Wave pattern question

Discussion in 'Boat Design' started by rambo!, Aug 3, 2009.

  1. rambo!
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    rambo! Junior Member

    Hi, when walking every day over a high bridge in my hometown I´ve started to look at boats wave pattern, read a lot about the Kelvin pattern and phase and group speed in dispersive medias. Two questions still no understod:

    1. Is the wave pattern of a planning hull going into the same pattern as for a displacement hull, only that it will appear in a distance after the hull? Will a planning hull still produce a Kelvin pattern?

    2. Is the wake buildup behind a planning hull more depending on the prop and the disturbance of the flow/pressuer? If the hull was towed into planning, would I see the same wake pattern then?

    Thanks for any answer
    Regards
    Olle
     
  2. marshmat
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    marshmat Senior Member

    Hi Olle,

    Whether the hull is classed as "planing" or "displacement" is not really a factor in determining the wake shape. (I know this observation will draw some flak- let me elaborate.)

    The Froude number(s), the immersed volume of the hull, and the distribution of that volume are probably the main factors that determine the overall shape of the wake. "Planing" just means the hull generates dynamic lift- this makes the underwater shape and volume change with speed, thus altering the properties of the wake-producing body, but the physics of the wave system remain the same.

    You'll notice, if you look at wakes from above, that the diverging wave system looks fairly similar regardless of what made it or how fast that boat is going- the angle changes, the spacing and size change, but the overall form is pretty similar from boat to boat, at least in the far field.

    The transverse wave system is where things get interesting. Generally speaking, the transverse waves will grow large with speed, up to a point- and will then shrink to almost nothing as the speed increases; the boat is now going faster than its transverse wave system can keep up. Planing hulls almost universally operate above the Froude number where this transition occurs, and you'll notice that when running at speed, they carry almost no transverse system.

    Props, sufficiently well submerged, don't tend to affect the far-field wake pattern too much. In the near field, they can have a significant effect in some cases.

    There is, of course, a LOT more to say on this topic, and I'm not enough of an expert on the matter to go into too much detail. Indeed, I suspect what I've already said may appear confusing and contradictory to some- in any case, I hope it spurs discussion.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    Rambo/Olle

    Since you're looking at the wash (wake/waves) from a bridge, this implies you are looking down onto a river or a body of water that is "relatively shallow". As such the depth and speed of the craft all play a major part in the wash and its pattern.

    There are 3 types of wash in this case, relating to what is called Froude depth. It is the speed of the craft relative to the water depth. It is a simple equation

    Fd = V/Sqr.root(gh)

    h = water depth
    V= speed of craft

    1. Sub-critical depth Froude number, ie Fd is less than 1.0

    This is when you see the classic Kelvin wash pattern, of 19'28" angle of wash with the divergent and transverse waves all meeting at the cusp locus.

    2. Critical depth Froude number = 1.0

    At the critical Fd of 1.0 the velocity of the craft and waves are equal to root(gh). In linear theory the energy does not disperse back along the wave train aft, as in sub-critical. Consequently a significant proportion of the power of the craft is converted into wave energy in a few wave fronts nearly perpendicular to the track of the craft, ie almost 90degress, compared to the 19'28" of the classic Kelvin at sub-critical.
    At the critical point where the craft has reached this speed, the wave front will be curved and continue within a "cone" shape. The wash height is greatest at this Fd.

    3. Super-critical depth Froude number, ie greater than 1.0

    At this higher Fd the waves are long and non-dispersive and the wash pattern takes on a totally different appearance. The long waves cannot travel in the direction of the craft as the water depth limits their speed. Therefore the wave fronts subtend an angle of 'theta' to the course of the craft so that C=VxCos'theta', C= wave speed, V= craft speed. Consequently all the wash wave components radiate out in lines from the craft in a 'delta' like formation. The longer faster waves are on the outside of the wash and ahve larger values of 'theta' compared to the slower shorter waves with crests swept further back. Both the critical wave and the transverse waves, which existed before the craft entered the super-critical region, are now left behind as they cannot keep pace owing to their velocity being restricted by the depth of the water.


    This is just a brief summary, but perhaps it answers your question?

    This is why Flag states like the MCA in the UK, requires a route analysis prior to accepting a service of a vessel. It is to establish the depth and speed of the craft, to ensure no nuisance waves are created. That is to say wash that either hits a wall of no consequence or hits endless boats moored up for permanent residence etc. The former not being a problem, the later being very problematic!

    In practice you want to avoid the Fd range of 0.8~1.2. Also the amount of power required, on smaller boats, may not be enough to get over this 'hump'. So best to power through quickly rather than slowly.

    PS..just copied/scanned some images, for clarity, see attached.
     

    Attached Files:

    Angélique and Fanie like this.
  4. rambo!
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    rambo! Junior Member

    Thank you Matt and AdHoc for taking you time to explain this. I know that I´ve could have asked my initial thoughts here direct, but I have enjoyed the days of digging for information and learning the basics.
    I did check your inputs today and yes, of course you´re right, it´s just harder to see the pattern of a fast moving object.
    This old man standing on the bridge with all his carboard patterns, waiting to be taken in and put in some nut house...hehe

    As an amature to this (I work with Acoustic and Soundwaves), thak you for filling in missing parts.

    Best regards and thanx again
    Olle
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Olle

    Try taking photographs of boats that pass, paper cuttings are fine...but relies too much on memory ;)
     
  6. rambo!
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    rambo! Junior Member

    Two more questions regarding waves, your answers have led to a lot of interesting readings...thank you.
    1. a single water molecule is not moving in a circular pattern as often described, it´s a cluster of many molecules that together forms this circular movement of energy?
    2. If 1 length meter of a 1 meter wave holds about 10 kW of energy...if my hull produces 2 waves of 1m heigt (leave other waves out)...is that equal to that I need 20 kW of power per meter just to create those waves? If running at 4 m/s this will need 80kW/s just for the wave making.

    Olle
     
  7. ivor Bittle
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    ivor Bittle Junior Member

    I had a look at the bow waves made by displacement hulls on my web site under bulbous bows. It is at www.ivorbittle.co.uk You might be interested.

    Yesterday I saw a bird feather that was curved and floating with part standing free above the surface. It was windy and it produced the typical bow wave as it moved. I could scarcely believe it.

    Ivor Bittle
     
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  8. Submarine Tom

    Submarine Tom Previous Member

    Olle,

    I have observed both bow waves and stern waves from displacement hulls perhaps because the hulls are

    so much longer, generally, than planing hulls that seem to produce only one, much smaller wave, at speed.

    Matt,

    Why do most twin screw boats use rotations that move the tops of the propellers apart,

    while much fewer use rotations that move the tops together? I worked on a Dutch manufactured tug

    that was the later. Of course it's wake made it appear single screw.

    Tom
     
  9. TollyWally
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    TollyWally Senior Member

    Ivor,
    Absolutely fascinating website you have. Sadly I couldn't get your pictures to successfully transmit through. I shall be returning to your site in the hopes that the diagrams will magically appear. Extremely interesting!
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Olle

    1. Im not 100% sure about the difference between a single molecule and many molecules acting together, as such. But if the cluster is moving in the classic circular motion, see attached sketch, then it follows that what is in the 'cluster' as you call it must do so too. Ergo the single molecule follows the same circular path.

    However the clusters cannot become "too thick", otherwise observations/theory of the reduction in orbital motion with depth would not occur. That is to say the radius of the described motion (circular) decreases with increasing depth, because the pressure variation in depth (hydrostatic pressure, Z, in the picture) cannot be constant along the length of the wave at varying amplitudes ie changes with depth long its length. There would in effect be "surfaces" or "clusters" of equal pressure very close to each other...but once this distance between clusters increases, the theory falls down. Since hydrostatic pressure is there, hence it affects the motion of the molecule(s)!

    2. Waves are far more complex than the simplification. The energy of the wave you note, is the wave per unit area or per width, what is the period, wave length etc etc??. But to assist the calculations to make them 'simpler' to compute, the energy of waves, is related to infinite 2D waves, ie what you would draw on a 2D sheet of paper. Not a finite 3D wave produce from a boat. So to use this energy relationship, even in its simplest form, is not possible as the two are totally different.

    The properties of an equivalent regular waves series is also not the same as the normal nomenclature people use to describe the sea, ie significant wave height (or single wave of 1m as you ahve selected). Since a sea state or spectrum (which is where the energy calculations are coming from) is generally described as:
    " A collection of a great number of simple, regular waves of different lengths, all of small height and all mixed together with no apparent relation to each other except that they are all there and are all travelling in the same direction."

    Again, that is just the 2D spectrum. When looking at "normal" 3D waves, as in those from your hypothetical boat...it becomes more complex.

    So you see, in this simple example, your trying to compare apples with oranges.
     

    Attached Files:

  11. rambo!
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    rambo! Junior Member

    Hi again, I´ve just came back from 7 day´s of single handled kayjak in our costal waters, so waves has been a very obvious thing to look at. As you say Ad Hoc, in 3D it gets far more complexed. Thanks for a good explanation of something very complexed.

    Thank you all for taking you time to answer my questions, an Ivor....that link will keep me busy for a couple of eavnings....

    The idea that creating big waves cost a lot of power just helped to calm me down when some of these big wake boats got too close....they will pay for this at the next gas station.....


    rgds
    Olle
     
  12. yipster
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    yipster designer

    was allready impressed that f.e. chines dont give extra resistance above hullspeed and if water was compressable than hullspeed should give a bang i belive?
     
  13. ivor Bittle
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    ivor Bittle Junior Member

    Wave patterns

    Tolly Wally

    The pictures come through if you are patient, at least they do for me. They are big files and take time to down load.

    Ivor Bittle
     
  14. mark775

    mark775 Guest

    Ivor, I, too, am ignorant of how to see the pics and diagrams on your site - but thanks.
     

  15. ivor Bittle
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    ivor Bittle Junior Member

    Wave patterns

    I asked my daughter to open the offending section of my web site on her computer as being typical of others and only about half the graphics came down. I asked her to refresh the site and all the graphics appeared.

    I hope this helps because I do not know what to do if it does not.

    Ivor Bittle
     
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