VO 60 an AC keels...

Does anyone know anything about them?

Do they in any way resemble the Naca sections the rest of us might use or are they creating their own from scratch?

I also read som intriguing stuff about that NOT having a laminar flow along a body's lenght might give better results than having a totally laminar flow.
Ie that having some turbulence at about 60% of the cord would give better "release" at the aft end.

Any comments?

Erik

 

Bump!

Anyone?
:?:

 

Yes, the latest studies show that controlled turbulence creates less drag. The tests show that smaller eddies have less drag. What they did, for both water and air flow is to make either a ridge or a rough surface. They both seem to work equaly. Check "Elements of Yacht Design". They go into detail and give the numbers.

 

Elements of yacht design?

Do you really mean Skene´s?

If it's a recent discovery it wouldn't be there I guess.

Or do you mean Principles of Y.D.? I have the second edition and have not yet seen (or understood perhaps) that info there...

Erik

 

Yes "I meant Principles of Yacht Design". Sorry about the confusion. Skene is OK, but some of the dynamics are rather obsolete. The priciples behind eddie resistance is that the larger they are, the more resistance. However, eddies in the trailing edge make less resistance than those on the leading edge. The sections are designed to minimize this resistance. There is also skin friction. It increases in importance with speed. NACA sections are based on aerodynamic models. They work pretty good for underwater use even though is not the original intent. I think that some of the newer sections are better. Particularly because they take into account the less compressibility of water and cavitation.

 

I'm quite certain that the sections used on AC boats would be custom designed for the specific application, taking into account the size (chord) of the keel, the expected speed range, applied loads, and deflection of the "trim tab" (it's really a plain flap). Take a look at a modern book on section design, like Eppler's "Airfoil Design and Data" (Springer-Verlag; currently out of print). If you want to experiment with your own designs, the XFOIL program is pretty close to the current state of the art in 2D section design.

The problem with a laminar boundary layer is that it separates sooner when it encounters a region of increasing pressure. The after portions of all sections have this characteristic because the pressure has to increase from its minimum value (dictated by thickness and lift) on the forward portion of the foil to a little above the ambient pressure at the trailing edge. So you want the boundary layer to transition from laminar to turbulent before it hits that increasing pressure (called the pressure recovery region). Since a turbulent boundary layer can stand a more dramatic increase in pressure, the recovery region can be shorter and steeper, and therefore the laminar boundary layer can be carried farther aft, for less drag and a more energetic start to the pressure recovery.

It's like driving up to a stop sign. If you are driving on ice, you have to start slowing down much earlier and you can't drive as fast to begin with. On dry pavement, you can drive faster and slam on the brakes just before you get to the intersection. So just like good braking allows faster stop-and-go driving, transitioning to turbulent flow at the right time allows more laminar flow and higher maximum lift.

The whole art of section design is to shape the pressure distribution about the section so as to influence the boundary layer development.

The transition from laminar to turbulent flow in the boundary layer can be tripped by irregularities in the surface, like roughness or a ridge or a depression (the dimples on a golf ball) or even jets of fluid blowing out from small holes. The problem with turbulators is that they trip the flow at one place and there is an additional drag penalty for the turbulator itself.

At low lift coefficients, you'd like to carry the laminar flow as far back as possible for minimum drag. But at higher lift coefficients, the peak velocity is higher and the minimum pressure is lower - so the amount of pressure recovery is greater and it takes a longer distance to get it done. This means you'd like the transition point to be farther forward at higher lift coefficients. If you position the turbulator to handle the high lift case, then the transition occurs too early for the low-lift case, and this impacts the minimum drag. If you position the turbulator for low drag, the laminar flow will separate ahead of it and sharply limit the maximum lift.

A modern design will take into account the way the pressures change with angle of attack, and have a shallow region of increasing pressure that will trigger transition by itself before the steep pressure increase begins. This way, the transition moves smoothly forward at high lift and back at low lift, changing gears like an automatic transmission.

You can bet the AC boat designers will use these techniques to create a keel that can accelerate out of a tack and still have low drag upwind and down.