Midship interceptor

Discussion in 'Boat Design' started by HJS, Nov 11, 2008.

  1. HJS
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    HJS Member

    5,5 x 1,6 m 330 kg 8 hp 19 knots
    How to continue?

    HJS
     

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  2. daiquiri
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    daiquiri Engineering and Design

    Isn't it very similar to the Dynaplane hull concept?
    http://www.dynaplaneboat.com/

    If you think it isn't, the next step could be an application for a patent. ;)
     
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  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    It's different from Dynaplane, but I would have thought the turbulence would increase rather than decrease drag. Does either concept have the claimed advantages in other than perfectly calm conditions conditions?
     
  4. daiquiri
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    daiquiri Engineering and Design

    Well, it looks like there's a tradeoff between shear stress and pressure drag component behind that vertical plate.

    In case of clean hull, without the plate, you have a shear stress only which depends on velocity gradient near the hull bottom surface. Since there is no plate, the velocity near the hull rises quickly from zero to nearly free-flow velocity - hence the high gradient and high shear stress.

    With the "interceptor" plate it seems that there's a situation behind it where a turbulent wake is formed with low mean velocity, and therefore a low shear stress. But on the other hand there must be an increase in pressure drag, due to the low pressure inside the turbulent wake and a high pressure in front of the vertical plate, which is what gives the lift increase in the zone of "interceptor".

    Now we know (thanks to HJS' tow tests, but also thanks to Dynaplane test results) that the overall tradeoff is a net decrease in total (shear stress + pressure) drag.

    I agree with Kayaker that this is sufficiently different from Dynaplane to allow for a patent application (and I advice HJS to do it as soon as possible now that he has disclosed it here), but the physical principles behind the two are similar.

    But HJS' solution definitely has a big advantage of constructive simplicity.
     
  5. HJS
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    HJS Member

    to anciant kayaker:
    The boat behaves like other boats with same bottom beam, length, weight and dead-rise. That means speed reduction is as usual in waves. Can be calculated according to Hoggard.

    to daiquiri
    The bottom aft of the interceptor is dry when the boat goes over FnD 2,5.
    All is happening in front of the interceptor. There is no turbulence either forward or aft of the interceptor. If there is turbulence the interceptor will work as a brake.

    The drag-lift is about 1/10, that is the same as normal hydrofoils.

    js
     

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  6. daiquiri
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    daiquiri Engineering and Design

    Nice. I didn't consider that air intake. It is definitely different form Dynaplane. ;)
    It is similar to an old patch-solution for boats (semi-displacement, usualy) which need to rise the transom a bit, in order to trim down the bow.
    I've applied it once to the transom of a motoryacht which was unable to attain the speed that the designer had granted (12 kts for a 40 ft semi-displacement hull, but the boat didn't want to go over 10 ). We have welded a plate perpendicular to the flow, right at the trailing edge of the hull and it worked. Just like the one in this section view.
    And it worked well - over a 1.5 knots gained with same power and the visibility from helm station greatly improved.

    It was a good intuition you had to apply it to a planing hull. :)
     
    Last edited: Nov 19, 2008
  7. rwatson
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    rwatson Senior Member

    From a simple peasant mind, the effect of a vertical plate should create considerable drag.

    Why wouldnt an adjustable "step" work better , with no vertical leading surface ?
     
  8. daiquiri
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    daiquiri Engineering and Design

    Like I said before, a hull drag in full planing regime is mainly due to pressure and friction drag components.

    If you consider the pressure drag component, the answer is yes - the plate creates more drag than just a smooth hull. A vortex behind the plate would form, which would increase the drag. The air intakes behind the plate serve to avoid that - it is already being done for stepped hulls.

    The next part is my conjecture, based on my knowledge of fluid dynamics - no proof for this:

    The vertical plate reduces the friction drag component, imho, for two reasons.
    The first one is that you have replaced water behind the plate with air, which is less viscous fluid than water.
    The second reason is that the plate creates a region of high pressure in front of it, which creates an increase in lift.
    So, for the same boat weight a smaller wetted surface is needed to sustain the hull - and that again means less friction drag.

    Conclusion - the pressure drag increases to some degree, but the reduction in friction drag is much more important. And therefore the overall drag is smaller with the plate.

    I repeat, this is my back-step analysis of this invention, easy to do once you see it working.
    The physical principles are well known, the HJS' achievement is that he was the first one to apply them in this innovative way, so my respect.
     
  9. tom28571
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    tom28571 Senior Member

    This is all very interesting and also counter intuitive at first glance. It reminds me of some experiments made, by Prandtl I think, in aerodynamics where he tested a strange airfoil. It was a "V" laid on its side with the open end forward. Apparently, the high pressure air in the cavity serves as a surface to flow and it provided lift just like a normal airfoil. Don't remember how effective but think it was more velocity dependent than the normal foil. It is also a bit like the pick-up truck where the drag is lower with tailgate closed than open. Money made on bets on that one.

    Air introduction behind steps goes way back as far as steps do.

    Daiquiri, I think you implied that the boundary layer thickness in front of the interceptor is much greater and therefore the sheer stress (and therefore drag) is reduced. Is that right?
     
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  10. daiquiri
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    daiquiri Engineering and Design

    It was probably a precursor of modern RAM-air airfoils, which are used for parachutes and paragliders:

    [​IMG]

    You can notice that the leading edge is open, allowing the airflow to pressurize the internal cavity to the total flow pressure (p0 + 0.5 rho V^2). Since the outside surface is subject to static pressure only, this pressure differential gives the necessary rigidity and shape to the parachute. And the external airflow effectively follows a path of an "invisible", inexistant leading edge, as if it was actually there.

    A similar trick is employed at the trailing edge of aerodynamic bodies, when they are subject to limitations of total length. The back of the body is truncated, which creates a "bubble" of recirculating air behind the body. The external airflow "sees" this bubble as if it was a part of the body and follows its path around it, towards a virtual trailing edge. It allows the pressure recovery behind the body, and the consequent reduction of drag.
    Toyota Prius is an example of that solution, as is this Volkswagen concept:

    [​IMG]

    I think (my speculation, ok?) that the interceptor should be working inside the hull's boundary layer to give the maximum L/D increase. While inside of it, the pressure drag increase should be modest, while the friction should not depend directly on this, since it is mostly the air behind the plate which reduces the shear stress drag component.
    But it is also true that pressure increase in front of the interceptor plate is what provides the additional lift, which allows a smaller hull wetted surface (as explained in the first post). So from that point of view you would need to do exactly the opposite - to extend it outside the boundary layer in order to have more lift.

    So at the end it's a game of pros and cons. I guess that only the experiments (or CFD) can indicate the optimum plate length.

    Oh, by the way... These considerations are just my suppositions, like I said. It is based on my knowledge of aero/hydrodynamics. Serious people (I'm obviously not part of that category :D ) who care about their reputation usually don't expose themselves so much before seeing the complete test results (and letting someone else say the first opinion). ;) :D
     
  11. tom28571
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    tom28571 Senior Member

    Ok, I guess any protrusion can be considered to be working inside the boundary layer since there has to be a boundary layer making its way around it. What I probably meant was that the boundary layer in front of the interceptor plate is much thicker and flow hardly exists over a part of it.

    Does the fact that there is no actual water flow under a planing hull make any difference to the analysis? It's actually the water at the hull surface that is moving and water on the far side of the boundary layer that is at rest, or nearly so. I think all the diagrams I've seen explaining planing forces are wrong, even in respected books.:p
     
  12. daiquiri
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    daiquiri Engineering and Design

    Strictly speaking - yes, you're right. When I say that the plate is inside the boundary layer, I'm thinking of clean hull boundary layer. It will obviously be modified by the interceptor plate, but there will exist a plate length which will introduce a small enough modification that it can be considered effectively inside of hull boundary layer.
    It's a common expression in aerodynamics. Vortex generators (or turbulators), which serve to induce transition to turbulent flow on scale models during towing tank tests (or during wind tunnel tests on model airplanes), are dimensioned so that they don't protrude outside of the boundary layer. In that way they create turbulence (which serve to keep the flow attached to the wing or to the hull), without introducing additional pressure drag.

    It is the same thing. You just change your inertial system, i.e. your point of view.
    There is a principle of reciprocity in fluid dynamics which states that it is indifferent whether the fluid is moving towards the body with the velocity V or the body is moving with the same velocity through the fluid.

    It is also very intuitive. If you are on board of a planing boat, your reference system is the boat and you see that the water is moving under your hull with a certain velocity V.
    If you are another person swimming in the water and watching the same boat, your reference system will be some (still) point in the water, and you'll see the boat moving over the calm water surface with the velocity V.
    In both cases the boat is the same and it's hull is developing the same lift ad drag forces, it is just your point of view (your inertial system, to use a more technical therm) that has changed.
    That's the principle on which the wind tunnels or towing tank experiments are based.
     
  13. tom28571
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    tom28571 Senior Member

    I understand the reciprocity theorem and know that the math works out either way. The implication is that part of the kinetic energy (momentum) in the flow is transferred to the boat hull in the form of lift and drag. If the boat is moving, its momentum is transferred to the water and these must always be equal if there is equilibrium. I understand planing forces much better by applying Newton's laws with the boat in motion though.

    Newton did not help a whole lot at first glance, in looking at the effects of the interceptor.
     
  14. daiquiri
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    daiquiri Engineering and Design

    Our brains work differently. I NEED to see the hull standing still with fluid flowing around it. If the water flows my mind flows too, otherwise it doesn't. :D
     

  15. tom28571
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    tom28571 Senior Member

    Daiquiri,

    I sent you a PM.

    Tom
     
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