36 foot, $300,000 boat porpoises to no end

Discussion in 'Boat Design' started by gofastguy, Mar 26, 2019.

  1. Mr Efficiency
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    Mr Efficiency Senior Member

    No worries, he really has to establish whether this adhesion is occurring, but as I have mentioned, it should have created some obvious visuals behind the boat. One imagines, perhaps incorrectly, that the people who make these pods have settled on a slope that avoids the issue, and seeing the step up to the pod is said to be even a little more than specified, seems even less probable. But you have to know for sure, it is possible that an oscillation could be set up by intermittent "grabbing" of the passing water, especially when backed off a bit, but when I had a boat with a pod, that porpoised, the fix was generous tabs, and it gave no more trouble, but admittedly not a boat comparable to this one.
  2. BlueBell
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    BlueBell Ahhhhh...

    You are correct, I meant Post #23.
  3. fallguy
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    fallguy Senior Member

    Wouldn't it be great to computer simulate these problems?

    Even to simulate performance after ballasting? Or raising the engines? Or changing the props?

    We computer model flooding every year. The effort requires knowing every inch of snowfall and frost depths and takes a teams of federal employees.

    Doesn't the boat industry have the ability to enter this vessel and all her modifications into a simulated condition and recreate the porpoising on the computer?

    And then wouldn't we be able to modify inputs and calculate the changes?

    And wouldn't it seem an imperitive of the ballasting folks to enter the ship into a model software and validate the outcome?

    Twenty questions from an idiot.

    But I would love to see my boat modeled in such software and see what happens with bigger engines, for example.

    And here; it would be neat to see if dual 225s and less required? ballasting would change outcomes as well.
  4. DCockey
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    DCockey Senior Member

    The ability exists and is used in the marine industry. It requires a knowledgeable user, suitable software which is available commercially, and a computer with sufficient performance and memory. Also needed is a CAD model of the geometry of the hull and sufficient information to determine the mass and CG location. There are consultants who could do such analysis but someone would need to pay their bill.
    rwatson likes this.
  5. Barry
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    Barry Senior Member

    stepped hull pressure distribution.jpg Pressure distribution planing hull.png

    A reply to Rwatson
    You had said that the pressure on the hull bottom is "suction" and stated that this is pressure below atmospheric. Ie below Zero PSIG

    These drawings show that the pressure does not go below atmospheric pressure. The concept of a stepped hull is to provide another high pressure
    spike, a stagnation line, to get additional lift for the same wetted surface. (or less wetted area) In a stepped hull, the reason that you ventilate the step, ie the first transom from the front, is so that you create in essence another hull effect. (and reduce the drag that a non ventilated immersed short transom would produce.)The drawing on the right shows an unstepped hull, the one on the left a stepped hull as well as an unstepped hull. Certainly with lift strakes, and other protrusions, you can get lower pressure than the surrounding areas but these are localized and usually small compared to the entire hull.

    If the pressure under the entire hull was below atmospheric pressure, the boat would sink.
    Last edited: Apr 13, 2019
  6. rwatson
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    rwatson Senior Member

    I think we are talking Air pressure, not Water pressure. Certainly, if there is no water pressure, the boat would sink.

    I couldn't find any explanation or sources for those two diagrams, but it looks like the two big spikes on the Graph are initial drag, and the low pressure effect would be a big part of that. As the hull gets out of the water, planing on either a stepped hull or unstepped hull, it overcomes the low pressure.

    I spent quite a bit of time researching the topic, as I remembered researching the topic before. I was lucky enough to find the paper I had in mind.
    I also found another reference that I think is useful.
    These sources are where my opinions came from, so it is probably best to read them from the horse's mouth rather than my interpretation.

    Drag, Boundary Layer and Hull Roughness on Ship Hull Surface | Boundary Layer | Reynolds Number https://www.scribd.com/document/39037779/Drag-Boundary-Layer-and-Hull-Roughness-on-Ship-Hull-Surface



    Attached Files:

  7. Barry
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    Barry Senior Member

  8. Yellowjacket
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    Yellowjacket Senior Member

    Barry, Watson is correct, you can generate a huge amount of negative lift (pressure below atmospheric) under a hull with rocker or with tabs that are angled above the line of the planing surface. The pressure of the air around the hull has noting to do with it. If the hull has rocker (not a straight hull, but a convex shape when viewed from the side, the hull will be changing the direction of water and that momentum change is caused by a pressure over the surface that is negative. This "sucking down" is aptly named because it can be below atmospheric pressure. This negative pressure isn't over the entire hull area, it is confined to the surfaces with negative incidence. In the case of trim tabs that are angled up, the negative pressure is only on the tab area. On a hull with rocker this occurs only on the aft surfaces that are at a negative incidence, and that is only at the rear of the hull.
  9. Barry
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    Barry Senior Member

    We are not discussing a significantly rocker hull. I believe I made mention of this in an earlier post. Watsons references re hard chined hulls planing, supports positive pressure throughout. These
    pressure graphs have been presented in many articles on planing hulls.
    If there exists any point on the hull that the pressure is negative, and this area is close to the edge of the hull, it will more than likely ventilate.

    So if you agree with RW, then you would agree with his following comment from a previous post "The low pressure created by the fast moving water over flat hull sections, pulls the boat hull deeper in the water. That's why fast boats have parallel chines to the water, because at speed, the boat gets levelled out by the low pressure, as the "suction" exerts a downward force, even with no airfoil shape.."

    As a builder for many years of flat surfaced planing boats, excluding lift strakes and running rock strewn rivers, the faster you go, the higher the boat sits in the water. No suction, except for some very limited areas. Newton supports this, higher acceleration of the water = higher force

    The graphs that I supplied are from a couple of different papers where pressure distributions were recorded, and repeat themselves in many of the papers that I have read.
    It you drill a hole in any flat surfaced hull, water will flow in. (unless it is in an area behind a protuberance,) I am not arguing that.
    Last edited: Apr 13, 2019
  10. powerabout
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    powerabout Senior Member

    is it creating a hook in the hull when it is moving in the water as its longitudinal strength is not enough with all that weight hanging off the aft?
    Easy for ribs to suffer from this as no deck to support them and they have heavy narrow hulls.
    How wide is that at the chines at the transom?

    If that was an old narrow cigarette it would be dragging its arse at 30mph and proposing depending on the conditions
    A 38 cig weighs say 10,000lbs, you have the ability to almost double in same size hull or perhaps narrower?
    That all makes the tube drag in the water which creates huge drag.
    Hard to see from the vid but it looks like its digging a trench at that speed and the tube looks to be well and truly on the water
    Last edited: Apr 14, 2019

  11. Mr Efficiency
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    Mr Efficiency Senior Member

    I did ask that, I don't think he came back with it. You raise some interesting points, but this is a heavy beast for what bottom area is available.
    powerabout likes this.
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