TBPNews #13 - Jan. 06, 2002
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What factors influence Tunnel hull performance?

We have had many requests to explain each of the many factors that can influence the performance of a tunnel boat. There are literally dozens of factors that have an impact of performance, and most all of them also influence the other factors. This makes the prediction of tunnel hull performance a tricky business - to maintain the inter-relationships of all of these factors throughout the operating velocity range of the boat.   The good news is that most of these factors are controllable (by design and setup).  We usually rely on computers, and our "Tunnel Boat Design Program(c)" software (V6.5 is now released) to do the hard work for us.

There is a huge benefit, though, to understand what each of the influencing factors are, and how they can impact performance and stability of the tunnel hull.  There are many of these factors, and in the next few TBPNews articles will summarize most all of them. background.

- Tunnel Height - this is the Height of the tunnel at the aft most location, measured from the tunnel roof to the aft sponson bottoms (running pads).  The aerofoil of a tunnel hull is really a wing in what is called "close proximity ground-effect".  This means that the aerofoil is actually influenced by it's proximity to, in our case, the water surface.  A smaller tunnel height will increase Lift/Drag ratio of the tunnel hull "wing", improving lift characteristics.  It also brings the tunnel roof closer to the water surface, risking increased wetted surface if water conditions (waves) can cause "splashing" to the tunnel roof and sides. 

- Tunnel Width - Width of tunnel, measured from inside sponson to inside sponson. A wider tunnel creates a wing of larger "aspect ratio", which also increases Lift.  Generally there are only the practicalities of construction and user operating needs that limit the tunnel widths.

- Wing Chord - length of the "wing" or aerofoil, measured from the leading (front) edge of the deck to the trailing (aft most) edge.  This also impacts the length of the boat, of course.  A longer Chord will increase the overall area of the "wing", and hence, more lifting surface.  Unfortunately, increasing the chord will also reduce the "aspect ratio", which will reduce L/D ratio of the wing somewhat.  The increased surface area will also increase aerodynamic drag.  There's a trade off in the benefits of increasing Chord length.

- Wing Thickness - Maximum thickness of the aerofoil, measured from the top of the deck to the tunnel roof, at the thickest point along the length.  If we were to slice a tunnel boat in two, from bow to stern, we'd see the cross section of a wing or aerofoil that is made by the deck and tunnel roof.  It's this aerofoil that provides the tunnel boat with much of it's lift.  "Thicker" aerofoils - especially those in "ground-effect" - have higher  L/D ratios, and so are more efficient at creating lift.  So a thicker aerofoil is better.

- Deck Width - Width of the hull deck at the widest point.  Just as with the width of the tunnel, a wider deck creates a wing of larger "aspect ratio", increasing Lift.  Generally there are only the practicalities of construction and user operating needs that limit the tunnel widths.

- Aerofoil shape - The shape of the aerofoil and It's influence on the performance and stability of the tunnel hull is the subject of a full book, all in itself.  Someday I'll write such a book, based on all of the research that we have done on the subject.  The shape indeed has a great impact on tunnel boat performance.  The "camber" of an aerofoil can both change overall L/D characteristics, and change the position of the "dynamic center of pressure".  The C of P is the location of the aerodynamic lift, and it changes for each speed that the boat is traveling.  The effect is a dramatic influence on the dynamic stability of the hull.  This is one of the key design features that can optimize the performance of a tunnel boat.

- Angle Increment - Incremental angle between running surface (sponson pads) and 
 the wing chord. .  The angle of the "aerofoil" to the oncoming airflow (when the sponsons are parallel to the water surface), is not always zero.  Often an "incremental" angle of attack is built-in to the hull.  In such cases, when the sponson angle is 1 degree, the aerofoil angle could be 2 degrees - meaning the hull has a built-in "incremental angle" of 1 degree.  This is usually the additional (if any) angle of attack of  the wing chord compared to the angle of the running surfaces (e.g. the  angle of the wing chord if the running surfaces were at an angle of zero).  Higher angles of attack increase both lift and drag.

Well, that's enough for this issue.  Next time we'll continue to look at the factors that influence performance and stability, looking at cockpit design, sponson design and lower unit design.

2) New TBDP Version 6.5 Released!

The new version of the "Tunnel Boat Design Program" is now released.  Many of the features that our users have been asking for have been built in to this new release.  (We're trying to provide the stuff that you all would like). 

You can also see a new Version 6.5 listing of all the TBDP boat design input data, with explanations. Four full input screens, with over 60+ input variables, for very precise control of your design and setup. 
Have a look at:  http://www.aeromarineresearch.com/tbdp6/tbdp_controls.html

/Jimboat
www.aeromarineresearch.com
Jimboat@aeromarineresearch.com

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Note: The articles and references presented in TBPNews are made by Jim Russell, or are edited excerpts from the "Secrets of Tunnel Boat Design" book, author Jim Russell, published by AeroMarine Research.  The STBD book explains 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.

/Jimboat
AeroMarine Research
Jimboat@aeromarineresearch.com
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