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
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
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,
LAero + LWater
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
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 …
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.
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.
‘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!
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