Aerodynamic and Hydrodynamic Performance
design for Powerboats

Advanced analysis of the changing hydrodynamic and aerodynamic forces acting on tunnel hulls and vee hulls gives highly accurate dynamic stability, performance predictions & optimization.

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

Background...
The 'Static CG' of a hull is the location of balance of the hull, appendage and payload deadweights while boat is at rest. But this is a small part of the important balance of a performance hull - particularly since the performance boat usually operates at velocities greater than zero!  

The combined center of ALL the lift forces and all the drag forces (sponsons, center-pod, vee surfaces, center-pad, aerodynamic surfaces, lower unit, etc.) while a boat is under way, is called the 'Dynamic Center of Forces' or 'xCFDynamic'. The 'xCFDynamic' location changes throughout the operating velocity range and is the most important design measure to consider when 'balancing' a performance boat.

Some situations that can evolve as triggered by dynamic instablity include…

• Chine Walk- an aggressive, often rapid, side-to-side oscillation of the hull. Acute cases of chine walking grow more severe by themselves.

• Porpoising - the cyclic oscillation of a boat in pitch and heave, of sustained or increasing amplitude, occurring while planing on smooth water.

• Blowover - negative longitudinal instability that results in unbalanced 'bow-up' moment causing the hull to lose vital contact with water surfaces.

• Barrel Roll - lateral instability (sometimes seen as 'chine tripping') often caused by abrupt change in forward direction (momentum change) and higher than desirable CG.

• Nose-Dive ('Stuff') - positive longitudinal instability that results in a 'bow-down' moment causing the hull to nose-dive (bow-first) into the oncoming water.

• Airborne - simple imbalance of normal dynamic forces, e.g.: total lift is greater than static weight, also referred to as 'blow-over'.

Note analysis methods apply EQUALLY to all styles, sizes, configurations of hulls, and for all weights, power and velocities - just the same analysis.

All of the lift & drag (hydrodynamic and aerodynamic), thrust and weight forces on a boat act in different locations - and the location of each of these forces is constantly changing. (This is why you can't effectively "balance" your performance hull while it's still on the trailer). All these different forces at their different locations combine into a net resultant force that acts at the dynamic center of Forces (CGFynamic) and represents the delicate balance of the performance hull.

Ideally, we'd like to have the resultant of these forces acting at the same location as the Static CG, to make the boat dynamically ‘stable’. Since the xCFDynamic location constantly changes throughout the velocity range, this makes the task of ‘dynamic balance‘ of the hull one of optimization. Moving weight around can help the boat's balance in a key velocity range, improving handling and response. The goal is to move weight so that static CG is closest to the xCFDynamic at the most common or most critical hull speeds.

Example Analysis of XCGDynamic & XCGStatic

As an example, let’s consider a small performance hull with total weight 1600bs and a calculated static CofG located at XCGStatic=4.5ft forward of the transom. This means that the weight (1600lbs) of the hull while at rest is centered at the static center of gravity located +4.5ft ahead of the hull transom [see Figure 6 above].

Based on the design and setup of this hull at 20mph, the calculated location of the center of dynamic forces (XCFDynamic) is approximately +7.8ft fore of transom. This is +3.2ft FORE of the XCGStatic location, and represents a 'bow-UP' moment of +5120 lb-ft. If we were going to try to ‘balance’ the hull at just this one velocity (20mph), then we would have to relocate the static CG location (XCGStatic) FOREWARD by…5120/1600 = 3.2ft FOREWARD from where it is now.

Alternatively, the calculated XCGDynamic location at 72mph is approximately +1.7ft fore of the transom. This is -2.8ft AFT of the XCGStatic location and represents a ‘bow-DOWN’ moment of -4480 lb-ft. If we were going to try to ‘balance’ the hull at just this one velocity (80mph), then we would have to relocate the static CG location (XCGStatic) AFTWARD by -4480/1600 = -2.8ft AFTWARD from where it is now.

The important thing to note with such analysis is that the static center of gravity (XCGStatic) of the hull obviously cannot be in two different locations. The important parameter to study for the performance of the hull is the Dynamic CofG (XCFDynamic), and XCFDynamic moves to different locations under all the different operating conditions of the boat. Thus, the hull can, at best, be ‘dynamically balanced’ at only one velocity. (If we were to move the static CofG to balance at 20mph, then the ‘dynamic balance’ at 72mph would be worse).

Many of the design features of a hull can influence the location of the Dynamic CofG (XCFDynamic) throughout the operating velocity range of the hull. The weight of many of the payloads of a hull can also be located so as to alter the Static CofG (XCGStatic). It’s best to try to locate XCGStatic and XCGDynamic as closely together as possible, and so as to minimize any dramatic changes in the shifting of XCGDynamic as it relates to XCGStatic.

An easy way to analyze just how “dramatic” the changes in shifting XCFDynamic is to plot a graph of the derivative of the XCGDynamic data…this is the “rate-of-change” of XCFDynamic…and is shown on the TBDP©/VBDP© graphic displays as the blue line on the chart. When this curve is increasing, this means that XCGDynamic is changing at a faster rate. This can indicate that the hull is becoming more unstable and may be more difficult for the driver to correct.) See more about “rate-of-change” graphic curves in Section 7.6.2 of the TBDP/VBDP manual).

Also considered is the 'direction-of-change' of the XCFDynamic location as velocity changes.  For example, if XCFDynamic is moving aftward as velocity is increasing, this movement generates certain stability characteristics (see Figure 4, above).  This same observation works in the opposite direction when velocity is decreasing. In the example (Figure 4, above), a deceleration or velocity decreasing will cause XCFDynamic to move forward, which may have detrimental handling and stability characteristics.

This example illustrates the “compromise” of the design process for performance powerboat hulls. A hull cannot normally be in ‘dynamic balance’ throughout its operating velocity range. So, the best approach for the designer is to attempt to locate the static CofG (XCGStatic) at a location that helps create the best ‘compromise’ of DYNAMIC BALANCE at most all speeds in the velocity range, paying particular attention to the speed regime that will be most utilized by the operators.