Advanced Aerodynamics and Hydrodynamics for Powerboats

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AR© develops advanced Whisker Spray drag technique for Vee hull and Tunnel hull powerboat performance optimization.
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  BREAKTHROUGH!



Steps can generate required wetted surface with improved Lift and Drag characteristics.


Stepped hulls operate with a larger effective angle of attack due to the inherent step angle 'built-in' to the planing surfaces.


Wetted surfaces of each stepped area have a controlled location of forces, and usually enhanced Lift efficiency due to local 'angle of attack' and higher local aspect ratio.


Multiple Steps can improve performance through greater velocity range.


Stepped Vee hull model.


Stepped hull geometry is complex, and generates better Lift/Drag ratios due to higher Aspect ratio for similar wetted surfaces.


Typical stepped hull wetted surface, water channel proof testing.

 
AR® has developed advanced analysis techniques that accurately calculate the performance effects of single or multiple steps in tunnel hulls of vee hulls.

Making a Step Work
A stepped hull can be visualized as a Vee or flat bottom with longitudinal "offsets".  When functioning, the area immediately aft of each step is void of water - an air or air/water mixture.  At low speeds, the entire hull bottom is in the water, but at higher speeds only a portion of each step is wet, so theoretically we should get less hull drag.  

The stepped hull creates 2 (or more) planing surfaces that helps maintain a constant (trim) angle of attack for more efficient lifting.  The stepped hull has less bottom surface in contact with the water creating less drag and allows for more speed. 

The 'fore' and 'aft' planing surfaces created by the 'step' helps maintain a constant trim angle Stepped hulls have two main advantages...
1) Steps can maintain near optimum angle of attack throughout a wider speed range; and, 
2) Steps can reduce the amount of wetted surface that is not near the leading edge (and would otherwise produce less efficient lift).  

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

Step Design
The design of an effectively performing step will most always achieve optimum 'benefits' (more than the 'losses') at only one velocity.  A step design is generally only good for a single (trim) angle of attack with a single center of gravity (LCofG). This is why it is complicated to find a step design that can "help" the performance throughout the entire speed range of a performance boat.

The planing surfaces of the stepped hulls operate with a larger effective angle of attack due to the inherent 'Step Angle' that is 'built-in' to the planing surfaces. Even though the hull 'Trim' angle is often less, the higher total angle of attack can generate improved CLW (more efficient Lift). The result is a lesser (total) wetted surface requirement and less Drag.

Location of LW (planing surfaces hydrodynamic lift) is normally located further forward in a stepped hull design, as compared to a non-stepped hull. This can sometimes have a desirable effect on XCGDynamic (Dynamic Center of Forces).

Here's How It Works
The hydrodynamic Lift generated by planing surfaces is influenced, in part, by the surface area, aspect ratio, and trim angle of the wetted surface(s).  In the case of a stepped hull design (say 2 steps), there can be 3 separated wetted surfaces.  If effectively designed, these 3 separate lifting surfaces can be more efficient than the 1 single surface that a non-stepped hull could deliver.

The step design imposes a set angle of attack for the planing surface area aft of the step.  This 'step angle' incrementally increases the total 'trim angle' that the planing surfaces see (hull trim angle + step angle = total 'planing trim angle').  Higher trim angle generates more lift.

At the same time, the 'Aspect Ratio' (width/length ratio of the planing surface) for each of the 3 lifting surfaces of the 2-step hull are greater and so contribute to more efficient lift (more lift, less drag) than the shape of a longer, single (non-stepped) planing surface.

Finally, the imposed 'step angle' helps maintain a minimum angle of attack for the surfaces that, if properly designed, can optimize the lifting efficiency (L/D) of each surface.  Any change in trim angle affects each of the 3 planing surfaces - which also changes the wetted surface and AR too, and thus affects the Lift & Drag generated by each of the 3 surfaces.  So the performance analysis of step design is quite complex.  Overall, however, there are benefits of the application of steps to a hull design [TBDP© /VBDP© can do all of the engineering performance analysis to optimize effective multi step design].

Issues with Steps
The most significant issue with step design is the engineering challenge of properly locating an efficient step on the hull.  The length of planing surface behind the step (i.e.: the location of the step) and depth of the step have an impact on the performance of the setup.  To design the step improperly can actually decrease performance.  The issue of multiple steps makes the challenge even trickier. 

Some designers stay away from including steps in designs because of degradation in performance at low and moderate speed range.  At lower speeds the steps are entirely immersed, so each step actually adds drag to the hull.  At moderate speeds during the transition to full planing, air must get back behind the steps or the boat will suffer the penalty of continuing high drag.  So the step is really only ideally functional at its single design velocity, and thus, it can potentially generate a penalty at all other velocities. Use of ventilating steps by design can cause the hull to "trip" on the irregular chines causing a dangerous stability problem with serious handling results.

Complex Step Performance Analysis
Although the geometry of stepped hull lifting surfaces is extrememly complex, the multiple lifting surfaces can generate more efficient Lift and Drag due to higher Aspect Ratio of multiple surfaces for same wetted surface.  Most importantly, the location of Dynamic CG of stepped surfaces is shown to be foreward of non-stepped surfaces, improving Dynamic Stability in key regions of velocity range.  Instead of a single longer non-stepped wetted length, stepped planing surfaces are shorter (thus higher Aspect Ratio for similar planing width, b) separated by aerated (non-wetted) surfaces.  While complex to calculate, these multiple surfaces can be more efficient and can move CGDynamic further forward, improving Dynamic Stability.

Step Height and Step Angle
A properly designed step can help maintain a constant trim angle (WAngle), reducing cyclic changing trim angles under operating conditions. A designer can even design the step height and step angle to optimize the planing trim angle of the planing surface(s). A step angle that is too large can impose a step angle that is too high, causing additional drag and, more importantly, resulting in very low hull trim angles (even WAngle = 0), exposing other surfaces to increased wetting and extra drag under many conditions. Step height should be considered so that step angle does not result in very low operating WAngles. A good guideline is a step angle of less than 1/2 of optimum trim angle (example, if optimum trim angle = 5 degrees, then max step angle could be set to less than 2.5 degrees).

Multiple Steps
There are additional challenges with multiple steps.
1) If the steps are located too close to each other, the water attaching to the second step is "contaminated" by aerated low-density water from the first step, so the aft step does not produce the high lift forces desired. 
2) Where do we locate the center of weight (LCofG) so that the weight is balanced across the steps?  Remember, the running trim angle can change throughout the speed range and this makes a huge difference in the lift-force distribution on your steps.  It takes only a small change in the relative locations of the CofG to change your boat from a stable, efficient boat to one that porpoises at several velocities.


[TBDP©/VBDP© performance design software helps to optimize step design and placement, and provides performance analysis of any hull with or without steps.]
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 [also see Jimboat's article on Step Design]


Research results now included in performance analysis by TBDP©/VBDP©

[more about AR's research     more about AR's publications    and    technical articles/papers]
 
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