What Makes A Tunnel Boat Work? Part Three

What Makes The Tunnel Hull Work?
Part III: Dynamic Force Balances
by Jim Russell, AeroMarine Research

Note: This 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 9.x and XP performs 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. If you missed part one of this series, please click here.

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.

Lift = Weight (Discussed in Part 1)
Drag = Thrust (Discussed in Part 2)
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.

Click On Image For Larger View

Figure 1 - Dynamic Instability

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 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 pretty 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'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. It’s important to do this “dynamic balance” at all speeds through the boat’s operating range – since balance at one speed just isn’t 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 can’t use an auxiliary device effectively (even if it was allowed by the rules). STRIKE ONE!

Figure 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.

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 we’re 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 ‑ 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.

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

"PropWorks2©" software for propeller selection and powerboat speed prediction - http://www.aeromarineresearch.com/prop2.html

Article Information

Jim Russell's third installment in his Tunnel Hull Tech series explains the dynamics outside forces on the tunnel hull.

Created:

May 7, 2002

What Makes The Tunnel Hull Work?
Part I: Lift and Weight

Aeromarine Research Tunnel Boat Design Software

Aeromarine Research Secrets Of Tunnel Boat Design Book, 12th Edition

By:

Jim Russell

Category:

Hull Tech

Company Website

AeroMarine Research