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The inherent design of the tunnel hull or power catamaran makes this type of boat capable of outstanding speed and performance. Some performance enthusiasts even feel that tunnel boats are the ‘fastest’ hull designs compared to other hull types. It’s interesting to look at the design characteristics that give the tunnel hull design it’s ability to ‘fly’ and to achieve great speeds.
How is a tunnel boat able to go so fast?
There are several design features that give a tunnel hull design an advantage in the way it converts it’s available power to speed. Tunnel boats and power cats gain added LIFT because they have a "wing" or “aerofoil” built-in to their design. Their inherent design gives them some performance opportunities that other hull designs don’t have.
Every boat must generate exactly enough lift to overcome its weight. Not enough lift and the boat sinks – too much lift and the boat takes off like an airplane. This total lift can be produced from hydrodynamic (water) lift or aerodynamic (air) lift, or both.
Also, there is only a certain amount of horsepower available with your boat setup, and this available power is converted to thrust and used to overcome all the drag created by the hull.
| The design elements of a tunnel hull |
The aerofoil portion of a tunnel boat creates lift in the same way that the wing of an airplane generates lift. Aerodynamic forces are created by the “wing” that is formed by the deck surface and tunnel roof (between the sponsons) design. This extra lift helps support the total weight of the boat. The hydrodynamic lift created by its planing sponsons provides the rest of the lift needed.
Less is More!
With any kind of LIFT there comes some DRAG - it is an inescapable law of energy. Both hydrodynamic (water) lift and aerodynamic (air) lift come burdened with a certain amount of drag generated. The cost (in horsepower) of water-drag is 800 times more than that of air-drag, so tunnel hull designers can optimize the use of aerodynamic drag so as to ultimately reduce the need for water-lift and its associated drag.
If we can generate ANY lift by virtue of aerodynamics, it will be just that much LESS lift that we have to generate by our ‘water-lifting surfaces’. Every pound of lift that can be generated by aerodynamics is one less pound of lift that doesn't have to be supplied by the water-lifting surfaces. The drag generated by a lifting surface in water is MUCH more than the drag generated by a lifting surface in air.
| The tunnel hull design of wing thickness, surface area, angle of attack and aerofoil shape all work together to optimize aerodynamic lift for the boat. |
Any performance hull will perform better when taking best advantage of "aerodynamic" lift. Any amount of 'aero lift' will improve a boats performance - even a seemingly small amount.
If we compare two boats each weighing 1500 pounds, that means 1500 pounds of total lift must be generated by the hull. At say, 50 mph, the first boat with no aerodynamic lift capability requires all of its lift to be supplied from an area of wetted planing surface. If the second boat can contribute (even only) 100 pounds of aerodynamic lift (that's only 6% of the total lift required) then only 1400 pounds of water-lift remain to be generated, which requires less wetted surface area and a reduction in hydrodynamic (water) drag. Less drag means more efficiency and better performance. In this case, the reduced wetted surface of the second boat results in a 9% reduction in water drag - just like gaining 10 to 15 hp and much improved fuel economy, or an additional 5 mph!
Some tunnel hulls generate 30-50% of their total lift from aerodynamic design.
| A tunnel hull's aerofoil generates lift in the same manner as wings on an airplane. |
Aero Lift Is Complex
You may have heard different explanations of how a tunnel hull’s aerodynamic miracle works, sometimes using odd terminology. Sometimes colloquial descriptions such as a hull design that is “packing air” or has “higher compression” can be misleading or confusing, so let’s clarify what’s really happening.
We’ve done a good deal of research and performance testing to understand the factors that can affect the performance and stability of power cats of all sizes and shapes. In particular, significant research has focused on “low aspect ratio aerofoils in close-proximity ground effect” – just like tunnel hull designs. Here’s some of our findings...
Tunnel Height is the height of the tunnel section formed between the sponsons. This is a major contributor to the efficiency of lift generated by a tunnel hull design.
The aerofoil of a tunnel hull (formed by the deck surfaces and the tunnel roof surface) is really a “wing”, flying in what is called “close-proximity ground-effect.” This means that the aerofoil is actually influenced by its proximity to, in our case, the water surface. We have done extensive wind tunnel and water channel research modeling the effects of tunnel hulled performance boats. A smaller tunnel height (closer to the water) increases the Lift/Drag ratio of the tunnel hull "wing", improving lift characteristics significantly.
Sponson Sides Enhance Aero Lift
A wing derives its lift by the difference between a high pressure on the underside of the wing compared to a lower pressure on the topside. This difference in pressure results in an upward force. Some airplanes attain improved lift by adding ‘wing-tip-ends’ or ‘winglets’ that prevent airflow from escaping around the end of the wing, causing ‘wing tip vortices’ and reduced lift/drag efficiency.
The configuration of a tunnel hull has ‘built-in’ wing-tip ends formed by the sponsons on either side of the tunnel section. This dramatically increases the efficiency of tunnel lift with more lift and less drag.
| Lift-generating airflow below and above the tunnel hull's aerofoil design. |
Tunnel Width/Length
‘Aspect Ratio’ refers to the relative width of the ‘wing’ compared to its length (wider is better). A higher aspect ratio (all other things being equal) will give us more lift. Glider planes have very ‘wide’ wings because their ‘high aspect ratio’ generates much more efficient lift (more lift for less drag). The ‘aerofoil section’ of a tunnel hull is a comparatively ‘lower-aspect ratio’ wing, but research shows that ‘more is better’ and these hulls take full advantage of enhanced aerodynamic lift. In a tunnel boat, a wider tunnel dimension or ‘aerofoil’ section is more efficient for making aero LIFT, as compared to a narrow tunnel width.
Other Lift influencing factors
There are other features of the tunnel hull design that can have an influence on the amount and efficiency of LIFT generated by the tunnel configuration. Wing thickness, surface area, wing angle of attack, aerofoil shape all work together with the other design features to optimize aerodynamic lift for the boat.
Cats have Two Keels
The tunnel boat configuration gives it two keels (one on each sponson), improving handling and maintaining a more level ‘bank’ during maneuvering. This ultimately can dramatically increase potential cornering speeds (if this is important to you).
Low Deadrise Planing Surfaces
There is another characteristic of many tunnel hull designs that contributes to an ability to go fast. Sponsons on many tunnel boat designs often have flatter, low deadrise (10-15 degrees) bottom surfaces that provide very efficient hydrodynamic lift. Vee hulls typically have deeper deadrise (18-22 degrees) planing surfaces that have advantages of softer ride and good performance in heavier waves, however this higher deadrise planing surface is less efficient and can limit top speed. Some vee hull designs use a low-deadrise ‘Vee-Pad’ that contributes very efficient lift for higher speeds. Lower deadrise planing surfaces generate more lift, less drag, and faster speeds.
| Tunnel boats typically have low-deadrise bottom surfaces, providing highly efficient lift |
Vee Hulls Can Have Aero Lift too
On vee hulls, a well-designed deck and forward hull surface can also produce aerodynamic lift. For these vee hulls, more aero LIFT contributed means there is less total DRAG for the engine to overcome – and more power is available to go faster (or operate more efficiently).
The aerodynamic forces generated with tunnel hulls or catamarans are even more significant, however, with lift generated by the “wing in ground effect” that is formed by the deck surfaces and tunnel design.
The Bottom Line
More air lift = less water lift = less drag. The tunnel hull design is able to maximize the use of aerodynamic lift can turn this into a performance advantage. There are certainly ‘pros’ and ‘cons’ of the tunnel boat design as compared to other hull types, and I’ve not endeavored to address these comparisons. There is no disputing, however, that the tunnel boat is capable of achieving very high speeds and exciting cornering capabilities.
We’ve had a quick look and just why the tunnel hull is able to achieve its performance, and how that’s different than some other hull types. The tunnel hull is really part ‘boat’ and part ‘airplane’, and now we’ve examined the design characteristics that give the tunnel hull design its ability to ‘fly’ and to achieve great speeds.
About Aeromarine Research
In the forefront of Tunnel boat and catamaran design, Vee hull & vee-pad hull design and high-performance hull design technology for over 40 years, Aeromarine Research® has high performance clients in over 50 countries and in every US state. Tunnel boat and performance vee hull design publications and the world's only computer design program for tunnel boats, offshore cats, modified tunnel, modified vee-bottom (Mod-VP) designs, performance vee hulls and pad-Vee hulls.
Jim Russell's world acclaimed papers on tunnel hull and vee hull design, performance and setup tips are popular in dozens of powerboat publications and addressed in the Aeromarine Research® books, software and articles and newsletters.
For detailed information on the Tunnel Boat Design Program and the Vee Boat Design Program, articles, and books, visit the Aeromarine Research website.
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