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AR© advanced Dynamic Stability analysis for Vee hull and Tunnel hull performance optimization.
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BREAKTHROUGH!
Aerodynamic and Hydrodynamic forces
Figure 1 – Aerodynamic and Hydrodynamic forces combine to ‘balance’ the hull – and it’s a different balance at every velocity.




Figure 2 – Tunnel hulls see a unique balance between aerodynamic Lift generated by the deck and or tunnel configuration, and the hydrodynamic Lift generated by the sponson surfaces.




Figure 3 – Vee-pad hulls transition to significant Lift generated by highly efficient aftward “pad” at higher velocities. 

Figure 4 – Vee-pad hulls transition to significant Lift generated by highly efficient aftward “pad” at higher velocities.

Figure 5 – An airplane is 'inherently stable', since a slight raising of the nose results in a self-correcting nose-down moment. 


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 'Dynamic CG'. The 'Dynamic CG' location changes throughout the operating velocity range and is the most important design measure to consider when 'balancing' a performance boat.

How it Works...
Lift = Weight = Performance - All boats must generate enough lift to balance the weight of the hull. Performance tunnel and vee hulls generate lift by hydrodynamic 'planing' surfaces and aerodynamic surfaces. As a boat goes faster it needs less wetted surface to generate it's required Lift – but that Lift is always equal to the weight of the boat – and the Lift always comes with a certain amount of drag.


Tunnel hulls see a unique balance between aerodynamic Lift generated by the deck and or tunnel configuration, and the hydrodynamic Lift generated by the sponson surfaces on the water.

Vee hulls (and Vee-Pad hulls) gain lift from the balance of lift from planing vee surfaces and/or center-pad surfaces and also from aerodynamic surfaces.

Tunnel hulls...
As the hull velocity changes, so does the source and location of lifts and drags. For example, a typical high-performance sport tunnel hull will, at lower speeds, see most of its Lift generated by a length of planing sponson pads, with the center of Lift located well forward. As speed increases, aerodynamic lift generated by the deck and tunnel configuration becomes more significant, allowing the sponson wetted surface area to decrease to only the aft-most parts of the sponson pads. The hull is now supported by the hydrodynamic Lift from a small aftward surface of the sponsons, combined with the aerodynamic Lift from deck/tunnel surfaces - and the overall center of Lift now located well to the aft of the boat.

Dynamic CG and the Hump Zone... The “hump zone” for a tunnel hull represents the speed at which a more significant amount of Lift changes from sponson lift to aerodynamic lift. Vee hulls see a similar "hump zone" transition when a more significant amount of Lift changes from vee surfaces lift to aerodynamic lift and/or vee-pad lift. This "hump" or "transition zone" occurs at a different velocity with each boat and setup.

The change in location of the center of Lift (with increasing velocity) is often quite dramatic, moving from initially, well forward of the hull’s center of gravity, to later, well aft of the CG – and causes the onset of potential instabilities - like Porpoising or chinewalking.

Inherent Instability...
Any vehicle in 'flight', such as an airplane or a high performance powerboat, will experience minor changes to the forces that act on it, and to its speed. If such a change tends to restore the vehicle to its original speed and orientation, then the vehicle is said to be “inherently stable”.  If such a change tends to drive the vehicle away from its original speed and orientation, the vehicle is said to be “inherently unstable”.

An airplane is 'inherently stable', since a slight raising of the nose results in a self-correcting nose-down moment [see Figure 5].  Performance powerboat hulls are all 'inherently unstable" ‑ that is, a slight raising of the bow at high speed will usually result in a bigger raising of the bow, and soon, the boat can become unmanagable.

This isn't necessarily a big problem for most boats.  Consider that we could try to balance a pencil on the end of our finger.  It's possible to get it to balance - it's just that any small disturbance is likely to upset the pencil, causing it to fall.  This setup is 'inherently unstable'.

We can always balance all of the acting forces on a boat so that total Lift balances total Weight. The boat may still be 'inherently' unstable, but we can design a performance hull to optimize it's dynamic stabilitly and minimize adverse reactions to small disturbances (such as waves, wind gusts, etc.)  

Vee Pad hulls...
The similar transition of the Vee-Pad hull to  “running on the pad” can also cause a dynamic instability. Here, the hump zone represents the speed at which the amount of Lift from the highly efficient “pad” section of the hull (aftward, center located, flat planing surface) becomes significant compared to the Lift generated by the veed (full length, higher deadrise) portion of the hull. For example, a typical high-performance Vee-Pad hull will, at lower speeds, see most of its Lift generated by the higher deadrise vee section through the length of the hull, with the center of Lift located well forward. As speed increases, the more efficient Lift generated by the flat, centrally located pad becomes more important, with the overall center of Lift shifting to the aft of the boat. This change in location of the center of Lift is often quite dramatic, moving from initially, well forward of the hull’s center of gravity, to later, well aft of the CG. This change can initiate dynamic instabilities.


What to do...

The transition velocity can be accurately determined (by engineering analysis) for any hull design and setup. The transition can be altered and the best arrangement can be determined to "smooth out" the “hump zone” by optimizing hull design and setup characteristics.

Detailed analysis of a hull’s dynamic stability behavior at the hull-design stage is the best way to ensure that the transition through this speed range is dynamically stable, safe and comfortable for the passengers. But it is sometimes difficult for the designer to consider all the various power, payload and setup scenarios that an individual hull design might have to perform to. Consequently, all such boats, will exhibit some kind of a “hump zone” – some more noticeable than others.


Figure 6 – The important 'Dynamic CG' location changes dramatically throughout the operating velocity range of the hull; while the 'Static CG' of boat remains in the same location.
Weight Distribution Matters...
The distribution of weight in your boat makes a difference to performance. Proper weight adjustments can improve or correct handling issues such as porpoising, chine walk and lower unit blowout. This is important for lateral (side-to-side) distribution.

Dynamic Balance is key...
The same goes for the fore/aft static balance, although it's not as easy to know what is just "right" when the boat is at rest. You can't balance your boat on the trailer. 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 gravity (CGDynamic) 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 CGDynamic 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 Dynamic CG 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 (XCGDynamic) 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 = -2.8 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 (XCGDynamic), and XCGDynamic 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 (XCGDynamic) 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 XCGDynamic is to plot a graph of the derivative of the XCGDynamic data…this is the “rate-of-change” of XCGDynamic…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).

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.


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

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