NACA Foil Sections

[Christian Williams]

I'm completing the Cape Horn vane installation, and find the pendulum oar oddly clutzy looking. But I may be wrong.

In the photos it's in the storage position, not hanging vertically in the water slipstream. The oar functions by rotating in the water when turned on its vertical axis. The force of the passing water thrusts it off centerline, and that force is used to steer the boat by lines to the ship's rudder.

Most daggerboards or rudders are not as high-aspect ratio as such an oar, so maybe the extreme camber of the leading edge is necessary. The foil always stalls anyhow, it just gets shoved aside like a barn door, more simple deflection than lift.

But how about that blunt cutoff at the tip? Dinghy rudders are tapered, and so is the wingtip of a Spitfire. Yet many trainer airplanes have blunt wingtips, as if it didn't really matter at low speeds (Wing tip vortices have been much studied, just not by me).

Just wondering if anyone with an interest in NACA sections has an opinion. I could easily reshape this chunk of mahogany to look more like what I'm used to--which might not be smart. The Sailomat foil was certainly not as thick and much more graceful.

......

(Yes, I'll report on the installation soon. Had one test sail, and it works OK. Took about a month to get this far).

 

[gabriel]

Since that foil is wood what keeps it from floating back up while deployed?

 

[Christian Williams]

It's fixed to its own quadrant and held by control lines.

 

[markvone]

Christian,

The truncated foil is a Kamm tail. Developed in automotive design by Wunibald Kamm for aero benefits with a practical shape vs a long tapered tail. I believe the steering oar benefits from the elimination of the delicate tapered tip which is prone to damage. The best description of the aero benefits (and picture) I found was actually from the Trek bikes site but there are plenty of automotive examples. Bike frames now have aero shape restrictions for the pros.

Kammtail Virtual Foil

www.trekbikes.com

Baby Kammback - A Short Synopsis of the Kamm Tail Design - WOB Cars

Take a look at a 1962 Ferrari 250 GTO, then a 1997 Chevrolet Corvette, now a 2006 Honda Insight, consider then the 1970 Citroen SM, and finally, look at Audi’s 2017 A7. Take all of those cars and evaluate their most significant design commonality – the rear fascia. Each of these automobiles, a...

wobcars.com

Mark

 

[Kenneth K]

The foil always stalls anyhow, it just gets shoved aside like a barn door, more simple deflection than lift.

I think that's the key--it's not really a lift-producing foil, but more like a barn door as you say.

You could taper the leading and trailing edges and claim that you theoretically reduced the drag of those blunted surfaces, but the effect would likely be completely immeasurable on a 7 ton sailboat. But it would look faster! Likewise, you could put a more Spitfire-like end on the foil tip, but the tip (being furthest out from the point of rotation) creates the most rotational force, so you wouldn't want to remove too much material there. As long as you don't materially weaken or shorten it, I think minor alterations would affect appearance only.

I got a kick out of that Trek marketing ad: "Designing airfoils for bicycles requires a great deal of creativity, as the traditional designs utilized by airplanes and automobiles do not consider the unique properties of cycling aerodynamics." Well, duh. Tour de France riders average 24 mph (edited), airplanes 75% the speed of sound. At the Olympic level, maybe it's worth exploring the benefits of airfoil shaped bike parts. The rest of the bike riding population would see a much much higher performance improvement (and save a lot of money) by shedding a few lbs of belly fat.

 

[Rocinante33]

Christian,

The truncated foil is a Kamm tail. Developed in automotive design by Wunibald Kamm for aero benefits with a practical shape vs a long tapered tail. I believe the steering oar benefits from the elimination of the delicate tapered tip which is prone to damage. The best description of the aero benefits (and picture) I found was actually from the Trek bikes site but there are plenty of automotive examples. Bike frames now have aero shape restrictions for the pros.

Kammtail Virtual Foil

www.trekbikes.com

Baby Kammback - A Short Synopsis of the Kamm Tail Design - WOB Cars

Take a look at a 1962 Ferrari 250 GTO, then a 1997 Chevrolet Corvette, now a 2006 Honda Insight, consider then the 1970 Citroen SM, and finally, look at Audi’s 2017 A7. Take all of those cars and evaluate their most significant design commonality – the rear fascia. Each of these automobiles, a...

wobcars.com

Mark

mark,

You are right about the Kamm-effect tails on cars. But quite a while before the ugly production cars were some beautiful and unique race cars with Kamm tails. Most notably, Ferrari GTO 250 and the fabulous, unique, Peter Brock designed Shelby Cobra Daytona coupe. Worth millions now, what a great car and way ahead of its time in 1963-64.

Peter Brock and the Daytona Coupe

How the Shelby Daytona Coupe Changed Racing Forever

While the Cobra gets all the glory, Carroll Shelby's innovative Daytona Coupe opened the door for a new era of American racing dominance.

www.motorbiscuit.com

 

[windblown]

mark,

You are right about the Kamm-effect tails on cars. But quite a while before the ugly production cars were some beautiful and unique race cars with Kamm tails. Most notably, Ferrari GTO 250 and the fabulous, unique, Peter Brock designed Shelby Cobra Daytona coupe. Worth millions now, what a great car and way ahead of its time in 1963-64.

Peter Brock and the Daytona Coupe

How the Shelby Daytona Coupe Changed Racing Forever

While the Cobra gets all the glory, Carroll Shelby's innovative Daytona Coupe opened the door for a new era of American racing dominance.

www.motorbiscuit.com

I love how a wind vane question becomes an exploration of today’s bicycle technology and classic automotive racing development and drama. Thanks, gentlemen.

 

[Christian Williams]

Good insights on the initial topic, too. I think I'll reshape the end a little just so it doesn't look like a cricket bat, but not worry about the overall section.

 

[Loren Beach]

Challenging.... to understand just how a wind vane works, and cricket is totally impossible !

 

[footrope]

It looks like a cricket bat or two would be cheaper spares than the real thing. Are you concerned about breakage?