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 a tunnel hull 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...
   L_A = ½ • ρ_A • V² • S_A • CL_A
   where:
   L_A = hydrodynamic (water) lift
   ρ_A = density of water
   V   = velocity
   S_A = effective wetted surface area
   CL_A= lift coefficient

An advantage of tunnel hull designs is the entrapment feature of the lower aerofoil surface or 'tunnel' section, through the 'end-plate' provided by outboard sponsons.  Normally higher aspect ratio wings tend to increase net lift because mainly, of the reduced opportunity for the 'escape' of high pressure air around the 'wing tips' to the upper surface.  This 'escape' of air causes what is called a 'wing-tip vortex', reducing lift and increasing drag.  The outboard sponsons on each side of the tunnel section, act as 'wing end plates' that restrict this 'escape' of airflow, enhancing aerodynamic performance.  

The establishment of effective surfaces and related CL_A is necessarily complex. 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  CLA, total Lift and total Drag is tricky.

Aerodynamic Drag
The total drag created by the hull must be overcome by the available thrust - namely the engine/propeller.  The drag of a tunnel hull is made up of both aerodynamic or 'air' drag (drag from the tunnel/deck surfaces, appendage air drags such as driver, cockpit area, motor, etc.) and hydrodynamic or 'water drag' (from the planing sponsons and motor appendages under water). 

As with tunnel lift, the air drag increases primarily with the square of the velocity and as angle-of-attack increases. 

This air drag originates in three (3) forms:
a) Skin Friction
b) Induced Drag
c) Profile Drag

Also, the aerodynamic lift/drag is strongly influenced by the complex interdependence of hydrodynamic Lift/Drag and other appendage drags (cockpit aero drag, cavity drag, engine lower unit drag, etc.) - all affecting CL_A, S_A and L_A of aerodynamic lifting surfaces. 

[The analysis methods in 'Secrets of Tunnel Boat Design' (ISBN# 1-894933-30-3) and AeroMarine Research TBDP©/VBDP© software demonstrate the development of proven algorithms that solve all of these challenges accurately.]

Sponson Sides Enhance Aero Lift
A wing derives its lift by the difference between a high pressure on the underside of the wing compared to a lower pressure on the topside. This difference in pressure results in an upward force. Some airplanes attain improved lift by adding ‘wing-tip-ends’ or ‘winglets’ that prevent airflow from escaping around the end of the wing, causing ‘wing tip vortices’ and reduced lift/drag efficiency. 

An advantage of tunnel hull designs is the entrapment feature of the lower aerofoil surface or 'tunnel' section, through the ‘built-in’ wing-tip ends formed by the sponsons on either side of the tunnel section.  This significantly increases the efficiency of tunnel lift with more lift and less drag

Trim Angle vs Aero Angle
Some tunnel boats are designed so that the 'wing' portion of the hull will operate at an incrementally higher angle of attack (aero angle, αaero) compared to the hull 'trim angle' (αtrim).  This feature is quite common in many tunnel hulls and power catamaran designs, particularly weight and power sensitive setups.  The αaero feature is easily identifiable by observing the tunnel height at the bow of the boat at a higher dimension than the height of tunnel at the transom.  The higher angle of the 'wing' hull section allows for better seaworthy behavior in heavier waves and also achieves more aerodynamic lift.  So, when the hull trim angle (αtrim) is say, 3°, the 'wing' experiences a local angle of attack (αaero) of 5°. This feature generates higher aerodynamic Lift (and also higher Drag), and is often more beneficial to overall performance.

Russell's development of Advanced aerodynamic techniques were proven through wind tunnel testing, water channel testing and full scale hull verification testing.  

How it Works
Any performance hull will perform better when taking best advantage of 'aerodynamic' lift.  Any amount of 'aero lift' will improve a boats performance − even a seemingly small amount.  

If we compare 2 boats each weighing 1500 pounds, that means 1500 pounds of total lift must be generated by the hull.  At say, 50 mph, the first boat with no aerodynamic lift capability requires all of its lift to be supplied from an area of wetted planing surface.  If the second boat can contribute (even only) 100 pounds of aerodynamic lift (that's only 6% of the total lift required) then only 1400 pounds of water-lift remain to be generated, which requires less wetted surface area and a corresponding reduction in hydrodynamic (water) drag. 

Less drag means more efficiency and better performance.  In this case, the reduced wetted surface of the second boat results in a 9% reduction in water drag − just like gaining 10 to 15 hp and much improved fuel economy, or an additional 5 mph!

The performance effects of all design and setup features that influence aerodynamic lifts, drags and dynamic stabiltiy for all Vee hull, Vee-pad hull, tunnel hull and modified-tunnel hull types of powerboat applications is developed by Russell. The results are highly accurate representations of aerodynamic forces and inter-dependence with hydrodynamic forces associated with all 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.