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NACA Intake Duct

The NACA (National Advisory Council for Aeronautics) was organized during World War II. It is the predecessor of NASA. Most of what we know about the fluid dynamics of sub-sonic aerodynamics was codified before Chuck Yeager's October 14, 1947 sortie through the wall that is now known as Mach-1(. . .but that's another story).

The interesting thing about fluid dynamics, is that most principles apply to fluids, period, regardless of whether liquid or gas; however, there are some caveats in consideration of compressible vs. incompressible fluids, examples of which are atmospheric air, and water, respectively. Boats operate in both fluid states. Arguably, one can ask the rhetorical question, "When does a high-performance boat become a low-flying aircraft with its tail dragging in the water?".

The aspect of a boat's tail "dragging in the water" is worthy of pause, because that is exactly what is occurring when a boat is moving across the surface of water. Drag is the Devil. Actually, this is called parasitic drag. In real-world pragamatic terms, it slows you down.

The NACA Duct has an interesting performance phenomena of minimizing parasitic drag (because nothing sticks out into the slipstream), while producing significant pressure at the throat of the duct. Most of the research and application is with regard to air ducts and scoops as applied to aircraft and high performance automobile systems. We are currently evaluating designs for the intake of marine water jet propulsion systems.

The following graphics are typical of designs undergoing evaluation:

This intake design is to be cast-in-place with the hull. This has several advantages:

  • Improved intake efficiency with regard to the hull design.
  • Savings in manufacturing cost passed on to customer.
  • Fewer maintenance tasks.

The throat aspect ratio is 3:1. The width of the throat at the intake flange is 6.75" and the throat ramp angle is 7 degrees.

In the following examples, the intake flange plane is parallel to the horizontal keel plane; however, this installed angle is subject to change. We are evaluating a five degree cant which would reduce the intake ram angle from 35 degrees to thirty degrees, while allowing the four degree transom flange angle of the AT SD309 pump to be installed in our ClassX 18 series without a molded flange recess on the transom plane.

  NACA Intake Duct Displacement Model



  NACA Intake Duct Design Hatched Perimitives



  NACA Intake Duct Wire Diagram


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