Advanced Aerodynamics and Hydrodynamics for Powerboats

Center-Pod Analysis - Traditional software and analysis techniques often fail to address the interaction of coincident lifting surfaces, such as sponsons and center pod surfaces. While many design and configuration elements affect the lift affectivity of each individual surface, the contribution from each surface also influences the action of all other surfaces. This interdependence of lifting mechanisms is much more complex to model and analyze.

TBDP©/VBDP© solves this balance effectively and accurately. This capability allows the user to simulate overall performance and dynamic stability of the total hull design, and ability to optimize the contributions of each surface.

For example, a center-pod 'height' relative to sponson bottom surfaces can be designed such that lift contributions of the center-pod is significant at lower speeds (fully wetted) and lesser at higher speeds (fully un-wetted).  Or the alternate behavior can be employed, where lift from the center-pod becomes more significant as speed increases.  These changing conditions must be reflected in the performance, loading analysis and dynamic stability analysis for the hull. [see Figure 2a, 2b and Figure 6].

Center Pod design - Many designs of present-day tunnel hulls now sport a center pod, a centrally located third hull or 'pod' [see Figure 1]. This additional hull section gives additional lift and is often located higher (or lower) to the water than the outboard sponsons for diverse lift advantages at early (or later) velocities. Recreational hulls routinely use a center pod feature; even F1 racing hulls sport a style of center pod for their unique needs.

The addition of a center pod to a pure (conventional) tunnel hull design can have these affects:
-Hull can plane earlier
-Better weight support
-Better acceleration
-Improved handling at lower speeds
-Feel bumps more in waves
-Reduced aerodynamic Lift
-Less ultimate top speed
-Reduce porpoising
-Dynamic stability altered

The performance results of the center pod are very different depending on the tunnel hull design, weight, power and operating speed; and on the design of the center pod itself. Accordingly, the design and configuration of a center pod should be quite different, depending on the desired outcome.

Design Features of Center Pod can be configured as:
-Height of Pod bottom surface - can be above sponson bottoms (shallow) or below sponson bottoms (deep) or the same as sponson bottoms.
-Pod bottoms deadrise - can be flat or low deadrise or higher deadrise.
-Pod shape - lengthwise configuration can be 'rectangular' (straight) or 'Delta' shaped (width varies from wider aft to narrower forward).
-Pod length - can extends full length of sponsons or shorter than sponson length or longer than outboard sponsons.
-Pod surface angle-of-attack, α_pod, can be the same as sponson bottoms (pod bottom and sponson bottoms are parallel to each other) or pod angle can be incrementally more than sponson bottoms (e.g.: if pod angle increment, α_incr = +1 degrees, then when hull trim angle, α_trim = 2 degrees then the effective pod angle of attack, α_pod = 3 degrees).

With the extra wetted planing surfaces that come from the added center pod, boats can often accelerate faster, particularly if the pod planing surfaces are of very low deadrise.

Tunnel hulls designed for 'drag racing' or quick acceleration are sometimes designed with a deep center pod (below the sponson bottoms) that can improve low/mid speed acceleration. At top speed a low-deadrise pod is designed to support the full weight of the boat, with sponson bottoms completely unwetted for less drag, and the hull tends to behave like a vee hull at very high speeds. [see Figure 3]

Multi-purpose recreational tunnels employ a center pod to increase weight-carrying capability. The additional wetted planing surface at lower speeds provides more lift, although the consequential added water drag limits the ultimate top speed.

Recreational Modified Tunnel hull designs sometimes employ a center pod, with differing pod design heights.  Some pods are positioned higher than the sponson bottoms that helps weight carrying and improves low-mid speed acceleration but allows for good aerodynamic lift at top end speeds. When operated at its higher design velocity, this center pod is no longer wetted and the hull achieves much (although not 100%) of the benefit of a full tunneled hull [see Figure 4, 2a].  Other recreational tunnels have center pods positioned lower than the sponson bottoms so as to carry increasing loading as speed increases [see Figure 2b].

For hulls where the ultimate top speed a goal, the center pod will reduce the maximum velocity attainable since the pod degrades aerodynamic lift and adds hydrodynamic drag. A tunnel design with a clean, undisturbed tunnel roof surface, and properly sized sponson surfaces, is always going to be more aerodynamically efficient. So a pure tunnel design (without center pod) is usually the ultimate fast machine.

Race hulls often employ a shallow center pod through the use of a 'built-down mid section'. These came into practice with the capsule design and is used primarily to reduce the height of the cockpit section, thereby lowering the CG of the driver and reducing the height of the cockpit or canopy. F1 race tunnel hulls have 'shallow' center pod that is designed to become un-wetted at top speeds. [see Figure 5]. The center structure also strengthens the tunnel span, as these boats can experience 4+Gs in turns and their tunnels spans can be highly stressed through the middle of the tunnel due to wide spans and thin sections between the sponsons. This feature can also improve acceleration through cornering maneuvers and can  improve handling in rough waters.

Center Pod Analysis - lift contributions of a center pod are strongly influenced on the relative 'height' of pod as compared to sponson bottom surfaces.  A pod surface that is 'higher' than sponson surfaces may be wetted at lower velocities and un-wetted at higher velocities. A pod surface that is 'lower' than sponsons may remain fully wetted at higher velocities while sponsons become less loaded with reduced wetted surface. [see Figure 6].

Similarly, pod performance is heavily influenced by the relative pod angle of attack, as compared to the sponson bottom surfaces.  Center pod planing surfaces can be designed with a 'Pod Angle' that is the same as sponson bottom surface OR a 'Pod Angle' that is greater than the sponson bottom surface (egg +2 degrees).  The latter feature can increase Lift generated by the pod surface due to the greater local 'effective trim angle' (hull trim angle + Pod Angle).

Pod surface deadrise can be designed to be different to sponson surface deadrise.  Higher Pod deadrise angle can provide wave-breaking benefits and anti-porpoising benefits, while contributing somewhat less Lift.

The overall performance result of the center pod addition to a tunnel hull design is a complex one because there are so many hydrodynamic and aerodynamic features working together, generating offsetting efficiencies. It is necessary to analyze the complete hull design in order to determine the overall performance comparison of different center pod configurations for a hull's weight, power and operating conditions. Some boats can benefit from a center-pod while others will not. What works successfully for one hull design and conditions may not benefit another design or operating situation at all.

TBDP©/VBDP© software analysis accounts for the use of a center-pod feature (tunnel hull) and also center vee-pad feature (vee hull) and calculates all contributing lifts, drags, force locations, wetted lengths and wetted surface areas, inter-related forces effects, dynamic stability - making performance optimization easy.