The Vee-bottom hull is the most common design in modern performance powerboats. The Vee design has a number of variations and near-endless modifications – including the pad-vee design - that can enhance particular handling characteristics of the boat. If going fast is the objective, it’s worthwhile knowing just how a vee pad can affect vee hull performance.
I’ve had many inquiries about vee hull modification, vee-pad hulls and pad modifications. High performance vee pad hulls must be carefully designed for each boat, as the balance of lift from the pad is crucial. The good news is that there are ways that we can engineer our way to the best performing vee hull or vee-pad design or modification.
Vee hulls have different variations of “vee” or “deadrise”. Deadrise is the measurement of the angle between the bottom of a boat (keel) and its widest beam of planing surface. A hull with a zero degree deadrise has a flat bottom; a deep Vee hull might have a deadrise of more than 20 degrees.
The deadrise angle of a Vee hull’s lifting surfaces can have a big influence on the performance of the boat. Like all aspects of powerboat performance, the design decisions are usually a compromise.
A shallow vee design (low deadrise) will generally produce more hull Lift, and consequently, a corresponding increase in ultimate boat speed. The shallow Vee, however, is generally less desirable for running in heavy waves as it tends to cause the boat to "skip" or “bounce” across the waves. This causes a rougher ride, and in extreme cases, can result in a loss of control.
The deeper "Vee-bottom” (high deadrise) boat is the most common of present-day hull designs - most manufacturers of performance boats built today use modifications of this design. Deeper Vees offer a good ride in rough water and softens some of the pounding, rough ride in heavy waves.
Vee-Pad Hull - Some "Vee"-bottom boats have a flat surface at the very bottom called a "pad." This pad allows a much more efficient planing surface aft, with an increase in top speed. There is some corresponding sacrifice of a little softness in the ride, but with this modified vee-type hull design, super-fast speeds are achievable! Alternatively, the use of the variable deadrise vee bottom, where the most central (keel) section of the vee is nearly “flat” functions similarly to a “pad”, generating high lift when planing at higher-speed performance conditions. Vee hull designs for high performance applications are using this design approach more frequently, where the variable deadrise bottom is configured so that the last part of the hull functions as a high-lift (planing) pad.
Hull designers have used the idea of a flatter planing pad with rounded chine hulls or hard-chined vee design hulls since the early 1900’s. During the past 30 years, however, the use of the pad has become the “secret” to high performance. Paul Allison and son Darris (Allison boats) pioneered pad-style vee hulls in the early 1960’s, and hold patents for their concepts of the performance enhancing vee-pad. (The original Allison test involved a fashioned water-ski on the bottom of a deep Vee plug).
Howard Pipkorn, founder of Hydrostream performance boats, patented a type of pad-vee design that has typified super-fast vee-hull configurations for the last 3 decades. Both Pipkorn and Allison have used several variations of their pad designs in construction of their hulls, which have become known as some of the fastest vee-bottom hull designs in the world.
Pad Design
The reason for the pad-vee design of hull and its relatively recent proliferation is that it really works!
The pad is a relatively flat (or perfectly flat) planing surface configured to the aft-most section of a vee shaped hull. The pad (low deadrise center-section) usually extends sufficiently forward so that the transition from the vee to the flat running surface pad is gradual, and usually exhibits some deadrise in the forward section of the extension.
This smooths out the pounding through heavier waves, and helps to more gradually transfer the lifting load to the pad as the hull accelerates. Some high-performance pad-vee designs, however, extend the flat pad virtually to the front of the hull bottom with the intention of initiating planing on the flat pad surface as early as possible – and foregoing the ‘problem’ of a rougher ride in heavy water conditions.
The pad has several performance advantages:
Balancing Act - the high-performance vee-bottom can be a real challenge to drive at high speed. Particularly deeper vees (15° to 20°+ deadrise) must be balanced on a thin keel edge, often exhibiting an unsettling lateral instability, as it “rocks” from side to side. Close driver attention is necessary to keep the deep vee hull “balanced”.
The pad provides a somewhat wider platform on which the hull will ride – making it easier to balance at higher speeds. Admittedly, some drivers (and passengers) will argue that the “balancing act” with a pad-vee hull can generate an even more dramatic ride – particularly at speeds around the transition from vee hull surfaces to riding solely on the balanced pad. While the hull can ride smoothly while evenly balanced on the flat pad, when the hull does “fall off” the pad, rocking to one side or the other side, the effect is more dramatic, to be sure.
All in all, however, the “pad-walk” is usually infrequent, whereas a vee-hull must constantly “waver” from side to side.
High Lift - The flat pad generates much more efficient Lift than the veed bottom shape. The theory of hydrodynamics dictates that, in general, a steeper angle of vee (for example 20 degrees) or “deadrise” creates less Lift than a shallow angle of vee (say, 10 degrees). The extreme case is the completely flat pad that has a zero (0 degrees) deadrise. This configuration creates very high Lift for its small wetted surface area. The result of this “extra Lift” is a dramatically reduced hydrodynamic Drag. Less Drag means more speed!
The hull planing surfaces must always make enough Lift to support the complete weight of the boat. During acceleration mode, the vee-pad hull gets Lift from the vee-hull sections as well as the flat pad section. It needs both of these lifting surfaces, at lower velocities, to Lift the weight of the hull. As speed increases, so does the Lift, and the amount of wetted surface required to “balance” the weight of the boat, is reduced (more Lift = less wetted surface = less Drag). As the speed increases more and more, the required Lift is generated more by the flat pad, and less by the vee surfaces. The pad takes on more of the Lift, and the vee surfaces become “unwetted”. Eventually, the hull reaches a velocity where the highly efficient “pad” alone (if it’s properly designed) can generate sufficient Lift to support the hull’s entire weight. Now there is MUCH less Drag, and the resulting speed increase is quite noticeable. Experienced pad-vee drivers will recognize the “pop” that occurs when the hull reaches that special velocity where the hull “breaks” away from the veed lifting surfaces and rides on the pad alone.
Less Trim - Because the pad is a more efficient lifting surface, the angle of attack required to generate weight-balancing Lift is less than it would be if the Lift were generated by a higher deadrise vee hull surface. This lower angle of attack makes the setup and operation of the boat more stable. When the “pop” occurs (transition from vee surfaces to pad running surface only), some hulls will noticeably “nose-down” to a lower angle of attack, due to the more efficient Lift generated by the pad.
Setup is important – particularly weight distribution, because the boat must balance on the pad. A well-designed hull can be possible to balance an improperly loaded boat to some degree, but since we have to balance the hull on only a narrow pad at high speed, there will always be some tendency for the hull to “fall off” to the unbalanced (heavier) side of the boat. So, it is important to do everything you can, to achieve an equalized weight distribution from side-to-side. Fuel tank, oil tank and battery can usually be located to help balance the normal running setup. Even propeller torque can cause a slight imbalance to one side at some speeds. Some boats have port and starboard gas tanks that can be filled or leveled as needed to offset an improper balance. Balance of passenger(s) and other payload will also help the stability of the hull at high speed.
Less Drag = More Speed - All the Lift of the hull must counterbalance the total weight of the hull. Think of it this way…not enough Lift and the boat sinks – too much and the boat flies! So designing the vee section and pad for just the right amount of Lift is pretty important. This Lift is created by the forces generated by the wetted surfaces (hull bottom) planing on the water surface at some trim angle or “angle of attack” (angle of the planing surface to the water surface). But with that Lift, comes some Drag - and that Drag must be offset by the thrust of the engine – horsepower. So more Drag means more horsepower required to achieve a required speed.
So, the engineering of the “balance” is simply…
1) Lift = Weight
2) Drag = Thrust
Some lifting configurations are more efficient than others. A measure of “efficiency” often used to compare different designs, is that of Lift/Drag (L/D) ratio. That is, the amount of Lift (good) created, compared to the amount of Drag (bad) that comes along with it, is called the L/D ratio. A low-deadrise flat surface is a more efficient Lift generator than a higher deadrise vee surface. This means that at given angle of attack, the low-deadrise pad could generate MORE Lift, and will be penalized by LESS Drag – so it has a higher L/D ratio.
Here’s How it Works - If we consider the example of a high-performance deep vee hull, we will see how the addition of a pad can increase speed.
A 1700lb (total weight) hull with a 20-degree deadrise vee hull design could achieve 90 mph at an angle of attack of approximately 2 degrees. In order to achieve this “balance”, the lifting force of the wetted planing surfaces would have to generate 1700 lbs of Lift. Along with that Lift, comes some Drag – approximately 1135lbs, in this case. And remember, that all the Drag must be overcome by the engine horsepower – so more Drag means more power! The efficiency of the design can be measured as the “Lift/Drag ratio” – in this case, an L/D = 1.5.
Alternatively, a 1700lb hull with a 12” wide flat (or very low deadrise) lengthwise pad, could achieve the same 90 mph, at that same 2 degrees angle of attack, but will generate less Drag because the efficiency of the design has a L/D = 1.9. The more efficient Lift generated, results in less Drag of the pad design that would need to be overcome by the engine thrust. And this reduced Drag represents, in our case, nearly 60 hp! So, our design can achieve the same speed for less power, less fuel consumption.
OR…alternatively, as most speed-hungry powerboat enthusiasts would consider, we could take advantage of our full power capability, and turn that efficiency into more speed! In this case, the same pad design could achieve an increased speed of 7-10 mph more, to 98 mph, at a slightly lower (and safer) angle of attack. An all ‘round winner!
Design the Vee hull and Pad - There is much that can be done to optimize the pad design. Marine engineers and performance hull designers will consider the hydrodynamic Lift & Drag of the running surfaces, as well as the aerodynamic Lift & Drag of the hull design, and optimize all of the design with the weights and power available at each stage of the operating velocity range desired. A pad design from one boat won’t have the same results on another hull design or application.
Porpoising and Dynamic stability are also affected by the relationship of all of these forces working together at the various speeds throughout the velocity range. So there is much compromise to the design process. Each vee hull shape/configuration, hull weight, expected payload, power used, and intended water conditions need to be considered to get the proper pad size, shape and dimensions custom-tuned for the best performance of the hull. (The Vee Boat Design software helps us consider all these factors to maximize vee-pad hull performance).
When a high-speed vee hull design is the objective, though, a well-designed vee-pad will usually result in more speed!
About AeroMarine Research:
Jim Russell is a professional engineer with a mechanical and aeronautics background. Currently living in Canada, he has done extensive aerodynamic research at University of Michigan, OH and University of Toronto, Canada and marine research at the NRC water channel laboratory in Ottawa, Canada. His published papers are highly acclaimed, and are specifically related to the aerodynamics and hydrodynamics of high performance catamarans, tunnel boats, vee hulls and vee-pad performance hulls. Russell has designed and built many tunnel boats. As a long-time professional race driver, he piloted tunnel boats to Canadian and North American championships. He has written power boating articles for many worldwide performance magazines and has covered UIM and APBA powerboat races. He has also appeared on Speed Channel’s ‘Powerboat Television Show’ speaking on tunnel boat design. Russell is the author of the "Secrets of Tunnel Boat Design" book, "The Wing in Ground Effect – Their relation to Powerboats©”, book, and the "Secrets of Propeller Design” His company has designed and published the well-known powerboat design software, "Tunnel Boat Design Program© " and "Vee Boat Design©" software specifically for the design and performance analysis of tunnel boats, powered catamarans, vee hulls and vee-pad hulls.
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