What is the effect of aerating a planning boats on it's performance

Discussion in 'Boat Design' started by BlunderBus, Jun 16, 2014.

  1. BlunderBus
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    BlunderBus Junior Member

    This question has come about as I'm considering changing out the venturi on my 14' skiff (a Javelin) to using a couple of one way tube type valves.

    The question is...
    By aerating a hull that is planning will it travel faster than before for the same force? And would the structures to create the suction of air through the boat add more drag that would counter the speed gain?
     
  2. PAR
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    PAR Yacht Designer/Builder

    If you put anything into the flow, you'll create drag. Air (bubbles) are less dense then water, so the boat sinks as the air to water molecule ratio increases. Some gains can be achieved, but usually a lot of models, followed by testing is required, to see any significant performance improvements. If you use the hit or miss method, exspect lots of misses before slight improvements show up.
     
  3. NoahWannabe
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    NoahWannabe Junior Member

    BlunderBus, I would think you would need some kind of hull shape that can capture air bubbles under the Javelin hull to minimize wet surface to reduce drag. Rapid movement of air bubbles won't lubricate hull enough to reduce drag. Without some type of schemes to capture air under the hull, air pump/compressor needs to be quite large and heavy to produce enough air bubbles under your hull to minimize drag.

    Interesting thread, will subscribe and learn...
     
  4. BlunderBus
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    BlunderBus Junior Member

    I was kinda hoping it might be similar to how a stepped hull works? What are the main differences between these two approaches?
    Would a stepped hull sink down too?
    Is it a close fit to assume that stepped hull or not that the same planning area would be required for a give load at given speed.
    So would aerating the hull via a vaccum be reducing the wetted area and thus you'd sink a bit fo that given speed until the same wetted area would be being used again?
     
  5. BlunderBus
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    BlunderBus Junior Member

    So in short...
    Putting an array of these little one way valves across the boat is not going to reduce drag.
    But probably putting a couple in and removing the current venturi (huge beast of a thing) will be less drag, and hopefully remove more of the last inch of water that tends to sit in the boat when going up wind.
     
  6. Michael Y
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    Michael Y Junior Member

    Typing out loud...these sorts of thought experiments are always fun...the pressure from the bubbles at a particular depth on the hull would be same as the pressure from the water at that same depth on the hull, no? Assuming the bubbles are small (microbubbles) so approximately no pressure change with depth for the depth of the bubble. And the buoyancy comes from the pressure?
     
  7. rwatson
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    rwatson Senior Member

  8. rxcomposite
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    rxcomposite Senior Member

    Yes, the 2nd step hull receive less water density from the wake of the first step. Not easy to calculate the density of water for the 2nd and 3rd step, so balancing is very very difficult.
     
  9. Jimboat
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    Jimboat Senior Member

    I agree with PAR...
    Adding air to a water mixture reduces lift, since the mixture has less density, thus more SF lifting area required. This means more hydrodynamic drag also. The drag of the additional area - even on air/water mixture is a penalty.

    Air introduction is only a potential benefit if prolonged laminar flow "sticks" planing surface to water, increasing wetted length unnecessarily, causing excessive drag. A slight perturbation or vent can "unstick"the surface and return the wetted surface to that necessary to provide required lift.

    Trying to force an air/water mixture as an alternate fluid overall can only reduce overall lift force, and reduce L/D efficiency. Lot's of studies on planing surfaces. Tested results increase drag.
     
  10. intrepid71
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    intrepid71 Junior Member

    If we are talking about a thin layer of air bubbles under the hull, then those air bubbles will assume the same pressure as the surrounding water, which will be based on depth from the surface. Air density is not the issue, air pressure is. The fact that air has less density just means that the bubbles will tend to rise, and in this case that is helpful since that will force them up the hull surface. I don't see why they would cause a significant loss of lift or cause the hull to sink further.

    I did an undergraduate thesis where air bubbles under a flat planing surface did cause a reduction in resistance. Demonstrating that was not the purpose of the thesis, but it was an interesting phenomenon that was clear from the data.
     
  11. BMcF
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    BMcF Senior Member

    There are working examples of "air-lubricated" vessels out there..but only a few that were truly successful. Success defined as having lower drag with the air lubrication than without it, of course.

    The one I am most familiar with was a mono hull crewboat of maybe 70 feet LOA. The addition of an air lubrication/injection system resulted in a significant increase in the top speed, although I've since forgotten the magnitude of the increase. The owner/operator used an array of used/surplus Roots blowers powered by a smallish auxiliary diesel engine to pump the air in to a plenum that fed the injection "step" on the bottom of the hull.

    And I know of other attempts to use air lubrication schemes where the end result was no drag reduction achieved at all.
     
  12. intrepid71
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    intrepid71 Junior Member

    My guess is that having a very flat planing surface is key. That is probably why it worked with a crewboat. Since most planing boats have a vee-hull or something similar, I don't think it would be that helpful in most cases.

    Certainly for recreational boats, the effort to save a few percent in efficiency, if it can achieve that at all, is most likely not going to be worth the trouble.
     
  13. Jimboat
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    Jimboat Senior Member

    End result of effective drag is simply...
    Dp = [½ ρ V*V S Cd]; ... with air/water mixture as planning surface, ρ is less; when ρ is less, Drag is less.

    As suggested before, there are indeed other ways (reasons) that air introduction can aid overall efficiency, but on major planning surfaces, air introduction reduces Drag and causes increases wetted surface to compensate.

    intrepid71 - I'd be interested to read your research on air bubbles under a flat planing surface". (maybe you can send to me by PM or email?) All the research & testing that we've done with flat planning surfaces result in reduction of both Cd and Dp, when air/water mixture substituted for standard water planning surface.
     
  14. intrepid71
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    intrepid71 Junior Member

    I would send it to you if I had a copy, but I am afraid I don't, as it was over 20 years ago. There is a hard copy in the Webb Institute library. It was only an undergrad thesis. We didn't measure lift but we were measuring drag. We found that towing these flat boards in waves was creating lower resistance than still water. Underwater photos were showing the large amounts of bubbles running under the boards when they were towed in waves. The bubbles acting on the planing surface were the only explanation as generally waves would increase resistance.
     

  15. NoahWannabe
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    NoahWannabe Junior Member

    Two issues here in simple English:
    1. Air lubrication sounds good but it is a foamy water. Not much floats in foam, a major drag.
    2. Stepped hull does introduce air bubbles to reduce wet surface drag (supposedly), but the hull is already supported by the solid undisturbed water on the planing hull surface area due to fast planing speed. So any non-planing wet surface may be benefited by bubbles and may reduce drag.

    Unless you can capture air bubbles and make a boat sit on the air bubble with air pressure supporting the hull like a hovercraft or an inverted hull canoe, the boat will not gain any benefit of air bubbles for decreased drag.

    Depending on a hull shape, a stepped hull increases drag until above certain speed (~20 knots +/-). I am not sure how fast you can sail your Javelin, but if you get above at least 15 knots then you may get some drag reduction, but until then you are dragging more.
     
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