TBPNews #21 - Mar. 27, 2002
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>>>>> Tunnel Boat Performance News >>>>>>>>>>>>>>
==================================================================

1) NEW Boat Design drawing programs FREE

Check out the links to new boat design drawing programs at our FREE download page:  http://www.aeromarineresearch.com/free_downloads.html 
****************************************  

2) AR WINS AWARD!

AeroMarine Research's website was presented with The Golden Web Award, by The International Association of Web Masters and Designers.  "The Golden Web Award" is presented to those sites whose web design, originality and content have achieved levels of excellence deserving of recognition.
http://www.aeromarineresearch.com
****************************************  

3) FEATURE ARTICLE

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

Part 1, Aerodynamics  

(How to increase your hull's design speed by 7+ mph with aerodynamics)

A high performance powerboat hull uses its running surfaces for the key portion of its LIFT.  In a tunnel boat or catamaran, this "hydrodynamic lift" comes from the sponsons.  In a vee-bottom hull, the lift comes from its veed surfaces or the narrow running surface (or "pad") to support its weight.  Any hull however, will perform better if it takes advantage of "aerodynamic" lift.  Moreover, ANY boat can benefit from proper use of aerodynamics - and here is why...


** Design Rules:
Let us start with 2 rules of design that must be met for any boat.  First, any boat must generate enough lift to over come it's weight, and keep it on top of the water.  If a boat weighs 1500 lbs, then we need to generate 1500 lbs of lift to make it perform on top of the water.  Second, the total drag created by the hull must be overcome by the available thrust - namely the propeller.

With any LIFT, there comes some DRAG - it is just the law of energy.  With hydrodynamic (or water) lift generated by a lifting surface, there is a certain amount of water drag that comes along with it.  The amount of drag experienced depends on a number of influencing factors, but we can calculate what this water drag will be (we'll see that later).  Now, with aerodynamic (or air) lift, there will also be an amount of air drag generated.  We can also calculate what this air drag will be.  It is these drags that limit how fast our boat will go, so we do not like to have any more than we absolutely must have.

NOTE - the drag generated by a lifting surface in water is MUCH more than the drag generated by a lifting surface in air.


** Water and Air Lift:
This is a key point to remember.  If we can generate ANY lift by virtue of aerodynamics, it will be just that much LESS lift that we have to generate by our "water lifting surfaces."  The important point to note here is that every pound of lift that can be generated by aerodynamics, is one less pound of lift that doesn't have to be supplied by the water lifting surfaces - and we have already noted that water lift brings with it allot of unwanted water drag.  So, the trick becomes to squeeze as much lift out of aerodynamic lift surfaces as we can so that we can take some of the load off the water lifting surfaces (like vee-pads, sponsons, etc.) 

Any boat hull has some surfaces that can be designed to generate some amount of lift (deck surfaces, tunnel surfaces, airborne bottom surfaces, etc).  Although many factors affecting aerodynamic forces generated can make this a complicated matter at times, the effort is clearly worth it.  Many effects come into play, but the main ones are airspeed, angle-of-attack, surface-area, aspect ratio of surface, surface profile (shape) and surface condition of exposed areas.


** Lift and Drag Calculation:
I mentioned earlier that we could calculate what these LIFT and DRAG contributions would be.  Let me prove to you just why water drag is so much more penalizing than air drag is.  Here is the design formula for drag, so that we can see the relationships we are talking about.  

D = [1/2 * pf * V^2 * S * Cd]   
where:	
D = drag (lbs) 
pA = density of the fluid (lb-sec^2/ft^4 )
V = velocity (feet per second)
S = surface area (sq.ft.)
Cd = drag coefficient (dependent on design of surfaces)

First, we can note that the drag (D) produced, increases very rapidly with increasing velocity.  This is because the drag force is a function of Velocity-squared (V^2).  So, the faster we go, the more and more important our ability to reduce drag becomes to our boat's performance.

BUT, here is the real catch...

- The density of water (pw) is = 1.94 (lb-sec^2 / ft^4 ).  
- The density of air (pa) is = 0.00237 (lb-sec^2 /ft^4).  
- This means that the drag in water will be OVER 800 TIMES more than the drag in air, for the same surface, same configuration, at the same velocity!


** Try an example
As an example of what this aerodynamic lift can do for us, let us make a simple comparison between a hull with some aerodynamic lift (Boat #1) and a hull of equal weight, but without any aerodynamic lift designed into it (Boat #2).  Each of these boats must generate, in their own way, the same total lift to support their weight.  Boat #2 will produce virtually all its required lift from water-lifting surfaces as hydrodynamic lift.  Boat #1 will have lift from water-lifting surfaces too, but will also contribute an amount of aerodynamic lift.  So, even if Boat #1 generates only a small amount of aerodynamic lift, this "extra" lift leaves a smaller part of the total to be made up as hydrodynamic lift by water-lifting surfaces.  

Simply, if our two hypothetical boats each weigh 1500 pounds, then 1500 pounds lift must be generated.  Let us say Boat #2 requires 30 square feet of running surface at 50 mph, to produce this 1500 pounds of lift.  If Boat #1 can contribute (even only) 100 pounds of aerodynamic lift, then only 1400 pounds of water-lift remain to be generated.  Also this 1400 pounds of water-lift will require less surface area too - say only 28 square feet of running surface.  Clearly, by reducing the wetted area of the planing surfaces from 30 sq.ft.  to 28 sq.ft., a reduction in drag of about 7% is realized - and maximum velocity is a function of only the drag and available power.  

In our test case above, our 7% reduction in drag could result in an additional 3 mph!!!


** Results
Think on the above results.  Need we really say more?  We have noted that the water drag in any type of hull is what most heavily affects the ultimate speed, and so we can see that the boat that utilizes aerodynamic lift - with it's reduced wetted surface will have a clear advantage over the boat that uses only water lift.  

For the high performance power boater, water drag is 'Enemy No.  1'.  We accept it only because we are forced to by our need for simultaneous water-lift.  Because the cost of water drag can be 800 times more than that of air drag, for the same configuration, we should try to reduce the need for water lift (and drag) as much as we can.  Even a slight reduction in the wetted surface of the planing surfaces (like vee-pads and sponsons) will result in appreciable gains in top speed, and this effect becomes more and more important as the hulls go faster and faster.  Therefore, any air drag that we must accept to provide our air lift is well worth the price.

** Increased Speed Means Increased benefits
This effect becomes even more important as the available power and maximum speed, increases.  In our example, if we increased our power-plant (the engine) so that we could be achieving 100 mph, the lift savings for the hull design utilizing aerodynamics would more than double, the overall drag savings could be over 15% (reduced) and the resulting velocity improvement could be more like +17 mph!!  The aerodynamic drag also increases with velocity, although the advantages of the lift gained far outweigh the disadvantages of this added drag.


** In future "Rocket Science" articles, we will look at how you can design the water-planing surfaces to be as "lift-efficient" as possible, and to design our aerodynamic surfaces to be as "drag-economical" as possible.

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