TBPNews #25 - July 16, 2002
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>>>>>> Tunnel Boat Performance News >>>>>>>>>>>>>>
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1) 50%+ DISCOUNT POWERBOAT MAGAZINES

Good Discount on all powerboating magazines!  We have recently obtained great deals on Powerboat magazine subscriptions.  Many of our TBPNews members have already taken advantage of it!  Check out your favorites at: http://www.aeromarineresearch.com/boating%20magazines%20discounts.html

2) FEATURE ARTICLE

*** ROCKET SCIENCE! - A series of articles on high performance powerboat design, and the technical opportunities for performance improvements by design.

Part 3, Efficient water planing surfaces

(How to increase your hull's speed without more horsepower)

In Rocket Science! - Parts 1 & 2 (TBPNews issue #20 and issue #24) we looked at how use of aerodynamics in powerboat design can really improve performance characteristics, and we started the review of how we can design the water-planing surfaces to be as "lift-efficient" as possible.  In this article, we will complete our discussion of the factors affecting hydrodynamic or 'water' lift, exploring how Aspect Ratio, Surface finish and Lifting strakes can influence the efficiency of the water lift of your hull design. 

From 'Rocket Science! - Part 2' we saw that hydrodynamic or 'water' lift generated due to the 'planing' of the sponson or running pad bottoms on the water surface is a function of five (5) factors...  
(a) Velocity (from Part 2)
(b) Lifting surface area (from Part 2)
(c) Angle-of-attack (from Part 2)
(d) Aspect ratio 
(e) Surface finish

Let us look at the remaining factors affecting hydrodynamic or 'water' lift, and how it influences the efficiency of the water lift of your hull design...

** The Aspect Ratio
For any area of planing surface, at any given angle-of-attack, at any velocity, some 'shapes' of the lifting surface are more efficient than others.  For example, a wider surface (compared to it's length) is more efficient than a narrower one.  This means that we get more lift and less drag with this more efficient design of planing surface.

The Aspect Ratio of the planing surfaces is kind of difficult to design for, although it is never the less an important factor to consider.  The AR is the wetted lifting width divided by the wetted length, or in terms more easily calculated; the wetted width squared (width * width), divided by the wetted surface (width * length).  This is calculated for the full lifting surface (e.g.: on a tunnel boat, for each sponson; for a vee hull, it's the running pad).  Higher aspect ratios will give us lift that is more efficient.

** Lift Strakes
Lift strakes or spray rails also can generate some lifting component, however their contribution is often so small, that it may not be worth figuring.  (This is not to say that lift strakes need not be used - they are very beneficial when applied properly to the design).  Lift strakes benefit reduction of water drag even more significantly than their lift generating capabilities.  So, their contribution to improved performance is an indirect one, but beneficial never the less!

** Surface Finish
The Surface finish of the planing surfaces must be not only perfectly smooth, but also perfectly flat.  The methods of calculations herein reflect only perfectly smooth and flat surfaces.  Roughness of the planing surface can be accounted for in a more detailed analysis, but is totally useless to us in tunnel hull design since we simply must have smooth, flat surfaces - or there's no need to own, build or design a boat - getting the picture?

In general, we will design the plan form of our running surfaces, attempting to get as good an AR as possible, knowing the amount water-lift that is required.  The cornering ability and acceleration requirements should also be considered, since these will also affect the plan form of running surfaces.  

** The Design Secret
How do we 'add up' all these 'pros' and 'cons' from the different factors that influence the boat's performance?  The secret to high performance powerboat design is a good understanding of the quantitative effects on performance of all the various design features, so that a proper evaluation can be made.  For example, you might ask the question..."A wider 'pad' (running surface) should give me better acceleration, but will also reduce the wetted length, moving the center of lift further aft.  Will this upset the balance of the hull design?"  ..."How would it affect cornering?...Or top speed?"  All these aspects must be evaluated.  The tools we have to do so include the 'qualitative' evaluation of similar features and effects, design calculations and logical study of the hull design as whole, because of the change.

The understanding of the behavior of the running pads of the high performance powerboat is paramount!  Having the tools to properly design this part of the total lifting forces needed for the boat is just as important as the aerodynamic details.

Ok, that's enough for this week...

In the next "Rocket Science" article, Part 4, we will consider how to design our aerodynamic surfaces to be as "drag-economical" as possible.

7) Spread the Word!
 
Hey, when was the last time you forwarded my newsletter to someone?  That's what I thought.  So why not forward it to a bunch of your friends, club members and maybe your ex-spouse?  Then suggest they sign up for their own copy by going to: http://www.aeromarineresearch.com/join.html

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

