Reducing detachment drag

Detaching Flows Using Sharp-Edged Flexible Flaps: A Practical Drag Reduction Device

Inspired by the very large documented drag reductions from knife-sharp chines on planing hulls, I propose a simple, practical drag reduction device: a flexible, sharp-edged flap strategically bridging the line of fluid detachment.

Core Mechanism:

Drag is created when flow is deflected at separation lines by viscous attachment forces. At the leading edge of ventilated foils this deflection manifests as an adverse pressure gradient that thickens the boundary layer and leads to uncontrolled separation with turbulent vortex shedding, creating large cavity drag forces.

A flexible, sharp-edged flap mounted along flow detachment lines of all kinds creates a passively compliant, low drag detachment surface, where viscous attachment passively deflects the flap to comply with local momentum, then the flow separates cleanly from the flap’s sharp trailing edge. In other words drag is reduced because the flap passively deflects to comply with local flow momentum, instead of the local flow momentum deflecting through viscous attachment to a rigid surface. This enforces a stable, separation line, reduces shear layer diffusion, minimizes wake momentum deficit, and mitigates form drag.
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Mechanism of Action

• The sharp-edged flexible flap functions as a compliant, low-drag detachment line, precisely defining where and how separation occurs.

• By passively flexing to comply with local flow momentum near the separation point, it inhibits boundary layer thickening and turbulent detachment on the suction side.

• Flow detaches cleanly from the flap’s sharp trailing edge, producing a thin, coherent shear layer and reducing wake-induced form drag.

Comparison:

• Without flap: uncontrolled detachment, thickened shear layer, broad turbulent wake, high form drag.

• With flap: enforced sharp separation, coherent shear layer roll-off, reduced wake cross-section, lower form drag.

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Guesstimated Benefits for Ventilated Foils

• Suction side viscous drag reduced by ~50% in the detachment region.

• Pressure (form) drag from wake separation reduced by 5-10% of total foil drag.

• Overall drag reduction of approximately 10-15% compared to baseline ventilated foil.
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Broader Applications

1. Planing Surface Transoms and Chines:

Creating sharp release lines reduces form drag by enabling clean separation. Flexible flaps could retrofit existing designs or dynamically adjust to varying flow conditions.

2. Surface-Piercing Foils and Rudders:

Flaps along the entire flow detachment line can stabilize ventilation boundaries, reducing unsteady separation and improving efficiency.

3. Ship Sterns and Transom Flow:

Sharp detachment devices reduce base drag by controlling separation at the stern.

4. Hydroplane Sponsons and Tunnel Boats:

Flaps manage reattachment and ventilation effects at high speeds, reducing form drag.
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Conclusion:

Applying sharp-edged flexible flaps at flow detachment lines of any type offers a practical, modular approach to mitigating drag penalties inherent in flow separation. By ensuring clean, sharp separation, these flaps suppress viscous detachment drag and reduce wake-induced form drag, achieving an estimated 10-15% reduction in total drag.

This principle is broadly applicable across marine and other contexts where fluid detachment imposes drag penalties. Its flexible implementation makes it suitable for retrofits, adaptive flow control, and optimized design of lifting and planing surfaces.
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Technical Appendix: Force Breakdown and Pressure Effects on Ventilated Fools

Pressure Coefficient (Cp) Effects

• Without flap: suction side Cp flattens due to cavity exposure, followed by a steep drop at uncontrolled separation.

• With flap: Cp shows smoother gradients with controlled separation, reducing pressure losses and narrowing the wake.


AI Guesstimates of Force Breakdown

• Suction side skin friction drag reduced from ~5-10% to ~2-5% of total drag.

• Pressure (form) drag from wake reduced from ~80-90% to ~70-85% of total drag.

• Induced drag (from spanwise circulation) remains unchanged.

Momentum Deficit Reduction

• The wake momentum deficit is reduced with flap intervention due to a thinner, more coherent separated shear layer and smaller wake cross-section.

Lift-to-Drag Ratio (L/D)

• As ventilated foils lack leading edge suction, lift remains constant.

• However, by reducing total drag, the flap improves L/D by a guesstimated 10-15%.

• Without Flap:
  
  • Wake is wide, turbulent, with significant momentum loss.
    
    
  • Thick separated shear layer diffuses vorticity into the wake.
    
    
  • Results in high form drag.
  
  
• With Flap:
  
  • Sharp separation enforced by flap creates a thinner, more coherent shear layer.
    
    
  • Wake cross-section is smaller.
    
    
  • Momentum deficit behind the foil is reduced.
    
    
  • Form drag is significantly lower.
    
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Design Checklist: Implementing Sharp-Edged Flexible Flaps on Ventilated Foils


Objective:

To reduce drag on ventilated hydrofoils by enforcing clean, sharp flow separation at the detachment line using flexible, sharp-edged flaps.

1.
Flap Positioning

• Placement and Safety:
  Positively secure the flap attachment point near the natural flow separation/detachment line on the suction side. Ensure the flap cannot detach or easily tear, as this could cause a dangerous broach if it happened on one side of a high-speed craft.
  
  
• Goal:
  
  Allow upstream flow to develop normally. The flap should only influence the flow at or slightly before the detachment zone.

2.
Flap Geometry

• Sharp Trailing Edge:
  
  Ensure the flap terminates in a geometrically sharp edge to promote clean shear layer roll-off.
  
  
• Aspect Ratio:
  
  Flap length (chordwise) should be sufficient to accommodate boundary layer deflection, but not extend so far as to interfere with upstream flow development.
  
  
• Thickness:
  
  Minimize flap thickness to reduce interference drag while maintaining necessary stiffness and durability.

3.
Flexibility & Compliance

• Passive Deflection:
  
  Flap must be able to flex under local flow pressure, passively aligning with flow direction near the separation point.
  
  
• Stiffness Tuning:
  
  Flap stiffness should be optimized to:
  
  • Resist flutter at operational speeds.
    
    
  • Comply with local flow without excessive bending.
  
  
• Materials:
  
  Use resilient, hydrodynamically smooth materials (e.g., thin composite laminates, reinforced polymers).

4.
Spanwise Application

• Tip Coverage:
  
  Consider flaps extending towards lateral tips to address tip vortex effects.
  
  
• Spanwise Segmentation:
  
  For high aspect ratio foils, segmenting flaps may allow for adaptive detachment control across the span.

5.
Integration Considerations

• Mounting Method:
  
  Positively secure attachment at base while allowing free trailing edge deflection.
  
  
• Retrofit Compatibility:
  
  Design for modular retrofitting onto existing ventilated foil designs where possible.
  
  
• Cavity Interaction:
  
  Ensure flap placement does not interfere with controlled ventilation systems (air feed paths, cavity stability).

6.
AI Guesstimates of Performance Gain

• Viscous Drag Reduction:
  
  ~50% reduction in suction side viscous drag within the detachment region.
  
  
• Form Drag Reduction:
  
  ~5-10% reduction in total form drag due to wake narrowing.
  
  
• Total Drag Reduction:
  
  Estimated 10-15% improvement in overall drag for ventilated foils.
  
  
• Lift-to-Drag Ratio (L/D):
  
  Improvement proportional to drag reduction, with lift remaining constant.

7.
Testing & Validation

• CFD Simulation:
  
  Validate flap design via RANS/LES models focusing on shear layer behavior and wake momentum deficit.
  
  
• Tow Tank / Water Tunnel Testing:
  
  Empirically measure drag reductions, wake profile changes, and flow stability. Could mount on only one side of the model and measure for yaw forces created by drag inequality from one side to the otheron the model.
  
  
• Vibration & Flutter Analysis:
  
  Ensure structural dynamics of the flap are stable under operating conditions.

8.
Broader Applications

• Planing hull chines.
  
  
• Surface-piercing foils and rudders.
  
  
• Ship sterns and transoms.
  
  
• Bluff body drag reduction .
  
  
• Hydroplane sponsons and tunnel boat flow control.
• Any mobile or stationary application where there is a benefit from reducing viscous deflection of flows along detachment lines
• 
  
  Concept at a Glance: Sharp-Edged Flexible Flaps for Drag Reduction
  
  What is it?
  
  A flexible, sharp-edged flap mounted near the natural flow detachment line around the periphery of, for example, ventilated foils, acting as a passive, compliant separation fence.
  
  Core Function:
  
  • Prevents viscous deflection of flow at the detachment region.
    
    
  • Allows flow to deflect the flap, then separate cleanly from its sharp trailing edge.
    
    
  • Controls vorticity release, minimizes wake size, reduces form drag.
  
  
  Key AI Guesstimates:
  
  • 50% reduction in suction side viscous drag (local).
    
    
  • 5-10% reduction in total form drag from wake narrowing.
    
    
  • 10-15% total drag reduction for ventilated foils.
    
    
  • Improved Lift-to-Drag Ratio (L/D) with no lift penalty.
  
  Application Examples:
  
  
  • Ventilated hydrofoils
    
    
  • Periphery of any planing surface, for example boats, surfboards, water skis, wakeboards, kiteboards etc etc.
    
    
  • Surface-piercing foils & rudders
    
    
  • Transom drag reduction (ships, boats, displacement yachts)
    
    
  • Bluff body drag reduction (vehicles, aerodynamics)
  
  Why it Works:
  
  • Creates a stable, sharp separation line.
    
    
  • Reduces shear layer diffusion and wake momentum deficit.
    
    
  • Mimics benefits of sharp chines, applied adaptively.
  
  Implementation Checklist:
  
  • Sharp trailing edge geometry
    
    
  • Passive flexibility (tuned stiffness)
    
    
  • Mounted just upstream of separation line
    
    
  • Minimal thickness for flow conformity
    
    
  • CFD and empirical validation
  
  
  FAQ: Sharp-Edged Flexible Flaps for Drag Reduction on Ventilated Foils
  
  
  Q1: Does the flap restore lost lift on ventilated foils?
  
  A: No. Leading edge suction is still limited by the cavity’s internal pressure. The flap does not restore suction lift but improves Lift-to-Drag Ratio (L/D) by reducing drag, not increasing lift.
  
  Q2: How does a sharp separation line reduce drag?
  
  A: Sharp separation prevents viscous forces from causing diffuse, uncontrolled detachment. The flap enforces a clean separation line, leading to a thinner, more coherent shear layer and a narrower wake, which reduces form (pressure) drag.
  
  Q3: Is this just like sharpening the edge of the foil?
  
  A: Conceptually similar, but more practical. Sharpening the entire foil may be structurally or hydrodynamically impractical. A flexible flap selectively bridges the detachment line, achieving the same clean separation effect without altering the whole foil geometry.
  
  Q4: Does the flap eliminate separation?
  
  A: No. Separation still occurs (as it must on ventilated foils), but the flap controls where and how it happens, reducing energy loss into the wake.
  
  Q5: Is the flap active or passive?
  
  A: It’s a passive device. The flap flexes under flow pressure, passively complying with local flow deflection while enforcing sharp separation at its trailing edge.
  
  Q6: Can this be retrofitted to existing foils, planing surfaces etc etc?
  
  A: Yes. The flap concept is modular and retrofit-friendly, designed to be added without major redesigns of existing structures.
  
  
  Q7: Where else could this be useful?
  
  A: Beyond ventilated foils:
  
  
  • Planing hull chines and transoms
    
    
  • Surface-piercing rudders
    
    
  • Ship sterns & transoms
    
    
  • Bluff bodies
    
    
  • Hydroplane sponsons and tunnel boats
  • Any planing surface of any type
  • Any mobile or static applications where there is a drag or other benefit from reducing viscous deflection of flows at separation lines
    
    
    Myth-Busting: Sharp-Edged Flaps & Flow Separation
    
    
    Myth #1: “All separation is bad — good designs prevent separation entirely.”
    
    Fact:
    
    Separation is basically inevitable and usually necessary, especially on ventilated foils where the suction side flow detaches along the leading edgeto form a cavity. The key is not to prevent separation, but to control how and where it occurs. Sharp-edged flaps enforce clean, energy-efficient separation, reducing drag despite separation still being present.
    
    Myth #2: “Only streamlined, attached flow reduces drag.”
    
    Fact:
    
    While streamlined attached flow is ideal for fully wetted foils, in ventilated or cavitating regimes, cleanly managed separation with controlled shear layers can significantly reduce drag. The flap achieves this by mimicking sharp trailing edge effects, narrowing wakes and lowering form drag — even in separated flow conditions.
    
    
    Myth #3: “A flexible flap would just flutter and add drag.”
    
    Fact:
    
    Properly designed flaps have tuned stiffness and damping and minimal length to ensure they passively flex with the flow without inducing flutter. Their primary function is to align with local flow vectors at the detachment zone, not to oscillate. Think of it as a compliant separator, not a vibrating spoiler.
    
    Myth #4: “Adding anything to a foil increases drag by default.”
    
    Fact:
    
    While adding surface area can increase drag, a strategically placed sharp-edged flap passively reduces overall drag by eliminating larger losses from messy detachment and wake formation. The drag saved by improved separation control outweighs the minor added wetted area.
    
    Myth #5: “A sharp detachment line only helps at low angles of attack.”
    
    Fact:
    
    Ventilated foils often operate at moderate to high angles of attack, where separation is inevitable. The flap’s benefit comes from forcing a clean separation in these high-stress conditions, reducing turbulent wake losses regardless of angle of attack.
    
    
    Concept Diagram in Words: Visualizing the Sharp-Edged Flexible Flap
    
    
    1. Without the Flap — The Messy Detachment Scenario:
    
    • Picture water rushing over the front of a hydrofoil.
      
      
    • At first, the flow clings smoothly to the foil’s surface.
      
      
    • But as it moves aft, it encounters rising pressure (an adverse pressure gradient).
      
      
    • The thin layer of water close to the surface slows down and thickens.
      
      
    • Eventually, it can’t keep up.
      
      The flow detaches chaotically, tumbling into a swirling, turbulent wake.
      
      
    • This messy detachment creates drag, much like water swirling behind a blunt object.
      
      
    • Energy is wasted in large, chaotic eddies that trail behind the foil.
    
    
    2. With the Flap — The Clean Detachment Scenario:
    
    • Now imagine a thin, sharp-edged flap mounted near where separation would happen.
      
      
    • Water still flows smoothly over the front of the foil.
      
      
    • As it crosses the flap, viscous attachment of the flow begins to deflect the flap slightly, accommodating the local momentum.
      
      
    • Instead of “peeling off” unpredictably, the flow maintains its local momentum.
      
      
    • When the flow reaches the sharp railing edge of the flap, it detaches cleanly, with minimal disturbance.
      
      
    • The wake behind the foil is now narrower, more coherent, with less wasted energy.
      
      
    • The foil experiences significantly less drag, even though separation still occurs.
    
    
    3. The Net Effect:
    
    • Same flow physics (separation is inevitable).
      
      
    • But with the flap, the geometry of separation is disciplined and precise.
      
      
    • Result: Smaller wake, lower drag, higher efficiency — all from a simple, passive device.

 

Many words and some numbers, but no validation. References, pictures and test results please!

 

There are also the factors of scale, speed and pressure.

 

I’m just putting this out there so anyone interested can run with it. I’d be curious to hear any results.

Yes its a case of try it and see on this one

 

It can be tested with adhesive tape on the chines and transoms of a planing hull simply by comparing engine revs at a given speed in calm conditions

 

AI has a lot to answer for......

 

I wonder if AI gets seasick