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The Aerodynamics of Unlimited Hydroplanes

By Doug Ford
Reprinted from h1unlimited.com.

In the 1930's and 1940's, aerodynamics played little or no role in the performance of the Gold Cup boats and Unlimited Hydroplanes.  But, in 1955 the boat racing fraternity got a rude awakening as Lou Fageol and Slo-mo-shun V performed the first ever 360 degree blow-over while at high speed on the backstretch of his final lap of qualifying for the Gold Cup in Seattle.  The boat had shown a bit of a tendency to fly or kite in testing, so in following the generally accepted premise that most lift was generated on the upper deck like on the upper surface of a wing, the crew installed three spoilers across the forward deck in an attempt to kill the excess lift.  Clearly, it didn't work.  But, it got us thinking, "what happened?"

Several years later, German aerodynamicists learned that a very large amount of lift can be generated on the lower surface of a ramp with side skirts, oriented at some angle of attack and placed very close to a surface , for example  either land or water.  This effect, which is known as "ram effect", "ram wing", or "surface effect" generates its lift on the lower surface rather than the upper surface like a conventional airplane wing.  Further, it generates much more lift than the increase due to what is known as "ground effect" which can increase the effectiveness of the wings on aircraft when close to the ground.  Looking back, it is easy now to see why putting spoilers on the forward deck or "upper surface" of the Slo-mo-shun V didn't have the desired effect of reducing lift.  Things would have worked much better by blocking some of the air from getting under the boat, which is why "fences" placed under the leading edge of pickle-fork hulls work as a way of reducing lift, at least while they are not in a nose up attitude.


Today, I use wind tunnel models to determine the lift and stability of a design at various pitch, yaw, and roll angles and at various distances off the water.  I'm often asked how this works, and if we have to test the models at the same speed as the boat expects to go.  Well, the short answer is we don't have to test at 200 MPH to understand what aerodynamic the real boat will feel at 200 MPH.  Here's how this works:

Aerodynamic forces (and hydrodynamic forces as well), such as Lift and Drag are dependent on shape, area (for example wing area), and what is called dynamic pressure or what aerodynamicists call "q".  Dynamic pressure is the pressure in pounds per square foot you feel pushing your hand back when stick it out a moving car, and is a function of air density and velocity multiplied by velocity (velocity squared).  This means that the aerodynamic forces increase very rapidly with speed.  For example, an increase in speed from 140 MPH to 200 MPH doubles the aerodynamic forces if everything else is constant. 

Now if a wind tunnel model is the same "shape" as the full scale boat, then the only things that cause different forces on the model in the wind tunnel from those on the real boat are the model "area" and model "q" versus the full scale boat "area" and "q".  So, if we measure forces on a model, then divide by the model area and the model "q", and then multiply by the full scale boat "area" and the full scale boat "q" we can determine what the aerodynamic forces would be on the real boat. 

The "area" or "reference area" of the boat can be pretty much anything as long as we use the same scaled area on the model as what we pick for the real boat.  I generally use the area of the ramp on the bottom of a modern unlimited, that is, the width of the air trap, say 7.67 feet (92 inches) multiplied by the length of the bottom before the break, say 18 feet.  This example gives a full scale reference area of about 138 square feet.  My wind tunnel models are usually 1/8 scale, so the reference area of the model's ramp would be 2.16 square feet.  If I test the models and measure the forces at approximately 100 MPH, that gives me a model "q" of about 25.6 pounds per square foot.  Then, using lift forces as an example, I simply take the measured model lift force from the wind tunnel, divide by the model ramp area of 2.16 square feet and by the model dynamic pressure of 25.6 pound per square foot. This gives me what is called the Lift Coefficient or, CL.  I then multiply that CL by the real boat ramp area of 138 square feet and by the real boat dynamic pressure of 102.3 pounds per square foot (for 200 MPH) to get the lift forces on the boat at 200 MPH.  Of course, the real boat has to be in the same attitude and distance from the water as the model for this to be representative.  The same approach works for determining drag.

It sounds complicated, but it is really pretty simple.

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Comment by Jesse Alvitre on February 12, 2015 at 5:43am

What is the purpose of the "Deck Fences" or "Deck Trap"?  I noticed on the opposite side of the turnfin and only on one sponson.

Comment by Joe McCauley on May 13, 2012 at 6:36pm

Have you run any wind tunnel tests on tunnel boats? Too many are made with the center of pressure

far too far forward of the center of gravity. That could be remedied.

Comment by Robert Beresford on February 28, 2012 at 11:14am

A peek behind the curtain!

Comment by Marc Connelly on January 18, 2011 at 3:01pm
Fascinating stuff. Thanks, Doug (and Dale). I have a thousand questions, for later!

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