Aeration in tunnel

Discussion in 'Boat Design' started by weldandglass, May 31, 2013.

  1. weldandglass
    Joined: May 2010
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    weldandglass Junior Member

    I have a question about planing hulls with tunnels and aeration. As aeration enters the tunnel of a planing hull, How does it act as it passes through the tunnel? Does it have a tendency to be forced towards the top of the water column in the tunnel? Is it evenly distributed across the tunnel or does it have a tendency to be forced towards the corners? If anyone has any knowledge or links to information on this subject it would be greatly appreciated. I've done some looking and haven't had much success.
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    I could be like Stephen Hawking and say it behaves according to the laws of physics, but I doubt that will assist you ! What is the particular line of enquiry you are pursuing, and to what end ? My first thought would be it depends on what particular boat you are studying. Is there much aeration of water in a "box" tunnel hull's tunnel ? Seemingly not.
     
  3. weldandglass
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    weldandglass Junior Member

    Flat bottom, outboard powered planing skiffs with tunnels typically exhibit some sign of aerated water entering the tunnel when running in a head sea or a following sea. The air ingestion is obvious due to constant revving of the engine as the boat encounters chop and the tunnel ingests air and then feeds the aerated water to the prop. These boats run well in shallow water but suffer from inefficiency in deep water due to the air ingestion and resulting prop slippage. Boats with some dead rise tend to split the air bubbles away from the tunnel but flat bottoms just feed the air sttraight back. Just wondering what opportunities there are for directing or bleeding the air off once it's in the tunnel. Trying to understand how the air bubbles behave once in the tunnel is the first piece of the equation, at least in my mind anyways.
     
  4. tunnels

    tunnels Previous Member

    Tunnels are there to get aerated water through under the boat and if the bubbles in the water are causing a problem with the propeller then the propeller needs to be changes to cope with airated water flow !!. or you could just fill in the tunnel and do away with it completely !!!!:D
     
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    OK, I get the picture now, first thing to try would be different prop, possibly a five-blade stainless could hold on better in an environment like that.
     
  6. tunnels

    tunnels Previous Member

    Ask a prop guy !!

    Don't guess ask a prop person what is the best prop to use in that situation . if he's worth his salt he will tell you straight away !!:idea::confused:
    Some racing props work in this kind of environment all the time and need air to work properly !!
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Don't know that there are any props that fit "stock standard" outboard gearcases that "need" air, or benefit from it.
     
  8. tunnels

    tunnels Previous Member

  9. CDK
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    CDK retired engineer

    The air/water mixture spends very little time in the tunnel of a planing boat, so it is safe to assume it does not separate into layers.

    My boat has its props at the end of the tunnels. Only at very low speed does the wake look like the dark surrounding seawater; with increasing speed the color gets lighter because small air bubbles need much more time to escape than large ones. The slightly milky trail exists long enough to be visible when crossing it after a 360 turn.
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    If the prop experimentation thing hits a dead end, try installing a "splitter" in the tunnel ahead of of the outboard leg, two or three inches deep might be enough even. Of course, if that will make the draft too much for the shallows, you have to look elsewhere for a solution.
     
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  11. baeckmo
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    baeckmo Hydrodynamics

    The physical image of the bubbly layer is that the shearing forces due to turbulence in the water tend to split the bubbles into smaller size. Against this effect there is the influence from several bubbles "hoovering" together in a pulsating pressure field. This makes the bubbles shrink and expand simultaneously, creating a force that pulls bubbles together until they merge into bigger ones.

    Just as CDK describes, this leads to a surprizingly steady and longlasting bubble layer, following the tunnel aft. The bubble buoyancy tends to generate a "foam" with varying density; lower close to the top of the tunnel.

    The point with this foam layer is that it can be very effective in reducing slamming pressures on the flat panels in the tunnel. The maximum pressure peaks are proportional to the velocity of sound in the fluid. Oddly enough, although the speed of sound in pure water is about 1500 m/s, but the speed of sound in aerated ("foaming") water can be as low as about 30 m/s (by an air proportion of about 10 volume percent). This makes a drastic reduction in slamming pressure.

    The volume of foaming water in the tunnel is depending on a number of factors, but when it comes to the propeller there are two routes to take. First, as "Tunnels" has told you, is to dimension the propeller for an aerated flow. This requires special blade profiles, since the fluid velocity along the blade is more or less supersonic with peripheral speeds around 50 to 60 m/s and speed of sound in the fluid as low as 30 m/s; ie Mach 2 in fluid terminology. Along with this comes the problem that the fluid density is reduced, which requires larger prop disc area.

    The other strategy to make the prop to work is simply avoidance. Either you lower the propeller position until it is working mainly in non-aerated water, or you find means to divert the air mixture from the propeller disc. Best medicine?? Talk to an experienced prop shop as Tunnels already said. If they have some experience from racing, they certainly know the trick. But for the boat performance I would keep the tunnel intact all the way to the transom.
     
  12. tom kane
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    tom kane Senior Member

    Areation in tunnel

    Tunnels and pods are always a problem because people expect the wrong results. You can only run a propeller successfully in the correct position.

    surface propellers need to be at least 60 percent immersed below the mean water flow level when in motion. And subsurface prop tips at least two inches below the mean water level flow. An unenclosed prop can not suck up the water and change the direction of water flow but can only fling water everywhere. Tunnels and pods are surface propulsion just like jet units.Then you have all the other problems of rudder depth and not being able to steer. Any propeller can be run in any mode with varing results.

    And what moves a boat forward? it is the volume and velocity of the water pushed rearward.
     
    Last edited: Jun 1, 2013
  13. weldandglass
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    weldandglass Junior Member

    Thanks to all who replied. Baeckmo, your input is extremely helpful.
     
  14. richard gray
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    richard gray Junior Member

    what about if it is a jet drive with a 6 t0 8 degree dead rise , with narrow outboard low draft sponsons( like cat or boston whaler type hull 8 foot wide) 5 ft tween sponsons with a level keel with last 5foot with a rise of keel of 5 inches to transom ?
     

  15. baeckmo
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    baeckmo Hydrodynamics

    That will certainly be problematic, not so much due to the sponsons, but due to the low deadrise, which does not "separate" the aerated water coming from the spray root, away from the main flow. The air volume that "kills" a waterjet pump is about the critical 10 % by volume that generates low speed of sound in the mixture, which is not much.

    When a jet has to be used with a small deadrise, it may be necessary to mount the intake in a low pod so that the inflow is green water. Now, if the pod width is a substantial proportion of the tunnel, or distance between side walls, the pod will generate an unproportionally great increase in resistance. So, generally speaking, if there is a longitudinal pocket in the bottom, beware!!!!
     
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