TBPNews #47 - October 6, 2003 
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In this issue: 
1) Yamaha recalls 4-stroke models
2) Cigarette Racing moves to old Mako site
3) Gillmans' Winless F1 Streak Ends
4) World Publications to launch Power Cruising magazine  
5) Trading "Boat" Spaces!
6) 'Hot Boat' magazine - Save 51%! 
7) FEATURE ARTICLE - How does a tunnel hull work? (Part 4) 
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1) Yamaha recalls 4-stroke models

Yamaha is voluntarily recalling several high-horsepower 4-stroke outboard models following reports of excessive drag on the linkage bushings that could cause the throttle to stick at fast idle. The problem also affects engines supplied to Mercury Marine.  The recall covers Yamahas F225, LF225, F200 and LF200 outboards from model years 2002, 2003 and 2004. These engines all have throttle mechanisms made with plastic bushings.  Corrosion and/or lack of lubrication on the throttle linkage creates excessive drag on the linkage bushings, which could prevent the throttle from returning all the way to idle after operating at cruising speeds. Instead, the engine might stay at fast idle speed.

Yamaha has a modification kit with new bushings of a different material. Dealers are instructed to clean and lubricate the linkage.  Yamaha advises customers to use a silicone spray for regular maintenance, but not all consumers follow that advice. If they do, apparently this problem will not occur.

Mercury has arranged to replace the complete throttle control assemblies, including the bushings. Owners of the affected engines are instructed to contact authorized dealers for repairs. The parts required will become available starting Oct. 22.
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2) Cigarette moves to old Mako site

Cigarette Racing Team has purchased the former Mako/SeaCraft plant in Opa-locka, Fla., and plans to relocate all 110 employees from its current home in Aventuras Thunder Boat Row by the end of November.  The Opa-locka facility  situated on close to 11 acres  has two main buildings and several outbuildings. One building will serve as Cigarettes 100,000-square-foot corporate headquarters and the other will be set aside for future expansion, Braver told the Herald.  Cigarette Racing Team says it plans to bring additional boatbuilding processes in-house when the Aventura, Fla.-based company moves into its new facility in November.
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3) Gillmans' Winless F1 Streak Ends

KUALA LUMPUR, Malaysia: American Scott Gillman led from the pole and powered his way to a dominating performance winning by 26.79 seconds.  This was his first winning event of the year and stopped his 30 race winless draught at the 4th Annual Grand Prix of Malaysia the fifth stop 
in the U.I.M. F1 World Championship outside Kuala Lumpur.  Gillman, who last won in Cagliari, Sardinia during his World Championship year in 2000, had battled through a frustrating 3 seasons with 13 DNF's and 14 podium finishes despite not picking up a win until this one.  The native of Los Angeles, CA led off the start, never let up and lapped all but the top 4 racers in the event. With the win, Gillman moves into 3rd place in the championship with 47 points.  

Italian Guido Cappellini moved from 3rd to 2nd on the start and stayed there.  He now has a 15 point cushion in the championship with fellow Italian Francesco Cantando of the Singha Racing Team who dropped out with engine problems. Cantando now stays at 55 points with Cappellini holding down 70.  The next round of the 2003 U.I.M. F1 World Championship for powerboating will be on the 26th of October and the Grand Prix of Singapore the 6th event of the campaign.
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4) World Publications to launch Power Cruising magazine  

World Publications will debut a new magazine, Power Cruising, with the premiere issue launching in February 2004. The company says that at least 75 percent of the new inboard powerboats on display at the fall boat shows are designed for coastal and long distance cruising, but there has been no magazine to address the cruising needs and interests of the people who will buy those boats. That's where Power Cruising will come in.  Pierce Hoover will be the editor, having held editorial positions with Sport Fishing, Sport Diver, Boating Life and WaterSki. Hoover says they will create a magazine that celebrates the power-cruising lifestyle, with an editorial package that is entertaining, informative and visually exciting. Power Cruising will publish two issues in 2004 before moving to a bimonthly schedule in 2005.  
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5) Trading "Boat" Spaces!

One of The Learning Channels most popular television programs is taking its cameras to the water.  "Trading Spaces" is coming to Florida in December searching for people interested in taping a special episode with their boats.  The normal show invites two sets of families to swap keys to each others homes for 48 hours and re-do one room along with a professional interior decorator/designer for guidance and a $1,000 budget.  A marina in Tampa, FL will provide the new location. All the same rules for the land-based program apply, excpet that boat owners do not have to be neighbors.

There are 3 requirements to participate: The boat must be over 40 feet, with the main living area measuring more than 14 feet by 14 feet and the overhead at least 7 feet high; all teammates (three or four to a team) must be family members; and there must be at least one child between the ages of 9 and 19 aboard each team  but no one younger than 9 is allowed to participate.  The deadline to apply is Oct. 24.

Contact Laurie Fried, National Marine Manufacturers Association, at (312) 946-6204 or lfried@nmma.org .
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6) 'Hot Boat' magazine - Save 51%! 

We have obtained great deals on Powerboat magazine subscriptions. This month we have a great deal on the popular 'Family & Performance Boating ' magazine. 

Hot Boat is prop and jet driven boat enthusiasts bible. Each issue featuring tests, technical and product reviews, adventure travel, outboard and drag racing coverage, high performance tips, and outboard RPM tricks. 11 issues Published: Seasonal Cover Rate: $54.89 Our Rate: $27.00 (51% savings!) Get yours now at: http://www.aeromarineresearch.com/boating%20magazines%20discounts.html 
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7) FEATURE ARTICLE - How does a tunnel hull work? (Part 4) 

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 "Tunnel Boat Design Program" (Version 7 now released) to do the hard work for us.

In the last three TBPNews letters, we looked at the factors that influence performance resulting from lift & drag relationships, thrust relationships and the location of weights & measures influences on performance.

This week we will look at lower unit design and the influences on performance. The optimum setup of your gearcase can create a gain of 13 hp at 100 mph!  Read on to see how!   

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).  


"Appendage" or motor drag.  This component is not easy to calculate in a simple manner, since there are so many different designs of outboard lower units in use today, and since every boat will have the loser unit set up with respect to the hull just a little bit differently.  The chore of analyzing such a 'beast' can be split into the parts of the lower unit, that is - the skeg, the torpedo and the propeller.  Here's some of the design measures that we have control over:

  *  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. Normally, values of +0.5" 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. 
  *  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).  The Skeg is really just a kind of wing, flying through the water - sideways.  It can be analyzed as such, using traditional aerodynamic approaches.  It 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).  It 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).  The 'Torpedo', or the part of the lower unit that houses the transmission, etc., is really very well described by it's colloquially used term - torpedo.  This part is actually just a projectile moving through the water, usually generating 'zero-lift' - just drag.  The 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.
  *  The Propeller of course, is a multi-aerofoil component generating lift and drag in all different directions.  (One of these directions we call thrust, and we use it to 'propel' our tunnel boat).  As far as additional drag on the hull however, the propeller contributes very, very little and so we can ignore it!

The total drag of the motor then, will be determined in a very general way, based on the velocity of the hull, and the 'type' of outboard motor lower unit used (i.e. size of engine, stock/racing unit, etc.) 

Height of gearcase makes a difference.  A typical Mercury Sportmaster gearcase at 60 mph velocity, with the torpedo positioned exactly at the trailing edge of the running surface (height = 0) can have about 34 pounds of drag.  If the gearcase was 1" lower in position (1" below the trailing edge of the running surface, or height = -1") then we would generate 43 pounds drag - that's over 6 HP worth of drag!  The same gearcase at 1" above the trailing edge of the running surface (or height = +1") would however, have only 25 pounds of drag at 60 mph.  And think of that gearcase setup at 100 mph - it will create 112 pounds (30 hp) of drag at height = -1"; and only 64 pounds drag (17 hp) at height = +1"!  

As we explained earlier, the motor drag can be a very long and tedious contribution to calculate in detail.  This drag is made up of the drag on the 'skeg' on the lower unit, and of the drag on the 'torpedo' itself.  Naturally, the size, shape and length dimensions of both the skeg and torpedo, as well as the velocity of the hull, will influence their relative contributions to total drag.  

The actual details for calculating such contributions are complicated, and really outside the scope of 'hand-calculations'.  With the use of Computer-Aided-Design programs, we have reduced this calculation process considerably.  To save a lot of time, we will try to 'standardize' the characteristic hydrodynamic drags of the popular outboard engine lower units available and currently in use on tunnel hulls.

The smaller stock outboard engines can be categorized into one group of say less than 50 Cubic inch displacement (cid) to establish a group of similar lower unit characteristics.  The engines larger than about this size, even up to the engines with displacements greater than 125 cid, can be categorized into another group of stock engine sizes.  Two additional groups are represented by the Racing versions of these engine sizes, since the dimensions, shape and size of these lower units are substantially different again.  

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.  We use the "PropWorks2(c) " software for propeller selection and speed prediction.  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.

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

See you next time!

/Jimboat

