Aerodynamic and Hydrodynamic Performance
design for Powerboats

• Lift force (loading) 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 angle (α_h)

Step forces can influence hull performance as:

• Total Wetted surface
• Total Wetted length
• Centre of Dynamic Lift wrt CG
• Effective trim angle (α_e)
• Porpoising onset velocity
• Effective Step(s) C_L, C_D, C_P
• Effective Aspect Ratio (AR_step)
• Unwetted step surface(s) Velocity (V_LStep=0)
• Dynamic Stability
• Dynamic Centre of Forces (XCF_Dynamic)

[Note: All of these influencing dimensions, forces and factors are calculated by TBDP©/VBDP© software for complete performance analysis of 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.  Designing the step incorrectly 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 extremely 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 forward 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_planing) separated by aerated (non-wetted) surfaces.  While complex to calculate, these multiple surfaces can be more efficient and can move XCFDynamic 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.  The load carried by each step changes dramatically through velocity range and hull trim angle changes, so performance and stability can also change considerably.

Step performance is influenced by the wetted length of the aft-step surfaces.  This length is influenced by many factors, including hull trim angle (α_h), effective (step) trim angle (α_e), 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 should be carefully considered when employed with steps.  The local deadrise angle (β) at step locations influences lift coefficient 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 stability, 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 stability.  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).

Step Height that is too low can sacrifice potential efficiency of the step, and in some cases, can even cause additional drag with degraded performance.  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 high Step Angle (Step Height) can generate higher lift resulting in very low Trim Angle (WAngle=0), which may cause lack of responsive trim control (you can't trim down).

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 (forward 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.].