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Seamanship - World of Wave Craft

Discussion in 'Ask All Sailors' started by Will Gilmore, Feb 13, 2018. Add this thread to a FAQ

  1. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,129 posts, 834 likes
    O'Day 19
    US Littleton, NH
    I came across this interesting U.S. Coast Guard publication.
    http://webapp1.dlib.indiana.edu/vir...4302894/FID1851/BOATCREW/files/chadd/adda.htm
    It looks like it was meant as a training or information tool for CG personnel.

    I have been taught that waves break at half the height of their depth, so, if you see waves starting to break at 4' in wave height, they are rolling across a bottom 8' underneath them. Out in open ocean, waves break when the wind forces build them up to such a height that the wind actually knocks them over. It can be particularly bad when that happens to be right on top of your boat.

    You may have seen this video before, but notice there are no obvious forces causing that wave to break.

    Another interesting point made in the article is about wave interference and looking for regular lulls when entering or exiting restricted passes.

    A quote from the above article: "The ability to recognize wave patterns and characteristics is essential to safe operation in surf and heavy weather. A coxswain operating in these conditions must be able to determine the timing of lulls and series, and estimate wave heights accurately."

    What do you all think?

    - Will (Dragonfly)
     


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  2. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,416 posts, 1,853 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Besides it was bigger that the rest?
    Much bigger than median waves are common and in fact predicable in their frequency over time. No new news there.

    Wavestats.svg.png
     


    Last edited: Feb 13, 2018
  3. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,129 posts, 834 likes
    O'Day 19
    US Littleton, NH
    Jackdaw,
    Are you saying that waves much bigger than the median are cause for breaking?
    I think there is something to that but just being big isn't enough. Waves break when they have a height to length ratio past a certain point or when the wind actually blows them over.
    The regular or predictability of interference patterns is interesting too.

    - Will (Dragonfly)
     


  4. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,416 posts, 1,853 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    What I'm saying is that due to the depth where he was, the average wave was was not big enough to break. But statically speaking one big enough to break there was bound to come along. And it did.
     


  5. dlochner

    dlochner

    Joined Jan 11, 2014
    1,880 posts, 624 likes
    Sabre 362
    US Fair Haven, NY
    The wind doesn't blow them over. As the forces interact, the wave becomes steeper, at some point it becomes too steep and collapses. Wind does blow spray off the top, but this is not the same as a breaking wave.

    Watch some movies of waves breaking on shore or a sand bar and waves breaking in deep water. In deep water the waves collapse into a more stable form. There is some foam at the top, but that dissipates pretty quickly and doesn't usually make it to the bottom of the wave.

    At least this has been my experience sailing in 10+ foot waves in several hundred feet of water.
     


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  6. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,129 posts, 834 likes
    O'Day 19
    US Littleton, NH
    Would you then say the breaking waves in the video were from bottom drag?

    - Will (Dragonfly)
     


  7. Jackdaw

    Jackdaw

    Joined Nov 8, 2010
    8,416 posts, 1,853 likes
    Beneteau First 36.7 & 260
    US Minneapolis MN & Bayfield WI
    Open water waves break due to wind.

    In shallow water, waves break as a function of their height to water depth. The bottom of the wave is slowed down by the friction due proximity to the bottom, and the top falls over it. Usually when the water depth is 1.3 to 1.5 times the wave height. This is exacerbated by shoaling, which has the effect of increasing the wave height. This shoaling happens when the depth is 1/2 of the wave LENGTH.
     


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  8. rgranger

    rgranger

    Joined Jan 19, 2010
    4,608 posts, 685 likes
    Hunter 26
    US Smith Mountain Lake
    I wonder if his shredded headsail and the lines dragging in the water had something to do with his motor not working.:oops:
     


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  9. dlochner

    dlochner

    Joined Jan 11, 2014
    1,880 posts, 624 likes
    Sabre 362
    US Fair Haven, NY
    Waves and Water Dynamics

    Waves will break when the height of the wave is equal to 1/7 of the length of the wave. This can happen for several reasons, the bottom slows the wave down causing the height to build, wind drives the waves to a height that exceeds 1/7 of the length, or when intersecting wave patterns drive the height of the wave above the 1/7th limit.

    On a deep water wave, once the wave breaks the height declines to a point below the 1/7th threshold and the swell continues but the wave stops breaking.

    Here in the southeast corner of Lake Ontario we get a lot of intercepting wave trains after a cold front passes. These trains create tall steep waves that foam and have slight curls and then collapse, but not to flat water. An 8 ft pyramidal wave will peak, foam, and then drop to maybe a 4 ft wave. What's cool is sometimes the winds and waves oppose enough that it is possible to surf a boat upwind.
     


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  10. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,129 posts, 834 likes
    O'Day 19
    US Littleton, NH
    Wave interference patterns far out at sea are generally the result of two or more seperate wind systems sending waves out on an intersecting course. When the two different systems generate waves with different wave lengths, you see a pattern of large waves being generated in a syncopated pattern to the regular average waves. This means you get that one 'rogue' wave that is nearly twice as tall as what you have been sailing in. It's hard to tell how often or even if you can expect them.

    Where it really affects you and your boat are in places like inlets and narrows that have colliding currents and wind generated waves. That's when a larger wave can come almost out of nowhere and knock you onto a nearby object or even suck the water or from under you and hammer your keel into the bottom. Intersecting waves even reflect off shore, especially steep shorelines like sea walls.
    Watch and time it, because it is mathematical. There will be a pattern, if complex.

    - Will (Dragonfly)
     


  11. jon hansen

    jon hansen

    Joined May 25, 2012
    795 posts, 327 likes
    john alden caravelle 42
    us sturgeon bay, wis
    don't forget thermal cline. in the summer the thermal cline will act as a shallow bottom and cause waves to break. so along the shores the waves will always be shorter then say out in the middle. on erie the whole lake acts like a pond in the summer time. by november the waves on the great lakes are a whole different animal. way taller.
     


  12. dlochner

    dlochner

    Joined Jan 11, 2014
    1,880 posts, 624 likes
    Sabre 362
    US Fair Haven, NY
    Because Erie is so shallow it will act a bit differently than the deeper lakes, like Ontario. A big factor in the fall winds is that the air is cold and dry and thus denser and more powerful that summer winds, which are hot and moist and therefore less dense.
     


  13. jon hansen

    jon hansen

    Joined May 25, 2012
    795 posts, 327 likes
    john alden caravelle 42
    us sturgeon bay, wis
    yes the thermal cline on lake erie is more vast. in the summer. whereas on northern lake mich you may find the cline drops off 5 to ten miles out, sometimes more. hot years, may cross the lake, or change from upwellings and currents.
    malcolm never dropped the anchor, opps. the coast guard did not mention that either, figures.
    99% percent of boating accidents are trying to get into harbors. you have to be ready.
    99% of kids boating accidents happen at the dock.
     


  14. jon hansen

    jon hansen

    Joined May 25, 2012
    795 posts, 327 likes
    john alden caravelle 42
    us sturgeon bay, wis
    out of sturgeon bay, we can sail green bay or lake mich. on those prefrontal gradiant winds that blow hot from the south. green bay gets a steep 5 foot wave. oh they are fun. but to reach out into lake mich. the shorter steeper waves drop off and you can get these delightfull 12 to 15 ft swells that are great fun to sail in. like a skate boarder working the sides of a empty pool.


    both of those patterns are formed by the false bottom of the thermal cline, not the actual hard bottom
     


    Last edited: Feb 14, 2018
  15. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,129 posts, 834 likes
    O'Day 19
    US Littleton, NH
    I've never heard of a thermal cline. How does that work?

    There was an interesting point made about shallow lakes verses shoaling. It takes a shoaling bottom to push waves taller, otherwise, they are just limited by the depth before they even form.

    - Will (Dragonfly)
     


  16. capta

    capta

    Joined Jun 4, 2009
    2,275 posts, 501 likes
    Pearson 530
    XX where ever we are anchored
    I've seen waves in the Stream between Fla and the Bahamas in a norther that you'd swear were breaking on a beach or bar. Big round curlers. Occasionally the water under the boat seems to just disappear and the boat drops into a hole. I've never witnessed this, though I have experienced it a number of times, so I have no idea what caused it. It is rather disconcerting to be on a sailboat and feel her drop for what seems like minutes, but of course is probably less than a couple of seconds. That's when it's really nice to have faith that the boat under you can take anything.
     


  17. Hayden Watson

    Hayden Watson

    Joined Apr 5, 2009
    493 posts, 94 likes
    Catalina '88 C30 tr/bs
    US Oak Harbor, WA
    I was on this trip on the bow of the upper level deck looking out the front when the water dropped out of sight in front of us. You can see green water hitting the window on that deck just below the pilot house. That was when I decided to move back a bit farther in the boat. The force of water down the car deck moved most of the first 5-10 in each row with lots of car alarms going off. Such fun.
     


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  18. jon hansen

    jon hansen

    Joined May 25, 2012
    795 posts, 327 likes
    john alden caravelle 42
    us sturgeon bay, wis
    thermal cline, different areas of water of different temps do not mix. just like oil and water do not mix. so as the season warms up and the sun gets higher the water along the shores warms first. the shore is a water heating zone. it then spreads out away from the shore. the warmer on top, the colder pushing down. BUT, there is a cohesion to each zone. so as it spreads it clings to the shore and itself. the lines between the zone are not horizontal but deeper towards shore shore and finally come to the surface way out. you can get visual markers that you learn to recognize, called scum lines. the different zones can be going different directions as well.
    have you ever gone swimming when the surface is 75 but then you swim down 20 ft and it 50. thermal cline.

    all fish like a very specific temp. king salmon 54 d., stealhead 42 d. serious fisherman understand that fish will school up in their temp next to a warmer zone where their baitfish are. then dash into the warmer water snag lunch and return. you learn to read the signs and pay attention to the temps. if their temp is deep then they are deep. way off shore they may be near the surface as the thermal cline comes up. hence, why some always catch their limit and most only catch a few. this is the jist for fishing. nieve sailors might think the fisherman could be anywhere, when in fact his zone to catch is very small.
    for sailing, bethwhait teaches that understanding the surfaces of the lakes we sail in can improve your ability to get the most out of our boats. maybe we want to favor the bigger waves down wind to get a bigger push. or mayby fast tack the edge of a straight so to not have to fight bigger choppier waves. he tells of going to the chicago library and reading 100 year old logs of sailing masters of cargo boats as their knowledge of the water patterns was keen. he tells of a class race on lake huron , at the gun the wind was very light but there was an old chop hitting them on the nose. the whole fleet just sat there. he, trying to get going tried having his one crew sit on the stem while he sat on the transom. the cadence of the waves and the change in the weighting, he alone started to move ahead while all others still stayed in one place. by the time the others copied him it was to late. to win a sailing race, start first and increase your lead from there. right. bethwaite is the person that taught me about thermal cline acting as a shallow water sea. salmon fishing books taught me how to hunt the deep water fish.
     


    Last edited: Feb 14, 2018
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  19. jon hansen

    jon hansen

    Joined May 25, 2012
    795 posts, 327 likes
    john alden caravelle 42
    us sturgeon bay, wis
    google great lakes thermal cline. you might get some nice graffs.
     


  20. Will Gilmore

    Will Gilmore

    Joined Oct 19, 2017
    2,129 posts, 834 likes
    O'Day 19
    US Littleton, NH
    Jon, you are my new hero. :worship:
    Awesome post, thanks.

    - Will (Dragonfly)
     



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