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AR© develops advanced Step design analysis technique for Vee hull and Tunnel hull powerboat performance optimization.
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Stepped hull geometry is complex, and generates better Lift/Drag ratios due to higher Aspect ratio for similar wetted surfaces.

 
For Vee hulls (left) and Tunnel hulls (right), 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.

Step Dimensions

Dimensions of Step configuration are referenced relative to the 'Water surface'.


Multiple Steps can improve performance through greater velocity range.


Stepped Vee hull model.

step design pressure distributions
Simulation - Stepped vee hull pressure distribution


Typical stepped hull wetted surface, water channel proof testing.


'Steps Analysis' report [excerpt TBDP/VBDP performance analysis report, pg 1 of 4pgs].

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

Complex Analysis and Results
TBDP©/VBDP© software provides a step analysis through the full operating velocity range, including trim angle, dynamic stability, changing wetted lengths for each step(s) section, calc'd velocity when each step becomes 'unwetted', location of lift/pressure forces for each stepped planing surface, total hydrodynamic lift force map, step angles, even effects on porpoising. Comparative/optimizing analysis is simple provided by graphically comparing results of different design alternatives.

Making a Step Work
A stepped hull can be visualized as a flat or deadrise 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 potential speed. 

The 'fore' and 'aft' planing surfaces created by the 'step' have two main advantages...
1) Steps can maintain a constant and 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.  Your design inputs are pretty simple, but the analysis is complex and the performance results are outstanding!

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 XCFDynamic (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 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].

Contributors to Step Design/Performance
Hydrodynamic forces and effective lifting surface shapes, areas and attack angles at each individual step surface must be calculated, based on key contributors and local conditions throughout the entire operating velocity range: 

  • Step Length
  • Step Height
  • Step Angle (Τ)
  • Surface Deadrise angle (β) at local step
  • Trim Angle (α) at local step
  • Planing surface configuration forward of step

Step Performance can be determined based on:

  • Lift force at each local step
  • Drag at each local step
  • Whisker spray drag at each local step
  • Unwetted void length aft of forward step
  • Wetted width (bwidth) at local step
  • Wetted Length (LWetStep)
  • Centre of Pressure of local step lifting force
  • Hull trim angleh)

Step forces can influence hull performance as:

  • Total Wetted suface
  • Total Wetted length
  • Centre of Dynamic Lift wrt CG
  • Effective trim angle (αe)
  • Porpoising onset velocity
  • Effective CL, CD, CP
  • Effective Aspect Ratio (ARstep)
  • Dynamic Stability
  • Dynamic Centre of Forces (XCFDynamic)

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.  Designing the step improperly can actually decrease performance. The issue of multiple steps makes the challenge even trickier.  So, it is critical to have a defined understanding of how much effective lift that a stepped planing surface will generate and at what (longitudinal) location each stepped planing surface will be acting.

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, (bplaning) 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.

As trim angle changes, each stepped planing surface can change it's wetted length and thus, the lift force generated.  It is possible (frequent) that a forward stepped planing surface can become fully un-wetted at a specified velocity and particular trim angle.  This scenario can significantly change the magnitudes and longitudinal location of hydrodynamic lift forces, and thus Dynamic Stability characteristics of the hull. Step Loading (lift force/load at each lifting step location) is key to understand and balance with other active forces, for dynamic balance throughout velocity range.

Step performance is influenced by the wetted length of the aft-step surfaces.  This length is influenced by many factors, including hull trim angle (αh), effective (step) trim angle (αe), and the reattachment point of water flow as it flows from forward step to aft-step surfaces.  All of these characteristics must be accurately accounted for when determining the performance and dynamic stability of step designs.

Variable deadrise hull bottoms sould be carefully considered when employed with steps.  The local deadrise angle (β) at step locations influences lift coeficient and thus local lift and step loading balance.  Flatter sections of a stepped hull can overpower the deeper forward sections, and force the bow down at speed. It's preferable to keep local deadrise at multiple steps similar to avoid step loading balance and dynamic stability issues.

TBDP©/VBDP© provides a step analysis report for the full operating velocity range that includes the trim angle, dynamic stablity, changing wetted lengths for each step(s) section - even highlighting the velocity at which each of your steps become fully 'unwetted' (or not). 

Location of lift forces from each stepped planing surface are calculated and used to generate total hydrodynamic lift force map for performance analysis including dynamic stabliity.  This analysis allows us to show when your steps are effective and at what velocity the steps become most effective.  Comparative/optimizing analysis is provided by changing step designs and graphically comparing results of different design alternatives.
'Steps Performance Analysis' including step wetted length, dynamic stability, and porpoise analysis graphs [excerpt TBDP/VBDP, comparison of two alternative step designs; 3 of  graphs]

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. Step Height and Step Length set the Step Angle, for each step design.  A designer can design the step height and step angle to optimize the planing trim angle of the planing surface(s). A step height 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 reasonable guideline is a step angle of less than 1/2 of optimum trim angle (example, if optimum trim angle = 4 degrees, then max step angle could be set to less than 2 degrees). TBDP©/VBDP© performance Reports provide analysis of step design, reporting for Optimum Step Height(s) and Optimum Step Angle(s), and recommendations for Step location, step height and step angles, as well as leading planing surface design (foreward of step surfaces).

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]


All above research results included in performance analysis software by TBDP©/VBDP©

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

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