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

    I said "as a rooster tail-like display", thanks for changing the text :rolleyes: , if there is passing water dragging the boat down, there will be a corresponding lifting of water. Think Newton ! In any case, the area is vented to the atmosphere, making it even more unlikely that a low pressure develops to "suck" water to the pod.
     
    Last edited: Mar 30, 2019
    rwatson likes this.
  2. rwatson
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    rwatson Senior Member

    Just detail. That underwater shot show no tendency for any kind of rooster tail (tall or subtle) and is truncated sharply by the final transom as it should.

    The is no "venting" or any kind of aeration showing beside the Pod either. The fact that the flow off the Pod is uniform and laminar is a good clue that the water from the boats chine is re-attaching, otherwise there would be curls and vortexes beside the Pod cheeks.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    Obviously, I was referring an intermittent "rooster tail like display" being created by the pod, not the hull proper. But nothing of the kind, is evident on the video, as the boat pitches. If the boat is being pulled down by pod contact, it will drag water higher behind it.
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    I have no idea what you are looking at to reach that conclusion, the pictures show the pod clear of the water.
     
  5. rwatson
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    rwatson Senior Member

    No it doesn't. You can see the white flow from the final V of the Pod., and that flow continues all the way up to the top of the chine.
    The rightmost red arrow is showing the flow off the Pod


    . Flow.png
     
  6. rwatson
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    rwatson Senior Member

    No, that's not the case.
    The low pressure is created simply by fast water across the pod "cheeks". If the pod cheeks were were curved, like traditional sailboat lines ( or upper surfaces of plane wings ), then the effect would be even more pronounced, and the "suction" could create ventilation and all sorts of problems including "rooster tails". As it is though, the water cuts off cleanly at the pod transom, as it is meant to simply because the "cheeks" are flat.

    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..

    They have the same problem with Flying Boats taking off. Their "Pods" "stick", so they have to put "steps" into the flats of the sponsons, to break the low pressure. High speed racing boats with stepped hulls achieve exactly the same effect, unsticking from the low pressure against the flat hulls.

    Ideally, if the chines of the Pod are parallel, it doesn't matter, because the low pressure vector (at right angles to the surface of the Pod) of the Pod is identical to the main hull.

    But when the chines on the attached Pod are so steeply sloped up, the low pressure vector is slightly rearwards, and "levers" the stern down.

    It might be as YellowJacket mentioned, that the low pressure gets stronger at certain hull angles, but it is most certainly there.
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    The pod is foreground, top of picture, as I am seeing it. Your red arrows are behind the hull proper.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Naah, rwatson, you are looking at the transom step, and the separating flow. The pod bottom is the reflecting panel above the free flow. The problem here is that the inflow to the propellers does not have a fixed depth; it varies with speed and angle of attack. These props are quite highly loaded in terms of [thrust/dynamic pressure/disc area], making them sensitive to inflow variation and aeration.

    That said, I still consider the pod bottom to be part of the problem as I said before.

    Looking at the pics showing the propellers, it seems there is some cupping of the blades. I'd like to know the diameter and pitch (dia 16" ?). A slight increase of cup might improve the operation, since it can reduce the effect of aeration at high loading.
     
  9. Mr Efficiency
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    Mr Efficiency Senior Member

    That is the way pods of this kind are, the flow depth varies, and the idea (originally) was to get a little extra speed out of the boat, by props working in water that had lost a bit of the forward speed imparted by hull drag, sure the revs will fluctuate, but thrust not much, as the prop becomes more exposed, revs go up, thrust does not change drastically. But of course, the props need to be suitable.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    If the pod is implicated, the fix is to raise it slightly, there is room to drop the engines if needed, but I'd be surprised if that is the issue, if indeed there is an issue, the OP is used to other boats, and this one may be a different kind of beast, if it had the pods that are a continuation of the hull, it no doubt would be different, but might still be more inclined to porpoise than the boats he is used to.
     
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  11. fallguy
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    fallguy Senior Member

    Despite liking the post for presenting some good thinking; he said he porpoised without pax. And he said he failed to get on plane until 30 kts.

    Neither of these are going to change with the fuel tank.

    That said, as awful as this boat is performing, the manufacturer ought to be working with him to fix it, and there is nothing wrong with running the fuel tanks down low and testing near empty.

    I am betting the porpoising will worsen as the variability of centers increases.

    The easy fix for the other boat I saw in the Thames video is a seaweed catcher(hydrofoil). It would stop that ridiculous bow hop and probably at a cost of 1-2knots top end. And consider the cost; less than a new prop try. And, Yellowjacket suggested the pod is down creating downforces in the hole (I think not, but perhaps or in heavy seas, or a bit in the hole as well) This could be videoed, I should add. But a hydrofoil would work against that premise as well.

    Moving the liferaft was good.

    Moving the spotters chair another easy test.

    Gf- Did the manufacturer ever give you the hydrostatics? Not me, but some of the fellows here might spot other issues in those numbers. And then it would be important to note whether the hydrostatics were for the raw hull or the modified version you are running.
     
    Last edited: Mar 31, 2019
  12. Yellowjacket
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    Yellowjacket Senior Member

    The question isn't what is happening at 30 knots where the boat begins to behave. It's what is happening below that speed where the dynamic instability occurs and why is the efficiency so poor at lower speeds. This is a long pod and with these heavy motors the CG is aft of where you probably want it, but if you have a good bit of weight forward and the boat is still porpoiseing, then it likely isn't likely an issue with the CG being too far aft. The step at the transom isn't as large as I've seen in other cases and that could be an issue if it allows the flow coming off the transom to reattach to the pod. This should be pretty easy to see just by looking over the transom at the pod and seeing if the flow is clean with no attachment and the pod is out of the water then it isn't the problem. But if you see spray attached to the v of the pod, or is it dragging in the water as I expect it is then this is most certainly a big issue. If it isn't reattaching then I'm wrong and it should be obvious if you just observe it. There isn't going to be a big rooster tail, but if the flow is touching the bottom of the pod then it is certainly going to be a problem. Since the boat behaves above a 30 kts, you have to look at what is happening below that speed.
     
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  13. Mr Efficiency
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    Mr Efficiency Senior Member

    Not only would there be water shooting high behind the pod, but water shooting vertically skywards from the outboards legs above the splash plate, if water was attaching to the pods and following the upslope, this should be obvious with a glance behind. Seeing there has been no mention of this from the OP, I am inclined to doubt it is happening.
     
  14. Mr Efficiency
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    Mr Efficiency Senior Member

    if you look at the trim tabs in the opening post, you notice that a fair bit of length is hidden behind strakes, this would be hindering their usefulness, and means they are probably needing to be depressed more than ideally. One possible option if tabs are not effective, could be to just attach a wedge to the hull, outboard of the outer strake, between it and the chine, to "fill in" that area out wide with a straight wedge or a curved "hook", and have the tabs as that little extra. But if boats with ample power still struggle to get on plane, and hold plane at fairly brisk speeds, it is usually a combination of too much weight, not enough bottom area, unfavourable LCG, and deadrise angle too steep, and there is a limit to what tabs and wedges can do to help.
     

  15. Barry
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    Barry Senior Member

    Some explanation is required here. Suction to me is a pressure below atmospheric pressure. The pressure on the bottom of a hull is positive or it would not rise upward when planing. If
    you drill a hole in the bottom of a boat while planing, water flows in, not out.

    The purpose of steps, is to create another stagnation line, ie another place for high pressure in the same hull length. And of course the step, must be ventilated to atmosphere for this
    to work.
     
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