From: TBPNews [webnews@aeromarineresearch.com]
Sent: Saturday, October 27, 2001 11:58 PM
To: jimrussel@sympatico.ca
Subject: TBPNews - Oct. 27, 2001

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What makes the tunnel hull work? (Part 3)

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!)

Although the Tunnel hull design is clearly the most efficient design of high performance powerboats there are a few inherent traits that we should all recognize.

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).  Basically, 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 (thats right, the motor) bolted at the transom, this isn't very likely!  So the conclusion is that the tunnel hull is inherently unstable &#64979; that is, a slight raising of the bow at high speed will usually result in a bigger one, and pretty soon the boat can blow right over backwards.  (Well, that is how a tunnel boat behaves, isnt it?)

Now, before we pass judgment on this concerning conclusion, let's have a closer look at what all this really means.  

Balance of Forces - There is much that 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.   Its important to do this dynamic balance at all speeds through the boats operating range  since balance at one speed just isnt 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 cant use an auxiliary device effectively (even if it was allowed by the rules).  STRIKE ONE!

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 &#64979; restoring the 'flight' of our wing.  The set&#64979;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 &#64979; which results in a little more increase in the attack angle, which causes a faster rotation, which ...etc.   

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.  And, when we know what were 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 that bad after all is it? 

We have now defined the three rules of design that must be satisfied in our tunnel hull design &#64979; 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.  

/Jimboat
www.aeromarineresearch.com

Note: The multi-part article presented over the past few weeks was an edited excerpt from the Secrets of Tunnel Boat Design book.  The STBD book details the theory in full, and outlines example design calculations, step-by-step.  The Tunnel Boat Design Program for Windows 98, software, does all the force calculations, dynamic force balances at all speeds, and reports the analysis automatically, including complete graphical performance results for any tunnel or modified vee hull design.

Let us know any ideas you have, requests for articles, questions or comments on our TBPNews.

/Jimboat
AeroMarine Research
Jimboat@aeromarineresearch.com
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Let us know any ideas you have, requests for articles, questions or comments on our TBDPNews.
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You can get the world known "Secrets of Tunnel Boat Design" book, the "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 at the Aeromarine Research web site.  
GO TO: www.aeromarineresearch.com

"Secrets of Tunnel Boat Design" book
"History of Tunnel Boat Design" book
"History of Propellers" e-book
"Tunnel Boat Design Program" for Win98 software
"PropWorks2" software for propeller selection and powerboat speed prediction

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