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What Makes The Tunnel
Hull Work?
Part I: Lift and Weight
by Jim Russell,
AeroMarine Research
 The Tunnel
Hull is a strange 'bird'. While the Tunnel derives much of its high
performance from air lift, it depends at the same time on its planing
interaction with the water to maintain a stable and controlled 'flight'.
This interdependence of water and air force dynamics is the key to the
approach to Tunnel design.
The Tunnel Hull is a
strange 'bird'. While the Tunnel derives much of its high performance from
air lift, it depends at the same time on its planing interaction with the
water to maintain a stable and controlled 'flight'. This interdependence of
water and air force dynamics is the key to the approach to Tunnel design
Tunnel boats
demonstrate such exceptional performance because they have a "wing" or
aerofoil built-in to their design. The tunnel "roof" and the upper deck
surface form the lower and upper surfaces of the aerofoil, respectively.
When properly designed, it is this aerofoil, and the aerodynamic lift it
generates, that gives the tunnel boat its great performance.
What makes the tunnel hull work? This is a multi-part article on the
engineering basics of what makes the tunnel hull work. This week, we will
look at the principles of operation and Lift/Weight balance.
To understand the balance of hydrodynamic and aerodynamic influences on
tunnel boat performance and stability, we must examine the fluid dynamic
forces involved. Several requirements must be satisfied for an object to
maintain a steady straight-line velocity.
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Lift = Weight.
The weight for the hull must be exactly supported by
forces such as lift from the hydrodynamic planing surfaces and aerodynamic
lift.
- Drag = Thrust.
The drag experienced because of a velocity and all
the lift mechanisms must be overcome by the available thrust.
- Pitch = Null.
All of these various forces acting must act so that
the tendency to pitch about the center-of-gravity (CG) is eliminated.
Therefore, for
a tunnel boat (as for any boat) these forces must all balance out - and the
design of the hull can be thus generalized into the three areas of hull
lift, hull drag and dynamic stability.
1. Lift and Weight
The
hull weight (including engine, driver, fuel, accessories, payloads, etc.)
must be EXACTLY equaled by the lift forces generated. This is true for any
boat (or airplane, too) in stable flight. The tunnel hull must however
always be in 'stable flight', and so this balance is especially critical.
Too much lift and we take-off like an airplane - too little and we have more
"down" than we have "up", and this can be a distressing event for a planing
craft!
There is lift generated in two ways. The planing sponson bottoms create
'hydrodynamic' or water-lift (lift due to forces on and reactions with, the
water surface). Aerodynamic lift is generated by the relative air flow over
the tunnel and deck surfaces or "wing" (lift due to forces on and reactions
with the air, itself). This aerodynamic lift is affected by several
factors, just like a wing on an airplane. (thickness, camber, angle of
attack, etc.). The additional influence is that of the "wing" being in what
is called "ground-effect". With an airplane, it is experienced when the
craft flies close to the ground during landing and takeoff. With a tunnel
boat, it is experienced all of the time, due to the "wing's" proximity to
the water surface. The effects are complex, but generally, lift is enhanced
due to the "ground effect".
There are additional sources of lift in the tunnel hull rig, such as the
slight lift generated by surface piercing propellers, for example, but the
contributions of forces like these to the whole force 'picture', are
smaller, and beyond this article.
It is important to note that the relative significance of these forces
changes as the speed of the hull increases. We can see this reflected in
Figure 1-1 showing the increasing aerodynamic lift and drag as functions of
airspeed, for a typical Mod U/F1class racing tunnel boat of say 750 lbs.
total weight and a constant angle of attack of about 2°.
|
Air Speed (mph) |
Air Lift (lb.) |
Air Drag (lb.) |
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50 |
110 |
30 |
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60 |
130 |
35 |
|
70 |
160 |
45 |
|
80 |
220 |
60 |
|
90 |
275 |
75 |
|
100 |
340 |
95 |
|
110 |
410 |
115 |
Figure 1-1 -
Typical Air Lift/Drag vs. Velocity |
Generally,
under about 50 mph, the aerodynamic lift accounts for less than 10% of the
total lift, the sponsons supporting nearly all of the weight of the boat.
At the speeds now attainable by conventional racing tunnels, the tunnel lift
can account for well over 80% of the total lift. This tells us then that the
sponson lift is reduced accordingly which gives dramatic improvements in the
performance of the boat, as we will see later.
The percent (%) Aerodynamic Lift (of Total Lift) on recreational boats is
lower than it is on higher performance or race-type boats. I did a
performance analysis of an STV Euro 19'. This boat generates 18% LA at mid
velocity, and 29% (425 lbs) LA at maximum velocity. A similar analysis of a
full race boat, like a Seebold F1 boat, shows that it generates 65% LA at
top speed. The inherent design features contribute to the ultimate
performance of different tunnel boat design concepts. The selection of each
design feature is always somewhat of a compromise between top speed,
acceleration capability, stability, comfort, seaworthiness and reliability.
The 'air-lift' of the Tunnel Hull is what separates this type of hull form
from all the rest. Although the many factors affecting the aerodynamic
forces generated make this a complicated matter at times, the effort is
clearly worth it. Attention to detail in the design stages pays off many
times over, in the end.
The main factors involved in creating the lift generated by the tunnel and
the deck surfaces, or this 'wing' we have talked about, can be summarized as
follows:
- Air speed
- Angle of attack
- Surface area of
Tunnel
- Aspect ratio of
Tunnel
- Height of mean
camber line above the water surface
- Aerofoil shape of tunnel
cross-section
- Surface condition of
exposed areas
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Figure 1-2 - Forces on a
tunnel boat |
The
methods of calculations are presented in detail in the "Secrets of Tunnel
Boat Design" book, but let us take a 'sneak-preview' of the design formula
for air lift, so that we can see the relationships we are talking about.
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LA = [˝
ρAV2 SA CLA] |
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Where: |
LA = air lift
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ρA = density of air
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V = velocity |
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SA = surface area |
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CLA = lift coefficient |
About The Author
|
Jim Russell is
a professional engineer with a mechanical and aeronautics background.
Currently living in Canada, he has done extensive aerodynamic research
at Universities of Michigan, OH and 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 and tunnel
boats. Russell has designed and built many tunnel boats. As a
professional race driver, he piloted tunnel boats to Canadian and North
American championships. He has written powerboating articles for many
worldwide magazines and covered UIM and APBA powerboat races. Russell
is the author of Secrets of Tunnel Boat Design (reviewed
here),
History of Tunnel Boat Design, and History
and Design of Propellers.
His company has designed and published the well-known powerboat design
software, "Tunnel Boat Design Program©," specifically for the design and
performance analysis of tunnel boats and powered catamarans.
Get your
full, illustrated,
12th edition copy of the "Secrets of Tunnel Boat Design" book, with
over 165 pages of design practices and formulae and over 100
photographs.
The publications "History of Tunnel Boat Design" book, "History of
Propellers" e-book, the "Tunnel Boat Design Program© for Win98"
software, and the "PropWorks2" software for speed prediction and
propeller selection are available at the
Aeromarine
Research web site. |
 |
http://www.aeromarineresearch.com
"Secrets of Tunnel Boat
Design©" book -
http://www.aeromarineresearch.com/stbd2.html
"History of Tunnel Boat
Design©" book -
http://www.aeromarineresearch.com/history.html
"History & Design of
Propellers©" e-book -
http://www.aeromarineresearch.com/historyofpropellers.html
"Tunnel Boat Design Program© ",
V6.5 software -
http://www.aeromarineresearch.com/tbdp6.html
"PropWorks2©" software for
propeller selection and powerboat speed prediction -
http://www.aeromarineresearch.com/prop2.html
Copyright© 2002 AeroMarine
Research®. All rights reserved.
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