Wave height/swell height

Discussion in 'All Things Boats & Boating' started by netjaws, Oct 12, 2005.

  1. netjaws
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    netjaws Junior Member

    are they additive?
     
  2. CDBarry
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    CDBarry Senior Member

    Yes, could be
     
  3. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    Yup. Swell height refers to just that. Wave height is the measure of wind waves generated locally. Collectively they are refered to as "sea state". So when the forcast calls for 4 to 6 foot seas, this is the combined height of swell and wind waves.
     
  4. jehardiman
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    jehardiman Senior Member

    Actually, no. It is important to note that while both local wind generated waves (the sea) and remotely generated storm waves (the swell) are reported, they considered and measured seperately. This is because they effect different parts of the frequency regime. Wind waves (except in storms) tend to less than 6-7 seconds, swell tends to be greater than 9-10 seconds (this is not a hard and fast rule, it has a lot to do with spectral analysis of the measured seaway). For small vessels you are woried about the energy in waves, for offshore/large structures, you are worried about the energy in swell.

    While they both contribute to the power spectrum, and therefore contribute to the maximum wave height, the values reported cannot be mathematically summed, i.e. the H1/3 sea + the H1/3 swell does not equal the H1/3 of the seaway. This is because the sea is proportional to the sqrt of the area in the power spectrum that corresponds to the local sea, while swell height is is proportional to the sqrt of the area in the power spectrum that is generated by remote swell. To put numbers to it: H1/3 waves = 4*sqrt(Mw) and H1/3 swell = 4*sqrt(Ms) If the waves are 8 ft and the swell is 4 feet, then this implies that Mw = 4 and Ms =1 so M0=Mw+Ms=5 and H1/3 for the total seaway is 4*sqrt(5) =8.9...NOT 12. There is a lot of math behind this and if you want some references I can give them.

    However, note that it is M0, not Mw or Ms that determines the maximum expected wave height. while the average of the highest 1/3 waves (H1/3) is not increased that much, because it multiplied by the probability density of maxima (eyes glazed over yet?) of only 4, but, by the time you get to H 1/100 the multiplier is 6.5, by H1/10000 8.9. Therefore the expected wave height for evey 100th wave is 14.5 feet and for every 10000th wave 20 ft, both of which are greater than 12.

    EDIT:

    Netjaws...If you are at the USNA...go ask Bhattacharyya....he wrote a book on that stuff.
     
  5. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    Uh...Yeah, that's what I meant.:D
     
  6. cyclops
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    cyclops Senior Member

    My calibrated sea eyes only see ground swells and wind waves. Alone or stacked one on top of the other. What do YOUR eyes see at sea.
     

  7. jehardiman
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    jehardiman Senior Member

    Most un-trained observers when asked to say what the "wave height" is will come very close to the H1/3, which is one of the reasons it is selected as the "Significant Wave Height". This goes back to the days of Beaufort and his wind scale which was then converted into "sea state". When you look at the seaway, your eye is drawn to the waves that "stand out", i.e. the larger ones. It was only in the 1950's, when operational offshore wave gages began to be used in large numbers that they started to correlate measurements to "observed data" leading to what is reported. A person that has been to sea enough will be able to tell the difference between sea and swell (and sometimes able to tell the difference is direction, but this depends a fair amout on conditions). A well trained person will be able to pick out different sea and swell components and make a pretty good estimate of each.

    On the whole however, the concept of "Sea State" is very poor and must be handled very carefully in design. To make a design contract for "must operate in SS3" is just opening the door for confusion. An example conversation I was in....

    Contractor (who made the system): "The system is not woking, it must be greater than SS3 out there"
    Master Chief Diver (who will risk his life in the system): "What! #$%#$!! It's DFC out there. Your system is AFU'd"
    Me (technical observer): "Well...... it's actually about SS1 and a half, but there is a 2 foot 13 second swell"
    Contractor: "A thriteen second swell!!! That means it's SS7!! Of course it's not going to work!!!"
    MCD and Me:" Bwaaaahahahahah!......"
     
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