Waves

[Franklin]

I am reading "My old man and the sea" about a father/son trip around the Horn. They are between the Galapagos Islands and Easter Island, about 500 miles south of the Galapagos Islands. The wind is blowing 20 knots from the Southeast. The book states that the waves are 5-8', steep and some are breaking. Now from what I know of waves, that doesn't sound right. They are at least 500 miles from any land and mostlikely in extremely deep water. I had thought that it takes winds of excess of 35 knots before waves get steep in deep water.A few pages later and another few hundred miles they talk about 12' rolling waves on a calm day.Thoughts? Things seem fishy to me.

 

[BobW]

Is the wind blowing against the current?

That's been known to cause odd waves.Also, if they passed through a front, the wind and waves could be out of sync.12' rolling waves - swells - on a calm day..... how about a gale down in the Southern Ocean, whipping up 50' waves headed north towards the Galapagos Islands, which are still settling down when they hit our intrepid adventurers?Or maybe I just showed my complete ignorance about off-shore wind and waves?Incidentally, that is just about where that guy from here in California wrecked his boat and had to get rescued a few months ago. People still ask me about that one and I explain that amongst life's potential great adventures, there are:1. Sailing around the world2. Sailing around the world single-handed3. Sailing around the world the wrong direction4. Sailing around the world non-stop5. Sailing around the world with little off-shore experience6. Sailing around Cape HornAnd this guy wasn't content to try 1, 2 or even 3 of these, each of which is a huge adventure all by iteslf. He had to go for the 6-pack. It's probably a good thing he didn't make it all the way to Cape Horn, where rescue would have been much more difficult, even impossible.Also, another account of sailing in those waters is the adventure of the Tzu Hang, which somehow survived a knock-down.Those are interesting waters, for sure.Cheers,Bob

 

[Franklin]

current

that must have been it although I thought the current went north in that area. Maybe they were out of the main currnet but in an eddie. Thanks BobW.

 

[r.w.landau]

Franklin, in partial response...

I have sailed on nice days in the ocean when the seas had two different and harsh waves. The waves came from two different storm fronts. When they were in tune with each other, the waves built atleast half again what was average.By the way, I really enjoyed that book!r.w.

 

[Franklin]

The book

It does have it's good moments. My last book (first book I read and finished since college), Avenger, which has nothing to do with sailing was so good I would read for many hours without stopping. This book I find that my endurance isn't nearly as good.I was reading and wondering why I didn't hear anything about fishing, then later he talks about a fish that jumped into the cockpit and he threw it back into the sea because he felt sorry for it. I was like....ahhhhhhhh...ok...is the fish going to feel sorry for you when you are starving.I like the dad's sense of humor.

 

[tom]

current will make waves steeper.

they can get big swells in Hawaii from storms in alaska. My only experience is for a Hurricane near Key west and it was making big waves at Panama City . But the wind was light. The waves were really standing up fairly steep faced when they hit the current generated by the tide falling and rushing out of Andrews Bay. Once we were out of the current the swells were big but more of a gentle up and down . I think that it was Wilma.

 

[Stu Jackson]

A storm makes waves

they build into what is called swells. Those are the rolling ocean thingys.Wind waves happen over those swells.If one sails out beyond the SF Golden Gate, one finds that the swells are completely different than the wind waves. After a storm, the swells can be there for days even if there's no wind. Alternatively, with no storms and windy conditions, it can be flat with wind waves.No suspicious activity as far as I can tell. ;D

 

[Waffle]

Did you see the show on rogue wave

Wind blowing aganst the current around the horn could have made those waves.

 

[Ross]

Franklin,If you spend some time at a sheltered

pool of water that you can throw large rocks into and if you are patient, you can walk around the pool and create waves that come from different directions and watch them interact. Waves can add their amplitude or they can cancel each other and you can see both by making waves in a small pool. Almost everything that happens in the ocean can be duplicated in very small scale.

 

[Franklin]

pool

Ok...people are going to think I'm nuts throwing rocks into a pool Cool idea though. Maybe I can throw something else besides a rock.Waffle...he is about 2500 miles away from the Horn...way up north...only 500 miles from the equator.Just one more question. I believe that most eddies are a knot, give or take a half. If only a knot of current, would that be enough to cause steep breaking waves? My only experience with the current and wind fighting is in a 3 knot current through a jettie and 15 knots of opposite wind which produced about 5' steep waves but none breaking.

 

[Ross]

I just had an idea actually more of a question

Waves generated by the wind have a finite ratio between height and length if they are not influenced by currents. After the wind stops blowing the waves continue for some time. If the waves are generated by a ten knot wind the dieing waves will be shallow swells of the same frequency as the wind driven waves. Yes? Then does it follow that with ocean swells the greater the distance between the crests the stronger was the wind that generated the waves? Can we then make an inference as to the strength of a distant storm by the frequency of the swells? Further can we roughly estimate the distance to the storm by the height of the swells?.My experience with boat and ship waves would seem to support this. Small boats passing me while I am at anchor make very small swells a quarter mile from the channel but large boats make much larger swells that are farther apart.

 

[Franklin]

true

I read somewhere that old sailors could tell if a hurricane was developing by judging the waves...so I guess your theories are true. The part I remember them saying is if you see large rolling waves and not much wind, then there is a hurricane out there. It also talked about determining where it is and which direction is was going but I forgot all that.

 

[Franklin]

My mistake

I was thinking that the Galapagos islands were 1,000 miles from shore. Seems they are only 500 miles (charts are a great thing ). So they must have been in the outter edges of the Peru current. That makes sense.

 

[richk]

wave forecasting

not for the casual reader, but a study of the document at the end of this link may enlighten some participants in this discussion

 

[tom]

Beafort Scale

It seems that maybe Chapman's has a discussion of waves and storm distance. If you are really interested in playing with waves you can buy or make a wave tank. Back when I taught physical Science we spent a few days playing with wave tanks. Edmund's Scientific sells them. It probably would be more fun than throwing rocks. After starting SCUBA diving I stopped eating fish for a while. After spending time underwater I realised that they are animals with brains and personalities. Too bad they taste so good. Even now I feel a little bad killing them. Not too bad but I realise that my fish dinner was paid for by a life lost. Tuna are particularly hard for me to kill. They fight so hard to live even after you cut the arteries in their gills. You have to respect a tuna they are so perfectly evolved for their role in the ocean. Unfortunately for them they really taste good. So I give them a little respect as they are prepared for the grill.

 

[sailortonyb Allied Mis]

Ross and Franklin

Ross, I think that the waves from boats and their frequency are determined by their hull shape and size, which would not be the same as waves generated from the wind. Franklin, "The wind is blowing 20 knots from the Southeast. The book states that the waves are 5-8', steep and some are breaking."Could this be partly attributed to an error made by the author? Crew boat captains and supply boat capt'n's are notoriously wrong at stating wave height. I have to record all of the ambient conditions several times a day in the area Im in on the Gulf. The best way for me to judge is to watch how they ride up and down the legs of the platform or rig. Then guess at the height. In a small boat, your perspective will throw you off as the boat rides up and down, you tend to look up and down as opposed to a cross section view.The Gulf of Mexico is a relatively shallow body of water, and its this shallownesss that makes the waves steeper and closer together. Until you are way past the continental shelf and into really deep water, do the waves become longer and further apart. Closer to shore, say 50 to 100 miles or so, the waves tend to be steep and have a period of between 5 and 6 seconds. That is really fast for 10 to 12 footers to be comming at you. Also note, the author may have been observing waves from a localized quickie storm wherein the wind was blowing the tops off of them and they appeared to be breaking. The other thing that causes steep waves is when the offshore waves with all of their mass and momentum start to hit shallower water, the wave wants to keep its shape and size, so it will 'ride ' up the shallow areas and then gravity makes them fall down, thus the steep breaking waves. Also, I dont remember the exact amount of fetch at which the increased distance no longer make a difference in wave height (the waves max out) but i think its only about 20 miles or so, I might be way off. Also in the shallow GUlf Of Mex, the Beaufort scale does not tend to hold true. Strong cold front come quickly and the waves are taller than they should be. Until they finally pick up momentum, which could take all day or longer, the wind blows the tops off of them. Its the far offshore storms that bring what we call ground swells. Thats the big slow moving far apart from eachother waves.I'm being rushed to give up the puter here so I probably didnt get to say everything i had to or it didnt come out right. Hoped it helped some. Tony B

 

[Sandy]

height

I've heard some say that the trough doesn't count into the height so you take the observed height and then divide by two. All measurements I've seen though seem to include the trough. I was wondering how you guys determine the height?

 

[r.w.landau]

Sandy, for me it is like the chickens prespective.

I count the whole height then add the fear factor to that!r.w.landau

 

[newly anonymous]

Interval

The longer the interval between crests, the further you are from the system generating the waves. When you're actually within the storm, the seas tend to be steep because the waves are closer together. Eight foot seas at an eight second interval are going to seem much more threatening than 14 foot seas at a 30 second interval because the former are much steeper than the latter. Of course, steep seas are more likely to break than a gradual swell is. Which is why local winds produce white caps.

 

[r.w.landau]

John I have to disagree.

Wave length is directly affected by the wind. Hull speed was derived from the formula for wave speed. The longer the length between waves the faster the wave is traveling. The wave doesn't stop traveling, it loses it's fril like wind waves, but continues in it's full strength.The reason that storms near to you have steep waves is the lack of fetch or distance for them to work out the speed of the wind. Rereading, Maybe I do agree with you..r.w.landau