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1) What factors influence Tunnel hull performance?  (Part 3)

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 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 the advanced ": TBDP(c) (Version 6.5 now released) to do the hard work for us.

In the last TBPNews letter, we looked at the factors that influence performance resulting from sponson design and cockpit design.  This week we will review the influencing factors of the performance and stability, looking at the location of weights & measures, lower unit design, and the "outside" influences on performance.

Much can be done to optimize the balance of all the forces acting on the tunnel boat in motion.  This balance can be achieved for a range of speeds at the design stage, by optimizing the location 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).  Here's some of the design measures that we have control over:

  *  *  Boat Length - Overall length of hull, measured from sponson tips to aft most deck or sponson point.
  *  Driver/Passenger Length - Location of the driver and passengers, measured from the transom to the driver and passenger center location.
  *  Motor Length - Location (horizontal) of the motor, measured from the transom to the motor center of gravity (C of G).  May be either fore (+), or aft (-) of the transom.  Often adjusted by use of motor setback brackets.
  *  Fuel Length - Location of the fuel tank(s), measured from the transom to the fuel center.  Usually best to have the fuel located aft in high performance boats; can be located fore if optimum speed and control is not as important a factor.
  *  Miscellaneous Length - Location of additional equipment that is concentrated mostly in one location, such as hydraulic systems, batteries, ballast, etc., measured from the transom to the (average) load center.
  *  Motor Height  - Location (vertical) of Motor/engine from the water surface to the top of the engine casing/housing.  This locates the C of G of the engine for purposes of dynamic stability analysis, and defines the "form area" of the motor/engine if the engine (or part of) is exposed to the air flow and thus generates aerodynamic drag.
  *  Lower Unit Height - Height of lower unit "bullet" above/below sponson running pad.  (+) is above, (-) is below.  Most high performance setups start with as little of the lower unit bullet in the water as possible, thus reducing drag significantly.  Surface piercing propellers and low-level water pick-ups make this feasible. 
  *  Lower Unit Height - Normally, values of +0.5in to +1" (above sponson running surfaces) are 
 possible in very high performance applications. Without low water pickup, or when low-end torque is a requirement, then LwrUnitHeight values of -0.5 to -2" (below sponson running surfaces) is applied. 

  *  Boat weight - Total weight of hull (only), including rigging, but excluding motor, fuel, driver and other significant additional equipment.  This weight should include all rigging weights that have not otherwise been accounted for.
  *  Motor weight - Total weight of motor, drive unit, propeller, and accessories such as hydraulic trims, plates, etc that are attached to or built in at the motor.  
  *  Driver/passenger weight(s) - Weight of driver with all clothing and safety equipment.  Test analysis should also be done with higher and lower values of passenger weight, to understand the sensitivity of the design to weight and location of payload.
  *  Fuel weight - Weight of fuel tanks and normal fuel supply.  Test analysis should also be done with higher and lower values of Fuel weight, to understand the sensitivity of the design to weight and location of fuel.
 Miscellaneous weight(s) - weight of additional concentrated equipment such as hydraulic systems, ballast, batteries, etc. 


  *  Lower Unit Drives Quantity - The number of lower unit drives (no. of engines) will impact the hydrodynamic drag generated at all speeds.  It is a design trade-off as additional propeller thrust of multiple drives can reduce propeller slip, increasing overall efficiency.
  *  Skeg Width - average width of motor lower unit/outdrive skeg (leading edge of skeg to back of skeg).  Affects the friction and induced drag generated by the lower unit.
  *  Skeg Length - length of motor lower unit/outdrive skeg (top of skeg to bottom of skeg).  Also affects the friction and induced drag generated by the lower unit; but on the positive side, longer skeg can provide better stability and handling under high load conditions such as cornering.
  *  Skeg Thickness - thickness of motor lower unit/outdrive skeg (thickness of the skeg plate).  Affects the form, friction and induced drags generated by the lower unit.  Only reason for "thick" skeg design is a structural or reliability one - thinner is better.
  *  Torpedo Length - length of motor lower unit/outdrive torpedo housing (leading edge of torpedo to aft edge of torpedo, at prop shaft).  Longer torpedo is better, as it will have a higher "aspect ratio" thus generating less induced drag.  After-market nosecones are used for exactly this purpose, presenting a more stable water flow over the torpedo length, reducing drag and reducing propeller burning or blow-out.
  *  Torpedo diameter - diameter of motor lower unit/outdrive torpedo housing (in cross-section).  Affects the form, friction and induced drags generated by the lower unit.  Only reason for larger diameter design is a structural or reliability one - smaller is definitely better.


Well, that's enough for this issue.  I hope that we have seen the major areas of design control that we can concentrate our design efforts.  

At AeroMarine Research, we use the "Tunnel Boat Design Program(c) " 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 how these factors influence performance and stability.


2)  Design Article Published: 

Author Jim Russell writes Design article for BoatDesign.net on "What Makes a Tunnel Boat Work".

Check it out at: http://boatdesign.net/web/index.shtml  

And at:  http://boatdesign.net/articles/tunnel-hull-design/index.htm 


3)  New 11th Edition: SECRETS OF TUNNEL BOAT DESIGN 
                       1-WEEK LEFT FOR LOWER PRICES!
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The new edition of the "Secrets of Tunnel Boat Design" is now published.  Orders submitted before Feb 15, 2002, will get the NEW STBD book, at current prices.

This new edition has allot of new information; now with over 165 pages, and well over 100 photographs!  The new edition includes an added history of 'modified tunnel hull (Mod VP) design'; an added 'History & Design of Propellers'; and an added 'History & Design of 'Wing in Ground Effect' concepts, as they have impacted high performance powerboat and tunnel boat designs.

These new segments are added to the original STBD book features: The developments of the tunnel and V bottoms are interestingly chronicled, with detailed explanations of hull design, function, potential and characteristics.  The book also details ten design steps for analysis of hull performance and stability showing how the calculations are accurately performed, as well as providing detailed information about their relation to hull performance.  The ten steps range from layout design and dimensions, calculating aerodynamic and hydrodynamic lift and drag, power calculations, and stability, acceleration, etc.


4)  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 are trying to provide the stuff that you all would like). 

You can 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

