What makes the tunnel hull work?
Part III: Dynamic Force Balances
by Jim Russell, AeroMarine Research
This is a multi-part article on the engineering basics of what makes a tunnel hull work.
In the first two weeks, we looked at the fluid dynamic forces involved in making a tunnel boat work, and the Lift, drag, weight and thrust forces in action. Recall that several requirements must be satisfied for an object (boat) to maintain a steady, stable, straight-line velocity.
(1) Lift = Weight. (Discussed in Part 1)
(2) Drag = Thrust. (Discussed in Part 2)
(3) Pitch = Null. All of these various forces acting must act so that the tendency to pitch about the center-of-gravity (CG) is eliminated.
This week we will look more closely at the third part of the picture - the Dynamic Force Balances. (This is a complicated one!)
3 Dynamic Force Balances
Although the Tunnel hull design is clearly the most efficient design of high performance powerboats, we should all recognize a few inherent traits.
Figure 3-1 - Instability: the tendency for a small problem to become bigger!
To meet the requirements imposed by the laws of aerodynamic stability theory, not only must we satisfy the two static force balances (lift = weight; drag = thrust) - but a third criterion as well. The forces acting must all act such that the tendency to pitch about the center of gravity (CG) is reduced. This means we would have 'dynamic' stability.
For stable flight, a vehicle must simultaneously satisfy several momentum criteria. Discussion of each of these is beyond the scope of this article (a complete and full discussion is covered in full in the "Secrets of Tunnel Boat Design" book). We can summarize our requirements to say that, in a stable boat, we want two (2) things:
- The forces acting on the hull balanced at all speeds about the CG; and
- The placement of these forces such that the net moment they create about the CG causes a favorable reaction to small disturbances (such as waves, wind gusts, etc.).
(Note: A moment is the measure of the tendency of a force to produce rotation about a point, and is equal to a force multiplied by a length).
When we apply these rules to a tunnel hull, we will see that the only way to satisfy them is if the center of gravity is close to the bow of the boat - but with the heaviest part of the boat (that's right, the motor) bolted at the transom, this isn't very likely! So, the conclusion is that the tunnel hull is inherently unstable - that is, "a slight raising of the bow at high speed will usually result in a bigger one", and soon the boat can blow right over backwards. (Well, that is how a tunnel boat behaves, isn't it?)
Now, before we pass judgment on this concerning conclusion, let us have a closer look at what all this really means.
Balance of Forces - Much can be done to optimize the balance of all the acting forces. This balance can be achieved for a range of speeds at the design stage, by optimizing the location and design of the forces involved. By selective designing of all the aerodynamic and hydrodynamic surfaces that become critical at high speeds, each tunnel hull can be tuned at the design stage. It is important to do this "dynamic balance" at all speeds through the boat's operating range - since balance at one speed just is not enough! (So balancing your boat on the trailer, by moving weight around is only going to help if you boat never leaves the trailer).
Pitching Moments - When a positive cambered aerofoil (like in a tunnel boat) is used to produce lift, a stability analysis will show that some kind of auxiliary lifting device must be employed in order to satisfy the rule that "a created moment about the CG causes a favorable reaction". On an aircraft, they can use elevators to help out, but in the design of our racing boat, we cannot use an auxiliary device effectively (even if it was allowed by the rules). STRIKE ONE!
Figure 3-2 - Stable 'Flying Wing'
A stable craft is one where the moment resulting from a "change in angle of attack (caused by a wave or a wind gust) must be one that tends to restore the boat to a situation where these moments are again balanced". For example, if an aircraft experiences a sudden increase in attack angle from a wind gust, the moment induced is such that the attitude of the aircraft will return to the normal one, automatically (all by itself!)
To satisfy these criteria on a tunnel boat, we would need the CG to be located ahead of the aerodynamic center. Then, an increase in angle of attack, causing an increase in the lift (at the aerodynamic center), will cause an automatic decrease in the angle of attack - restoring the 'flight' of our wing. The set-up is then, stable. (Aircraft easily meet these criteria - but tunnel boats have much trouble)! STRIKE TWO!
Our problem arises when we hit an unexpected wind gust or flow disturbance in our 120 mph 'flight path'. As we know, a slight increase in the angle of attack will produce a rather substantial increase in aerodynamic lift, which is going to throw off our (apparently) nicely balanced hull. We can visualize what is happening, and you may have seen it in practice at high speed. The first small increase in angle of attack a uses a rotation about the CG (raising of the bow) - which results in a little more lift - which results in a little more increase in the attack angle, which causes a faster rotation, which ...etc.
In summary to all of the above then, there is one very important note that we should make at this point. A vehicle that is aerodynamically unstable is not necessarily one that will not work. All that is meant is that it will not fly all by itself. Aircraft will fly in steady, level flight once they are properly trimmed and will inherently maintain their stability through minor disturbances. Tunnel hulls will not. (They need drivers that are paying close attention.)
We are back again to the importance of the marine tunnel hull driver. A tunnel boat has to its advantage great control over the entire behavior of the hull, both aerodynamically and hydrodynamically. The driver (or perhaps more appropriately, the 'pilot'), is necessary to read the situation, and react to all these minor disturbances that would so quickly result in disaster in his absence.
Luckily, this same inherent design of the racing tunnel provides the driver with a great many tools with which he can make these adjustments. The easy adaptation of power-trim allows the driver not only to adjust the angle of the propeller thrust line (to re-establish a net moment balance), but also to make changes in the height of his transom (and thrust line) while under way. Ballast tanks have been used to change the CG by as much as two or three feet - all on the command of the pilot, and in less than a second. We have even seen the use of 'water-brakes' that can be actuated to create phenomenal drag at the extreme trailing edge of the running surfaces (causing an overpowering positive moment about the CG) to make a 'last-ditch' correction for a suddenly increasing lift component.
Further, the hull designer has a great deal of control, as we have seen, over the eventual behavior of the boat. If the aerodynamic lift reacts close to the aerodynamic center (AC), then the closer that the AC is to the CG, the less severe will be the inherent instability of the hull. Although it is difficult to locate the CG in it's ideal position fore of the AC, there are many ways that a designer can locate the AC in the most advantageous position possible. Additionally, an optimum positioning of the CG can be attained in the design location of the driver, fuel, etc. So, the situation is actually much rosier than it sounds when we say that the boat is 'unstable'. It actually works pretty well.
The Recreational tunnel is in even better shape. By the nature of it's intended use, this design of tunnel boat exhibits a CG closer to the bow. The requirement to carry a higher payload (passengers, skiers, etc.) creates a much more favourable CG location, and the physically larger boat, with (by design) a more limited use of the air lift behaves in a manner that is very acceptable to any driver.
By this time, you could well be asking, why tunnel hulls work at all? Well, the tunnel boat behaves like it does for different reasons when designed for different applications. Moreover, when we know what we are doing, we can design the balance of dynamic forces to make it easier for the driver to safely control his boat under the designed conditions.
THIS IS THE MOST IMPORTANT PARAGRAPH TO READ AND REMEMBER!!
To make the best of the stability characteristics in the design of a tunnel hull we need only do the following:
- Ensure all the forces acting net out to zero, at all speeds.
- Design the location of all forces such that the CG is as close to the AC of the tunnel wing as possible.
That's all! It's not really bad after all is it?
Figure 3-3 The Designed Tunnel Hull
We have now defined the three rules of design that must be satisfied in our tunnel hull design - lift = weight, drag = thrust, and the balance of force moments. We have also seen the major areas of design within these rules that tell us where we must concentrate our design efforts. At AeroMarine Research, we use the "Tunnel Boat Design Program© " software to make the analysis easy. The TBDP calculates all hydrodynamic and aerodynamic lift forces by all lifting surfaces, all drag contributors, and does a dynamic balance of the hull at every speed defined in the performance specification. Doing this all in seconds makes it very easy to make small changes to the hull design, power or setup, and to determine the effect on performance and stability. Whether done manually (as shown in the "Secrets of Tunnel Boat Design" book) or by computer with TBDP, designing a tunnel boat that will optimize performance and ensure stability is possible when we understand the inter-relationships of "what makes a tunnel boat work".
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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
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