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What factors influence Tunnel hull performance?  (Part 2)

1) We have had many requests to explain each of the many factors that can influence the performance of a tunnel boat.  There are 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.

It is helpful to understand what each of the influencing factors are and how they can impact performance and stability of the tunnel hull.  In the last TBPNews letter, we looked at the factors that influence performance resulting from tunnel and aerofoil design.  This week we will review the influencing factors of the cockpit design, sponson design and lower unit design.

Cowling or Cockpit type - the type of cockpit or cowling design will impact aerodynamic lift and aerodynamic drag significantly.  Designs are generally (1) OPEN cockpit with faired front and rear cowlings, with exposed driver cockpit.  (2) CANOPY type cowling with integral or closed, faired cockpit enclosure, like with a safety cell; or  (3) NONE - an open cockpit with no fairings.  and no cockpit cowlings fore or aft, to speak of.  The 2 former designs are usually seen on racing boats or very high performance recreational hulls; the latter design is usually seen on open cockpit recreational run-about and offshore hulls.

The features of a cockpit design that will impact performance are:

Height of REAR COWL - usually measured from the deck surface to the maximum point above the deck surface.  A well designed cowl will help present a less disturbed airflow to an outboard motor (drag body), but too large will create some air drag of it's own.

Height of FRONT COWL - such a cowl or 'fairing' can help smooth the airflow around the otherwise open cockpit of a recreational run-about or offshore hull.  Even some windshields can be effective fairings.  On a racing hull, the front cowl is usually quite dramatic, fairing the air flow neatly around the driver.

Width of COWL or fairing - measured at the widest point.  On a hull that has an airflow fairing cowling, the overall width will contribute to the amount of aerodynamic drag that the cowling generates by itself.

Open Deck - The length of open (unobstructed) deck fore of the cockpit, can be important at higher speeds.  Even a very flat deck, if unobstructed, will reduce aerodynamic drag, and even generate aerodynamic lift.  If the length of this open deck is less than 1/3 of the boat length, its beneficial contribution will be reduced.

Sponson Pad Width - The width of the sponson running surfaces (bottoms), measured from sheer (inside) to effective chine (outside).  Wider pads give higher aspect ratio (relative width to length) and thus, more efficient lift.  At higher speeds, however, there can be a greater pad surface exposed to extraneous water drag.  It is a design tradeoff.

Sponson Pad Deadrise Angle - The angle of sponson running surfaces, measured from sheer to chine.  A very shallow deadrise will provide the most efficient lift, but deeper deadrise angles (like 12 to 20 degrees) will make a much cleaner presentation to the water surface in rougher water conditions.  Again, a design tradeoff.

Sponson Steps - the use of steps on the sponson bottom design can be tricky.  Not all steps on production hulls are properly designed.- and even some racing hulls exhibit poorly designed steps.  A poorly designed step will generate a more dramatic separated water flow, with lower lift and higher drag characteristics.  Clean, properly designed steps, of an effective height, placed at the correct lengthwise location can indeed help acceleration, and generate more efficient lift in irregular water conditions.  

CENTER POD - the additional lifting surface in a "modified tunnel hull" or "Mod VP" design can improve handling and even acceleration under certain circumstances.  (The comparable hull with no center pod in the design is a conventional tunnel hull configuration).  Boats that are heavier or carry increased payloads can benefit from a center pod design.  The length of the pod will impact the performance and stability of the hull, as will the same factors that contribute to sponson pad design.  Additionally, the HEIGHT of the center pod's running surface, relative to the running surface of the (2) outboard sponsons can have the most significant impact on overall performance.  The center pod can be designed to become effective or non-effective at certain speeds, thus getting the advantages of both a conventional (2 sponson) design and the center pod design.

Spray Rails - will add hydrodynamic lift and reduce hydrodynamic drag, when well positioned.  Spray rails are often placed on the inside of tunnel walls, and on the outer chine walls.  

Spray Rail Height - is measured from aft most bottom of sponson running surface.  Usually are seen about 1/2 way between sponson bottom and deck (sheer clamp).  Outer spray rails can improve hydrodynamic lift at lower Clew's (lower velocities) and reduce spray drag. 

Spray rail width - the width of outer sponson SPRAY RAIL, are usually seen about 2 inches wide on IOGP and family (Mod-VP) type hulls - wider on offshore hulls. 


Well, that is enough for this issue.  Next time we will continue to look at the factors that influence performance and stability, looking at the location of weights, lower unit design, and the "outside" influences on performance and stability.

2) New Edition: Secrets of Tunnel Boat Design (11th edition)

Watch for the new edition of the "Secrets of Tunnel Boat Design" to be published NEXT MONTH.  This new edition has lots 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.

Orders submitted before Feb 15, 2002, will get the NEW STBD book, at current prices.
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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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