The tunnel hull design is able to maximise the use of aerodynamic lift and can turn this into a performance advantage.

In a tunnel boat, a wider tunnel dimension or ‘aerofoil’ section is more efficient for making aero lift than a narrow tunnel width.

Jim Russell, of AeroMarine Research, explains why a tunnel boat can be so fast …

The inherent design of the tunnel hull or power catamaran makes this type of boat capable of outstanding speed and performance. While some performance enthusiasts even feel that tunnel boats are the ‘fastest’ hull designs compared to other hull types, I’m not intending to jump into this debate during this discussion. It would be interesting, however, to look at the design characteristics that give the tunnel hull design its 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 its available power to speed. Tunnel boats and power cats gain added lift because they have a wing or aerofoil built into 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 aeroplane. This total lift can be produced from hydrodynamic (water) lift or aerodynamic (air) lift, or both.

            LAero + LWater = Weight

Also, there is only a certain amount of horsepower available with your boat set-up, and this available power is converted to thrust and used to overcome all the drag created by the hull.

            Thrust = Drag = DAero + DWater

The aerofoil portion of a tunnel boat creates lift in the same way as the wing of an aeroplane. Aerodynamic forces are created by the ‘wing’ that is formed by the deck surface and tunnel (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 optimise 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 from 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.

Here’s how it works

Any performance hull will perform better when taking best advantage of aerodynamic lift. Any amount of ‘aero lift’ will improve a boat’s 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, 50mph, 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 5mph!

Some tunnel hulls generate 30‒50 % of their total lift from aerodynamic design.

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 with a view to understanding the design factors that can affect the performance and stability of power cats of all sizes and shapes. In particular, our research has focused on ‘low-aspect ratio aerofoils in close-proximity ground effect’ – just like tunnel hull designs. Here are some of our findings …

Tunnel height

This 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 modelling 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’, thereby improving lift characteristics significantly.

Sponson sides enhance aero lift

A wing derives its lift from the difference between high pressure on the underside of the wing and lower pressure on the topside. This difference in pressure results in an upward force. Some aeroplanes 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.

Figure 4: Sponsons on a tunnel boat provide ‘wing-tip ends’ just like the lift-enhancing ‘winglets’ on efficient commercial aircraft.

The configuration of a tunnel hull has ‘built-in’ wing-tip ends formed by the sponsons on either side of the tunnel section (see Figure 3). This dramatically increases the efficiency of tunnel lift with more lift and less drag.

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 than a narrow tunnel width.

Figure 5: Glider planes have very wide high-aspect ratio wings that generate more efficient lift.

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 and aerofoil shape all work together with the other design features to optimise aerodynamic lift for the boat.

Cats have two keels

The tunnel boat configuration gives it two keels (one on each sponson), thereby improving handling and maintaining a more level ‘bank’ during manoeuvring. This can ultimately 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 the 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 the advantages of a softer ride and good performance in heavier waves; however, these higher-deadrise planing surfaces are 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 (see PBR March/April issue no. 146 on ‘Vee-Pad Design’). Lower-deadrise planing surfaces generate more lift, less drag and faster speeds.

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 ‒ but with lift generated by the ‘wing in ground effect’ that is formed by the deck surfaces and tunnel design.

Figure 8: Some vee-hull designs also take advantage of significant aerodynamic lift, which contributes to high performance on the water.

The bottom line

More air lift = less water lift = less drag. The tunnel hull design is able to maximise the use of aerodynamic lift and can turn this into a performance advantage. There are certainly pros and cons to the tunnel boat design compared to other hull types, and I’ve not endeavoured 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 at just why the tunnel hull is able to achieve its performance, and how that’s different to some other hull types. The tunnel hull is really part boat and part aeroplane, and now we’ve examined the design characteristics that give the tunnel hull design its ability to ‘fly’ and to achieve great speeds.

Safe powerboating! Wear your kill cord!

Linksfor reference:

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 the University of Michigan, OH, and the University of Toronto, Canada, and marine research at the NRC water channel laboratory in Ottawa, Canada. His published works and papers are highly acclaimed and are specifically related to the aerodynamics and hydrodynamics of high-performance catamarans and tunnel boats, as well as vee and vee-pad hulls.

Russell has designed and built many tunnel and performance boats. As a professional race driver, he piloted tunnel boats to Canadian and North American championships. He has written powerboating articles for many worldwide performance magazines and has covered UIM and APBA powerboat races. He has appeared on SpeedVision’s Powerboat Television as a guest expert on ‘tunnel hulls’, and was performance/design technical consultant on National Geographic’s Thrill Zone TV show and editorial consultant on the Discovery Channel’s What Happened Next?

Russell is the author of the books Secrets of Tunnel Boat Design and Secrets of Propeller Design. His company designed and published the well-known powerboat design software ‘Tunnel Boat Design Program’ and ‘Vee Boat Design Program’ specifically for the design and performance analysis of tunnel boats, powered catamarans, and performance vee and vee-pad hulls. ‘Jimboat’ is recognised for his advanced aerodynamic and hydrodynamic research and consulting on powerboat design, performance analysis, safety analysis, accident investigation, expert witness and education/training.

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