TBPNews #24 - June 8, 2002
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>>>>>> Tunnel Boat Performance News >>>>>>>>>>>>>>
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1) 50%+ DISCOUNT POWERBOAT MAGAZINES

Good Discount on all power boating 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
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2) FEATURE ARTICLE

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

Part 2, Efficient Water Planing Surfaces

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

In TBPNews issue #20 (March 27, 2002) we looked at how the use of aerodynamics in powerboat design can improve performance characteristics.  We now look at how we can design the water-planing surfaces to be as "lift-efficient" as possible.  

The understanding of the behavior of the running pads of the tunnel hull can really be a vital in achieving optimum performance from your hull.  Having the tools to properly design this part of the total lifting force needed for the boat is just as important as the aerodynamic details.

** Water drag and Air Drag

Before we look at the detailed influences on water lift generation, we should 'drive-home' the importance of the relationship between air and water lift/drag.  

Recall from Rocket Science! - Part 1, that air drag is steadily increasing as the speed increases.  While speed increases however, aerodynamic lift is also increasing rapidly and is lifting the hull off the water more and more.  This in turn, reduces the wetted surface of the sponsons.  Since drag in water will be OVER 800 TIMES more than drag in air (for the same surface), we will find that even at 120 mph, it is still the water-drag that is the major contributor.  

The air drag and motor drag of course, continue to increase in magnitude as long as the speed increases, whereas the sponson or lifting pad water drag will decrease as aerodynamic lift increases.
 
The key through all of this is to design sponson bottoms that are hydrodynamically 'lift-efficient', thereby minimizing their wetted surface, which in turn, lessens this part of our total water drag.  

** Water Lift
We have seen that water drag is Enemy No. 1'!  Let us now look closely at some of the factors affecting hydrodynamic or 'water' lift.  

The lift generated due to the 'planing' of the sponson bottoms on the water surface is a function of five (5) factors.  
(a) Velocity 
(b) Lifting surface area
(c) Angle-of-attack 
(d) Aspect ratio 
(e) Surface finish

The same fluid dynamic force equation that we used previously (in Rocket Science! - Part 1) can be applied to determine hydrodynamic lift:

Lw = [1/2 * pW * V^2 * S * CLW]			
Where:	
Lw = water lift (lbs) 
pW = density of water (lb-sec^2/ft^4)
V = velocity (feet per second)
S = wetted surface area (sq.ft.)
CLW = lift coefficient (dependent on design of surfaces)

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

** Velocity
The Velocity has a tremendous effect on the total lift generated.  We can note that the lift (Lw) produced, increases very rapidly with increasing velocity.  This is because the lift force is a function of Velocity-squared (V^2).  Therefore, the faster we go, the more and more is our ability to increase lift becomes to our boat's performance.  For example, the lift generated at 60 mph is 4X the lift generated at 30 mph, using the very same surfaces and design!

** Lifting Surface Area
The Wetted lifting area is an important term to understand.  The efficiency of the lifting surfaces depends largely on the angle of 'deadrise' of the surface and the 'angle of attack' of the lifting surface.  It can always be said, however, that more surface area means more lift...and more drag!

** Deadrise Angle
The fact that the hull's sponson bottoms or lifting pads are not necessarily flat or parallel to the water surface in section, but rather at some angle of 'deadrise', reduces the 'effectiveness' of the lifting surfaces.  This is because of the angle at which the surface presents itself, which allows a lateral flow of water to the outside of the hull.  Deadrise Angle can be thought of as the angle of the lifting surface across the beam (side to side) - a deep vee has a 'high deadrise' angle, where a flat-bottom boat has a very 'low deadrise' angle.

The lower efficiency of the lifting surfaces due to deadrise is reflected in a degraded lift coefficient.  For an average angle of deadrise (BDR) of about 10 degrees, this can represent a degradation of the lift coefficient of as much as 30% compared to a bottom design with deadrise of 5 degrees!  On many hull designs, this deadrise is necessary for cornering and handling requirements, but close attention to the design of the lifting surfaces will pay off, as we can plainly see.  

** Angle of Attack
The Angle-of-attack is the same story as for the lifting surfaces in air lift.  The angle between the sponson bottoms (on a tunnel boat) or lifting pad (on a vee bottom hull) and the water surface will alter the lift produced.  An increase in this angle will result in an increase in lift coefficient.  It should be noted here that the theoretically most efficient angle-of-attack for a planing flat plate (surface) on water is about 3.  (This gives us the best L/D ratio).  This should be considered along with stability considerations when designing and optimizing the hydrodynamic surfaces. 

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

In the next "Rocket Science" article, Part 3, we will conclude 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. 
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3) 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

