . . none of that is really all that practically helpful . . . . except to say that there are occasionally very big waves (bigger than the mean and significant wave), and they may come in sets, and sailors should know that and when picking tactics and they should hold some safety margin in reserve (vs. the current 'average' wave conditions) so as to be able to deal with it.
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Old 06-05-2013, 07:05   #297
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Re: Storm Sailing Advice

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Here's a graph that I've found to be very helpful for estimating wave heights offshore.
A very good point, thanks for the link. I use the one from the now slightly dated van dorn book "seamanship and oceanography". It includes time as a factor as well, which can be useful. I have been meaning to get a copy laminated to post someplace handy on the boat!
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Old 06-05-2013, 07:14   #298
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pirate Re: Storm Sailing Advice

I find cliffs pretty useful for gauging wave heights...
McNamara Surfing a 100 foot wave on 1-29-2013 - YouTube
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Old 06-05-2013, 07:40   #299
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Re: Storm Sailing Advice

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PS... not pickin on ya Raku... just never could resist a giggle...

You do realize I was asking Tom, because Tom was taking it so seriously (and aggressively) ... but lots of experienced sailors use their boom in unpredictable downwind weather (at least once!) -- because CS did, and I'm so glad he was willing to share that story.

Saturday (as someone pointed out it was very windy in this area on Saturday afternoon) a friend was goind straight downwind, wing on wing -- with the whisker pole on the headsail --, up the Manatee River. They were doing 7 1/2 kt. I'm not sure why his wife had her hand on the mainsheet, but she did, and she'd forgotten to put her gloves on and has second degree rub burns on her hand from the giant gybe. He wasn't the only boat that sailed the river that way, either, although the other skipper decided to take the whisker pole down. Don't know if he took his main down or not. He's sailed a very long time, and he would disagree with you. Me, I'm listening to CS on this one.

I particularly related to CS's story because my boat balances much better with some mainsail up. The bow lifts way too much in a gust if only the headsail is out.

I know you have a lot of fun joking around with issues, and that's OK, but I appreciated CS telling us what happened.
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Old 06-05-2013, 07:42   #300
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Re: Storm Sailing Advice

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Originally Posted by estarzinger View Post
Geek alert . . . only read this if you are interested in statistical details of waves
Thanks for this, I for one am very interested.

Quote:
2. One complication to the above is that waves in the real world (because of different interacting wave patterns) come in series or trains (several waves together that are similar height). So in the real world the 2x sig waves are more likely to be 3 close together and then none for a 36hrs, than they are to be one each 12hrs.
This is a very good point. And more closely aligns with my real world experience, that 2x waves are a rarer beast than statistics seem to indicate.


Ramble alert!

The first rogue wave I encountered was on trip from NZ to tonga on a 28 foot steel ganley double ender. We were in about 30 knots (force 7) broad reaching with a 3 reefed main and staysail. I was more or less hand steering as the trim tab windvane was struggling. We where pushing her harder than she really liked. The top drop board was out to let some air out and a small amount of spray in below. The sun was shining and life was good...

Suddenly I heard a roar behind me and looked behind to see a 10 foot wall of white water coming at us from about the beam, I really don't know exactly what happened next except I remember the skippers face in the galley as I yelled to him, and the impact of the wave as I tried to turn the stern into it (without any luck). He bravely held a chopping board up in the hatchway! When the water cleared from my eyes and I got a breath of air I found we where somehow perched and about to start surfing down the face of a very steep, and incredibly deep wave face. My guess is 30 or more feet? But who knows. I still reckon it was about three times the height of any of the other waves we had seen. I let her broach rather than risk pitchpoling or a high speed broach. And we where tumbled about for a second time.

We came up facing into the wind, on the opposite gybe, with the preventer bail ripped out of the boom (probably a good thing!). The trim tab was ripped off the rudder, and the two inch SS pipe tube for the windvane was bent, a shelf in the galley was torn off by the force of the water coming down the hatch. The weathercloths around the cockpit were badly torn and the boat had about 8 inches of water over the floorboards, and was wallowing badly.

Amazingly (and fortunately) it was very calm for about 15 minutes after these two waves, the sea had a fizzy feel, as all the trapped air bubbled out. We dropped the main, and slowly sorted out the boat, making it to Tonga about 5 days later having finally got all the bedding dry by hauling it up to the masthead away from the spray. Somehow both sides of the inside of the boat had been underwater, and all the electrics slowly failed.

At no time where the conditions before the "wave" anything other than brisk, fun sailing. And then it all suddenly turned pear shaped!

I should point out that we had safely (but rather uncomfortably) run before a force 8 earlier when bringing the boat up from wellington. The boat was tender but strong and had coped well with just a reefed staysail.

I still do not know if it was two separate waves, or a kind of double wave. it definitely had two crests. We were a few hundred miles west of the kermadecs, and the seabed there is pretty dynamic, maybe that was a cause... Since then I have read what I can find on waves.
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