Advanced Aerodynamics and Hydrodynamics for Powerboats

Complex fluid flow modeling specifically for tunnel and vee hulls allows accurate computer modeling of these complex designs that fly in air and on water.  

Important Considerations
Forces acting on performance powerboats must include the influence complex planing surfaces (steps, lifting strakes, centre-pad/pod surfaces, etc.), and of non-planing forces (aerodynamic contributions from low aspect ratio ground effect hull forms, etc.).

While the normalized derivation for lift from hydrodynamic surfaces is...
   Lw = ½ • ρ_W • V² • S_W • CL_W

   where:
   Lw = hydrodynamic (water) lift
   ρ_w = density of water
   V = velocity
   Sw = effective wetted surface area
   CL_W = lift coefficient

The establishment of effective surfaces and related CL_W is necessarily complex.

Hydrodynamic Lift and drag analysis for performance hulls must consider and accomodate for:

• Low Aspect Ratio (AR) planing surfaces (long wetted sponsons) require complex and specific CL_W algorithm development.

• Variable deadrise planing surfaces

• Variable Chine width

• higher AR planing CL_W's (short wetted area of high performance sponsons and reduced wetted surface vee hulls).

• effective planing bWidth

• effective planing Length

• drag determining pressure area, friction and whisker spray drag; interference drag

• Lift strakes, spray rails, trim tabs

• influence of complex/multiple lateral steps in planing surfaces

• center-pod (tunnel hulls) and vee-pad (vee hulls) shared lifting surfaces.

All of the above influencing variables are changing constantly with hull velocity, making the accurate accomodation very complex.  What's more, each of these variables are interdependant (on each other), so the accurate analysis of  CL_W, total Lift and total Drag is tricky.

Also, the complex interdependence of hydrodynamic Lift/Drag on AR, WAngle, Re/Fr, SWet is further complicated by the strong inter-dependence of aerodynamic surfaces Lift/Drag and other appendage drags (cockpit aero drag, cavity drag, engine lower unit drag, etc.) - all affecting CL_W, SWet, AR of hydrodynamic lifting surfaces. 

Russell's development of Advanced Hydrodynamic techniques included direct liaison with Prof. David Savitsky re: planing characteristics, and were ultimately verified through wind tunnel testing, water channel testing and full scale hull verification testing.  

Effective planing width (bWidth) indicates the progression of planing phases as velocity and Lift increases.  Analysis must precisely account for bWidth to accurately represent local Lift forces, Lift center of pressure, friction and spray drag contributions. Effective bWidth and wetted length of each planing surface have significant impacts on Lift and drag forces and locations for dynamic stability moments.

How it Works
The total wetted bottom area of a planing surface can be divided into two regions. One is aft of the spray-root line, commonly referred to as the pressure area, and the other is forward of the spray-root line, referred to as the spray area. The pressure area is the load-carrying area of the planing bottom, generating hydrodynamic Lift (and drag). The spray area contributes drag but is not considered to support any portion of the load.

The forward portion of this lifting area is a triangular shape (simply due to the geometry of the veed hull). Behind this triangle, defined by the "stagnation line" is the lifting area – this is the wetted surface that does the good work of lifting the hull out of the water. Ahead of the stagnation line, there is an area that experiences "whisker spray" from the high velocity water that flows along the hull surface. The "whisker" spray from highly loaded lifting surfaces can wet a significant portion of your hull surface, causing additional drag without the benefit of any lift.

When a performance hull is planing, particularly at higher speeds, the hull gets it's lift from aerodynamic surfaces and the wetted lifting area of the hull bottom. To the extent the 'aero' lift contributions are significant, the hydrodynamic lift requirements (and associated drags) are reduced. Because the cost (in HP) of water-drag can be 800 times more than that of aero-drag, the benefit in wetted surface (and drag) reduction can result in appreciable gains in performance.

Accurate computation of hydrodynamic lift and drag contributions rely on similarly precise analysis of the strong interdependence with contribution aerodynamic surfaces Lift/Drag and other appendage drags including cockpit aero drag, cavity drag, engine lower unit drag, etc. - as all of these dependencies affect the CL_W, SWet, AR of hydrodynamic lifting surfaces and resulting dynamic stability.

The performance effects of friction drag, profile drag, induced drag, sheer spray drag and whisker spray drag generated by all Vee hull, Vee-pad hull, tunnel hull and modified-tunnel hull types of powerboat applications is developed by Russell. The results are accurate representation of hydrodynamic drag associated with all performance powerboat configurations, using Russell's analysis techniques in the "Tunnel Boat Design Program" and "Vee Boat Design Program" software.

Russell applies these advancements in newest versions of AR's TBDP©/VBDP© performance analysis software.