Transom Hollow Wave Making

Discussion in 'Software' started by johnhazel, Apr 7, 2014.

  1. johnhazel
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    johnhazel Senior Member

    Just thinking this morning......

    Submerge a balloon half way in the surface of a pool. Pop it. Waves radiate.

    Use a long balloon (before some clown twists it up) and submerge that halfway. Pop this balloon and you have a 2D wave maker.

    Refer to many papers regarding 2D+t bow wave and try to apply this to the transom flow.

    Give yourself the best chance for success and specify the back half of the hull walls ahead of the transom are parallel to the direction of travel.

    Discover that the actual transom hollow in experiments is shorter than predicted. sometimes....

    Adjust the velocity used in the +t portions of the calculations so that the effective reduction in ship velocity due to the boundary layer is accounted for. If this violation of the 2D+t assumption is too great then adjust the effective inertia of the water according to the velocity component in the direction of ship travel.

    Find that there are times when the transom hollow is longer than predicted. sometimes....

    Take into account that the bow pushes a wave away, then a hollow appears, and then the water returns toward the hull creating the second wave crest along the hull (if Fn is low enough). Adjust the 2D+t transom hollow implementation to take into account the initial conditions of lateral and vertical velocities at the transom.

    Go ahead and take into account the slope of the hull walls now that the mechanism for doing that is pretty much in place.

    Starting with something like the 2D dam break demo in OPENFoam create a 2D+t ship wave making calculation that is implemented by starting with a still surface and no dam then contort one of the walls into the shape of a ship half cross section as it passes through a plane perpendicular to the direction of travel.

    A second implementation could be made with the full ship cross section in the center of the plane so that asymmetrical sections could be used to simulate leeway.

    I'd like to say that I'm a crow trying to soar like eagles here but it's not as good as that. It's more like a turtle trying to soar like an eagle. But a turtle can dream too. (even if elephants might stand on it's back)

    Has this approach been published by anyone? It would suck to "reinvent the wheel"....
  2. gonzo
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    gonzo Senior Member

    Can you explain "transom hollow". Also, how do you measure its length?
  3. johnhazel
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    johnhazel Senior Member

    If a boat that has a transom is moving fast enough, there is no water against the transom. That open space where water is on the stationary boat is the transom hollow.

    It's a thorny problem:
  4. DCockey
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    DCockey Senior Member

  5. SukiSolo
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    SukiSolo Senior Member

    Possibly an interesting area. Do you mean simulating wave flow by curving the rocker? or hull? If you look at the lines of 'Cheminees Poujoulat' (IMOCA 60 now sadly lost) she seems to have a slight concavity in her aft rocker. It is a Juan K design and if the line drawings (I've seen) are correct I surmise the concept is to make the water 'think' she is longer than 60' by using a gentle reverse rocker. This would simulate an earlier change of pressure from the 'hollow' at the midsection area. She is quite interesting also in the entry hollow waterlines static vertical though quite what they look like heeled I'm not sure, I'd have to model her.

    Hollow sections (horizontal when viewed from above) in the after part of keels and boards are quite common and work OK. Personally I am not overkeen on hollow entries on these fins but open to possibilities.

    On small dinghies that do not have SCP of 30% and above there are I believe still a few little tricks to learn. Above that the key seems to be a balance of keeping control, but avoiding the nose diving common to overwide transom buoyancy - common in skiffs. Not sure that is completely sorted but suspect it is a combination of things, as usual, a tradeoff.

    Thanks for that link David. I was very pleased to find my last dinghy can sail upwind with absolutely minimum transom wash in any sea state if I can keep her upright enough, in any wind condition. It's just me finding it hard to keep within 5 deg in 30+ knots...
  6. johnhazel
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    johnhazel Senior Member

  7. jehardiman
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    jehardiman Senior Member

    All that is just basic strip theory, and one of the reasearch topics/ways forward postulated by Korvin-Kroukovsky in Theory of Seakeeping (SNAME 1961). Strip theory has been surpassed by matrix methods due to the massive increase in computing FPO's since the early 70's, but I still use it as a quick and dirty method to get a ROM.

    As I have said in other threads, there is no "wave resistance" only the manifestation of the pressure disturbance at the density boundary. It is fairly easy to model the kelvin wave train as pressure disturbance using an impuse wave generated by the entry and exit (...apply slender body requirement here...). It only gets tough if you want to track the breaking crest, i.e. the near field effects. In most hydrodynamic and seakeeping requirements there is no need for that, the effects of the crest being lost in the random seaway encountered.

  8. johnhazel
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    johnhazel Senior Member

    After looking at my options it is so obvious to go to Flotilla.... :) Thanks again to LL!
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