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John Russell
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Initially Posted - 04/16/2007 : 10:11:49

I'm learning everyday. I can't seem to figure out what the "1000 FM" means in weather statements as in the following:

...STORM WARNING...
.THIS AFTERNOON...W OF 70W...W TO SW WINDS 30 TO 40 KT
INCREASING TO 35 TO 45 KT. SEAS 10 TO 18 FT...EXCEPT 18 TO 25 FT 
E OF 1000 FM. SCATTERED SHOWERS. E OF 70W...S TO SE WINDS 40 TO
50 KT EARLY...BECOMING SW AND DIMINISHING TO 20 TO 30 KT LATE.
SEAS 22 TO 29 FT...HIGHEST SW...SUBSIDING TO 19 TO 22 FT. RAIN
AND SCATTERED TSTMS...CHANGING TO SCATTERED SHOWERS.

Can anybody enlighten me?

John Russell
1999 C250 SR/WK #410
Bay Village, Ohio
Sailing Lake Erie
Don't Postpone Joy!

Edited by - on

tinob
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Response Posted - 04/16/2007 : 10:21:52

AND ME?

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Dave Bristle
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Djibouti
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Response Posted - 04/16/2007 : 10:38:18

1000 fathoms (6000 feet).

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John Russell
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Response Posted - 04/16/2007 : 10:56:13

That's kinda what I thought but location coordinates and depth coordinates didn't seem to be connected.

So does that mean, in the example above, that the wave hieght changes (increases in this example) as the continental shelf drops off? It's the "E" of 1000 FM that has me scratching dandruff on my keyboard.

Edited by - John Russell on 04/16/2007 10:56:40

Dave Bristle
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Response Posted - 04/16/2007 : 12:33:16

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by John Russell
 ...It's the "E" of 1000 FM that has me scratching dandruff on my keyboard.
<hr height="1" noshade id="quote"></blockquote id="quote">
That's the internationally recognized abbreviation for East.

As you go further east from the east coast, the Atlantic gets deeper--till you start approaching Europe or Africa. The deeper the water, the longer (and therfore higher) the waves can be. (Basic physics.)

(Try to imagine over a mile of water between you and the bottom, and what might be down there...)

Edited by - Dave Bristle on 04/16/2007 12:38:20

Steve Milby
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Response Posted - 04/16/2007 : 14:00:57

<blockquote id="quote">quote:<hr height="1" noshade id="quote">So does that mean, in the example above, that the wave hieght changes (increases in this example) as the continental shelf drops off? It's the "E" of 1000 FM that has me scratching dandruff on my keyboard.<hr height="1" noshade id="quote"></blockquote id="quote">There are different conditions that cause an increase in the size of waves. Waves can increase in size because the windspeed increases, or because of a shoaling bottom, or because of a long fetch. In this case, when the wind is out of the west, the farther east you go, the longer the fetch and the bigger the waves will grow. Even if the bottom is becoming deeper as you go farther east, that isn't the condition that is governing the size of the waves. The increasing length of the fetch is the culprit. That's why the waves at Cape Horn are frequently so huge. It's the only latitude where there is no land mass all around the world to interrupt the growth of waves.

Edited by - on

John Russell
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Response Posted - 04/16/2007 : 14:20:54

That's what got me confused, Steve. I know that waves are a function of wind over water. The confusion in my simple mind occurred when thinking about the continental shelf dropping off to 1000FM un-related to its distance from the shoreline. (Or, last wind obstruction) So, it's a matter of checking charts for depth in addition to lat and long when reading weather information.

Unfortunately Lake Erie has the other side of this issue, if I understand this correctly. Lots of fetch without much depth causing chop instead of big rollers.

Anyway, thanks Dave & Steve for the continued enlightenment.

Now if I could only figure out those other letters.... N W S SW .... man, so much to learn!

Edited by - John Russell on 04/16/2007 14:21:24

Dave Bristle
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Response Posted - 04/16/2007 : 17:01:17

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Steve Milby
 Even if the bottom is becoming deeper as you go farther east, that isn't the condition that is governing the size of the waves. The increasing length of the fetch is the culprit.<hr height="1" noshade id="quote"></blockquote id="quote">
I'll have to take the bait...

The 1000-fathom line is the generally-accepted boundary (approximate) between the continental shelf and the deep ocean. Sea states generally differ between the two--thus the distinction in most NOAA ocean reports. (If the wind is N or NE, the Gulf Stream has its own nasty state.) Note to John: That's North or Northeast.

Steve--I know you're a lawyer, not a physicist...

You're right that the W-SW winds were likely to build wave sizes as the waves moved further out into the Atlantic because of the essentially unlimited fetch. But no matter how long the fetch or how strong the wind, the length and speed of a wave is limited by the depth of the water. If the length is limited, then the height is also--at the point where the wave goes vertical and breaks. Thus, as a wave moves into shallower water, it slows down, becomes more vertical, and possibly falls over itself. A tsumami that is almost undetectable as it travels at up to 5-600 miles per hour in the mid-Pacific, slows way down and turns into a tumbling wall of water as it approaches land. And as John points out, shallow Lake Erie has short, steep chop while deep Lake Superior has large rolling waves which in turn are nothing like Atlantic swells beyond the continental shelf. The Chesapeake is more like the former, even with the long fetch of a N wind. (John--that's North.)

Now I've probably explained more than I know...

Edited by - on

Oscar
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Response Posted - 04/16/2007 : 17:33:37

All correct. NE winds on Lake Erie make for 6-8 foot chop in the South Western half....can't call them waves. Breaking all over the place. I remember a few long, wet sloughs in the C30....(Lady Kay I).

Indeed, a healthy wind NW through NE in the Gulfstream makes for very nasty conditions. THIRTY foot breakers are not unheard of....

Edited by - Oscar on 04/16/2007 17:34:25

Dave Bristle
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Response Posted - 04/16/2007 : 18:42:51

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Oscar
 All correct.<hr height="1" noshade id="quote"></blockquote id="quote">
Whew!!

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Champipple
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Response Posted - 04/16/2007 : 19:04:33

Being a Lake Erie sailor all my life my question is This

Why have I never seen that before?

John, can you give the source of the weather report...obviously it is a noaa report, but you don't show the region or zone. If its the coast then it is the 1000 fathom line...

Oscar - when an america's cup sailor calls them waves then they are waves...chop doesn't break and has no frequency.

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John Russell
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Response Posted - 04/16/2007 : 19:30:19

Yeah, Duane, it's NOAA Weather stuff. I've been following Donna Lange and wanted to read the weather warnings directly. By the way, her drogue has failed and she's attempting to make Bermuda, 250+ miles to the E (that would be East)

http://weather.noaa.gov/pub/data/raw/fz/fznt22.kwbc.off.nt2.txt

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Dave Bristle
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Response Posted - 04/16/2007 : 20:10:14

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Duane Wolff
 Being a Lake Erie sailor all my life my question is This: Why have I never seen that before?<hr height="1" noshade id="quote"></blockquote id="quote">
You answered your own question.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">...chop doesn't break and has no frequency.

<hr height="1" noshade id="quote"></blockquote id="quote">
Baloney. "Chop" is in the eye of the beholder. You're thinking of "chaos", as in an inlet or in front of a seawall. (Should I do a treatise on "standing waves"?

Never mind.)

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Steve Milby
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Response Posted - 04/17/2007 : 10:25:20

Most of what we know, or think we know, about waves was developed early in World War II, after a disastrous landing at Tarawa, in the Gilbert Islands, where over half of the landing craft were capsized or stranded on the reef under heavy enemy fire. At the urging of the military, scientists at the Scripps Institution of Oceanography developed methods of wind-wave prediction. In Oceanography and Seamanship 1st ed., William Van Dorn expresses his opinion that considerable disagreement remains even now with regard to some of the general principles, and notes that conditions that actually have been observed were often not predictable by generally accepted formulas.

As Dave observed, the depth of the body of water limits the possible height of waves. But, that isn’t the controlling factor that we sailors need to worry about, because bluewater sailors are unlikely to ever see a deep-water wave that has reached it’s maximum theoretical height. It has been calculated that the ocean is capable of sustaining a stable wave as high as 70% of the water’s depth. Thus, in the Pacific Ocean, a stable wave could possibly be formed about 14,000 feet in height. Oceanography and Seamanship, at pg. 149.

Waves are created when some kind of force or energy is applied to a body of water. The only thing that could impart enough force to generate that 14,000 foot high wave is probably a huge meteor strike. An undersea earthquake can create enormous Tsunami waves. But the force that creates most of the waves that concern sailors is the wind. I think it’s safe to say that the wind is not capable of creating deep-water waves anywhere near the theoretical maximum height. The depth of the ocean does not realistically impose a limit on the size of wind-driven waves, because the wind is incapable of imparting such a concentrated force as to generate a wave anywhere near the maximum possible height of an ultimate, deep-water wave.

But, the waves that we sailors really have to worry about are not so much the big waves, as the breaking waves. The height of a wave is not the factor that determines whether a wave will break. That is determined by the wave's shape. We have all seen some 3' waves break as they approach the shore. Likewise, most of us have at least seen photos of waves between 30 and 60' high that were not breaking waves.

The waves that most of us need to worry about are not the deep-water waves, because most of us will never sail more than a few miles off-shore, and we’ll head for shelter if it starts to get bad. The waves we need to worry about are shallow-water waves, which will break when their height exceeds about 80% of the water’s depth. Those are the waves which roll in from the sea, and, as the bottom rises, the height of the wave rises and steepens, until it can no longer support it’s own weight, and it collapses, rolling over anything in its way. That’s why, when you’re sailing along the shore, you should stay in water that is much deeper than the height of the waves. That’s also why sailors with a shoal keel should always resist the temptation to crank it up and take a short-cut across a shallow area.

As Oscar noted, there are other factors that can cause waves to build into breakers, such as a strong contrary current, as in the Gulf Stream. But the Gulf Stream is not the only place where you might find a strong contrary current. You also find them around bridges and passes and anyplace where the ebb and flow of the tides is restricted. They can also contribute to the creation of breakers, especially in combination with a shoaling bottom.

You don’t need to think about these things much when you sail an inland lake, but when you take your boat to tidal waters, or charter there, you can suddenly find yourself in trouble if you don’t include them in your planning.

Edited by - Steve Milby on 04/17/2007 10:41:10

Oscar
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Response Posted - 04/17/2007 : 10:38:39

Thank you Dave. The Annapolis Book of Seamanship defines "chop" as "Short steep waves."
Sailing on a close reach or close hauled, in the Western part of Lake Erie in a C30 was like taking a pick up truck, accellerating to 5mph and running it into a concrete wall, the speed would almost go down to zero, and when you got it going again you ran into the next one, well below their crests, the bow burying and the front deck covered in a foot of water These were short, steep waves, but on a very large scale. A very nasty chop indeed.

Says the guy who's been around sailboats most of the last 50 years......then again I've been wrong a few times before........

Steve types faster than I do......

Edited by - Oscar on 04/17/2007 10:41:47

tinob
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Response Posted - 04/17/2007 : 10:56:52

Thanks all and I thought the FM was frequency modulation

though the 1000 raised the question.

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John Russell
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Response Posted - 04/17/2007 : 11:12:56

Thanks for the lessons. I thought I asked a simple questionbut it sure generated an interesting discussion.

Oscar, I'm particularly interested in your last post since I'm about to put my "new" boat(read: first-ever serious sailboat) in the Sandusky Bay in western Lake Erie.

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dlucier
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Response Posted - 04/17/2007 : 11:30:24

Another thing is, you don't need a lot of fetch, relatively speaking, to experience large waves.

Once, I got caught in 50mph winds on Lake Erie a few miles off the coast of Canada. The wind was blowing directly from land and in no time, produced 7-8 waves of short periodicity. (My wife swears they were larger and maybe they were,...who knows?) These waves didn't need Lake Erie's 400 mile fetch to build, they only needed a few.

As to defining chop, to me, chop consists of small waves, couple of feet or less, that makes for a bumpy ride.

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Dave Bristle
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Response Posted - 04/17/2007 : 11:46:30

Steve: Interesting history--and I agree with all of that, but will add a bit (based on personal experience as much as physics courses, in which I did not excel, and history courses I didn't take)...

Block Island Sound (open water off RI) is mostly in the 100-200' range. I don't recall how far out the shelf is--it's way beyond where I go around here. (I'll guess it's 50-75 miles beyond Block Island.) But the length and speed of the swells out there is enormously greater than those in the "Sound", and thus their shape is very different. Fishermen and offshore cruisers alike pay attention to the conditions described inside and outside of the 1000-fathom line, because those are two different environments, sea-state-wise, even though there is nothing obstructing the seas except the bottom. And what's going on outside obviously can influence the inside.

Wave height, as you said, is indeed the smallest part of what that depth difference affects. But any given height turns from long swells to shorter chop as it enters the Sound, and vice versa, because of the change in speed. The 1000-fathom line is essentially at a big cliff where the depth goes from a couple of miles to a few hundred feet.

Oscar's "square waves" on Erie were probably very close to the maximum height that can be maintained for that short wavelength and slow speed (in what--15-20' of water?). Plus, a 30-footer might be exactly the wrong size in those seas. I recall being told that on Lake Michigan, a 36-footer was exactly wrong for that lake's wave period during or after a storm (in what I recall to be generally about 90' of water)--not big enough to ride comfortably through them but too big for the bow to pop up.

And you are certainly correct that lake sailors need to be aware of the surprising effects of currents and wind in tidal waters. Even a point can create some nasty surprises when the wind opposes the tide. Woe be to those who try to traverse "The Race" between the Atlantic and Long Island Sound when the wind opposes the monster tidal current that swirls over huge boulders 100' down--it is an awesome scene--one that nobody ever forgets but many write about! If you watch the weather and your Eldridge charts, The Race can be your moving walkway (albeit a bit lumpy)--otherwise you might think you're in a bathtub on the Colorado River in springtime!

That's about all I've got. John: The quiz is Friday.

Edited by - Dave Bristle on 04/17/2007 12:03:26

Champipple
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Response Posted - 04/17/2007 : 12:04:52

referring to why I've never seen FM in a forecast before...

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Dave Bristle
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Response Posted - 04/17/2007 : 12:16:05

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by dlucier
 As to defining chop, to me, chop consists of small waves, couple of feet or less, that makes for a bumpy ride.<hr height="1" noshade id="quote"></blockquote id="quote">
Come see Fisher's Island Sound on a busy weekend, with ferries, tugs, megayachts, ocean wind, currents, and God knows what else from every direction--I'll show you "chop"! (Here they call it "slop")

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dlucier
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Response Posted - 04/17/2007 : 12:20:20

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 Oscar's "square waves" on Erie...<hr height="1" noshade id="quote"></blockquote id="quote">

I'm not sure what you mean by "square waves". Most waves I encounter are more saw-tooth than anything else with a steep front and a more sloped back.

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Steve Milby
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Response Posted - 04/17/2007 : 13:05:42

Ocean waves are usually well organized. The wind blows from one direction, and the waves generally follow suit, all marching along in the same general direction, unless two different weather systems meet which are creating winds blowing in different directions. When that happens, the seas can get very confused and steep, and waves can combine with each other to create occasional rogue waves.

Short, steep choppy seas indicate a confused sea state. I think that largely accounts for the condition at Lake Erie. It's a closed system, with shore all around, islands, and lots of boats. On Lake Erie, waves don't just go in the direction of the wind. They strike the shores all around the lake and roll back into the basin, creating a new wave that starts rolling back out into the lake in a different direction. These waves are referred to as "reflected waves." As that new wave progresses, the wind causes it to increase in size. Eventually, it crashes into another wave, traveling in a different direction, and both of them combine and take off in yet another direction. This happens not only on the outer shores of the lake, but also on the shores of the islands in the lake. Also, every boat on the lake creates a wake that travels in yet another direction, and crashes and combines with other waves and goes off in another direction. The wakes of commercial ships can generate wakes that can be felt many miles away. This confused state of the seas is what makes Lake Erie so choppy. It's seas are highly disorganized.

Ancient Polynesian sailors became so proficient at "reading" reflected wave patterns that they could use them to find an island long before they were able to see it. There was a fascinating book about the Polynesian navigators, written, I think, by Dr. David Greene.

Edited by - Steve Milby on 04/17/2007 13:09:18

Dave Bristle
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Response Posted - 04/17/2007 : 13:06:52

Figure of speech, Don... Short, vertical, etc...

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Dave Bristle
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Djibouti
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Response Posted - 04/17/2007 : 13:30:47

Oh oh... Here I go again... Steve: In Oscar's case, it is about depth. Much of western L. Erie is only 15-25' deep, even miles out. All across the lake, you encounter very regular wind-driven waves that are short and steep, even with up to 200 miles of fetch. 8' is a very big wave on Erie, but it's especially big because it may be vertical, capping, and about to break. The beaches 50 miles away are not contributing to reflection.

Now, about intersecting waves... What actually happens is the waves combine at the point of intersection to create a larger peak, but they continue on their way, unaffected by each other--you can check it by throwing two stones into a puddle and watching the patterns. On the ocean, that's illustrated by the common occurrence of wind-driven waves going one way, swell going another, and possibly yet another swell (from another storm) going another direction. Random combinations of swells from different directions sometimes intersect in ways that add up to "rogue waves" and then separate to go on their normal ways.

Then there's the nasty version called "standing waves" that are common when a wave reflects back on itself, as from a seawall. There, the arithmetic, additive effect causes a single spot on the water to rise and fall roughly twice the height of the wave, like an elevator. The water looks like pistons going up and down next to each other, instead of waves traveling through the water. But that's a simple arithmetic affect of the original wave intersecting with the 180-degree reflected one of the same size and shape. Actually, both waves are traveling along their regular paths, unaffected by each other.

This is all why radio, sound, and light waves can intersect all over the place but still get to wherever they're going, intact.

That's the part of HS physics I remember... Back into my hole...

Edited by - Dave Bristle on 04/17/2007 13:42:22

Oscar
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Response Posted - 04/17/2007 : 14:39:28

<blockquote id="quote">quote:<hr height="1" noshade id="quote">In Oscar's case, it is about depth. Much of western L. Erie is only 15-25' deep, even miles out.<hr height="1" noshade id="quote"></blockquote id="quote">

Oops, yes, that is the very factor that creates the short steep wave pattern.....I failed to mention that. Sandusky Bay itself is fairly protected and a good place to start. (Don't go downwind of that big coal pile if it's still there......) Going out of the Bay onto the lake is less pleasant in strong(er) NE winds or the few days thereafter....it is a long fetch from Buffalo NY. Cold fronts from the West can also whip it up quite nicely. Before the C30 I sailed a J-22 all over that part of the lake.....quite sporting.

*******next story, illustration does NOT go with the above, but with the below....*****

Soundings in feet....(I know, a little hard to read.)

Interesting anecdote to illustrate the concepts, concerns and cautions mentioned above involving breaking waves....

Coming out of Beaufort, NC heading for the Chesapeake one time I was looking at the chart to see if I could cut across Cape Lookout Shoals. It extends almost 9nm out from the beach, and as I stil had several hundred miles to go, and was limited on time, every hour counted. As I cleared the "blue shaded area" coming out of the channel in the top left hand corner, I aimed for a spot about 4nm North of the red "4" that marks the tip of the shoal. There is no less than 11 feet of water across there, and with a five foot draft I thought that would be plenty. Tide was up and rising, wind was SW, about 12 knots. It was a pleasant afternoon.

In addition to the windwaves from the West, there was a gentle ocean swell comin in from the SE.....my guess is they were 6-8 feet, but they were long.....it was a pleasant ride.

As I approached the edge of the shoal I climbed up on the cabin top to take a look ahead, and saw......BREAKERS......big slow 100 foot wide BREAKERS.....(music of Hawaii-5oh).... I immediately realised that the slow gentle ocean swell was tripping over it's own feet in the 10-15 foot water in the middle of the shoal.......

Needless to say I hardened up and went around the red"4".......

Learning every day.......

Edited by - Oscar on 04/17/2007 14:52:33

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