AC75 : How to get rid of the additionnal parasitic drag when foiling ?

Hi everyone,

AC75 should take off in an 8 kts wind. So basically, AC75 spend most of their time flying. What does the flow around the flying hull looks like ? I've made a serie of simulations on bare hull shapes, skipping, for the moment, the crew zone details and the foils.


. Flow separation on a flat deck (top view)

As you can see, there is a quite large separation zone on the leeward part of the hull, starting fore of the mast and extending to the aft. In my opinion, this may be the consequence of AC75 being perhaps a little "too much archimedian drawn", in their fore part.


. Flow separation on a concace deck (top view)

Having a concave deck in this region do improve this phenomenon, but quite an important vortex is still formed behind the boat, fed by the flow passing under the hull.


. Flow separation on a concace deck (front view)

Got any idea on how can we manage to reduce this vortex ? If possible, I will use my calculations tools to quantifiy the effects of propositions you may have. Cheers,

 

Mikko recently posted this video showing the wind shadow effects from AC 72s.
(96) Sailing aerodynamics: Americas Cup/wind shadows/bad air behind boats sailing faster than the wind - YouTube
Maybe there is some graphic post processing that could be done to answer your question.

I was thinking it is just the visual aid we need to discuss high performance (low drag, faster than wind speed) sailing relative to conventional keel boats where drag is dominated by hull drag.

I also see that NZ has refitted last year's winning boat for the new rules to do testing and they are going for bikes in separate cowled cockpits.
(96) Te Rehutai Returns - YouTube

 

Deck plating the boom has turned the hull into a non deck plated boom? So which would create the most drag- letting some flow under the boom to smooth out the flow sucking up under the hull? Or figuring out some way to get the leeward flow around the hull to smoothly join the accelerated flow on the lee of the sail? Although I suppose you could argue that the hull is adding vortex lift like a triangular wing, but didn’t one of the teams have a concave keel on the boat? Maybe that‘s what they were attempting . Anyway, turning the hull into a lifting shape working in concert with the sails could be interesting- the trick would be to move the vortex so it’s pulling away from the sail? Would tightening the hull vortex away from the sail do anything in sum? Maybe the hull should be T shaped in a frontal view? Tuck the vortex underneath the top of the T, although I don’t know if the rules allow concaves like that. That shape does can add to stability, in Archimedean mode, and a wider deck. (Bolger explored this approach in a few of his own designs, and they are better than you might think, both in drag and stability.) And since these boats don’t heel much….. would be much room for all that machinery though? Although maybe all that could be flattened? Fun to think about the hull being a low aspect lifting wing, at any rate. A gigantic Madison Avenue boom?

Edit- come to think about it, the hull could be a lifting body in two directions, although would the drag be worth it? I suppose the teams have already beat this to death? Do the heads of the crew act as a trip on the windward side?

 

Thanks for the links Skyak. Now can we visualise what is like being behind a truck.
Reading the new rules left me with the feeling that, besides an heavy training if the crew, the margin left for passive improvement may be an hard work on the parasitic drag.

Listen to the AC72 flying on the water and it becomes obvious that a great amount of driving force just disappear in flow séparation. Next step for this investigation is to compute the total drag breakdown.

 

Intéressant remark, Paul. I sée no airplanes with big fat endplates at the tip of their wings. Wind catching with the deck in one hand... Creating bigger Görtler vortices in the other hand...

I like the idea of gently driving the flow under the hull. Possibilites are limited by the rules. For instance, concave keels are not allowed. But i am thinking of a shape inspired from jet fighters, although i'm not sure that the boat speed would be sufficient to skip the use of forbidden funny shapes.

I will make computations on T-shapes, V-shapes for comparison, and will highlight the potential gain obtained by reducing the parasitic drag. For fun. Stay tuned.

 

Prada’s keel is a small t, kind of- would this trip flow to a smaller vortex under the lee side of the hull? If it were touching the water, would it seal the sails to the water? Would water skin friction outweigh that advantage? I’ve been wondering about this for years…

 

The Bolger Box Keel- a Famous Naval Architect lauded it when I was in earshot!

 

A fighter with Big Fat Endplates.... Now what ? ... Excellent !

 

Why Does The Boeing 737 MAX Have Split Winglets? https://simpleflying.com/boeing-737-max-split-winglets/

The Boeing 737 Winglets http://www.b737.org.uk/winglets.htm
This is quite long

 

Winglets can also generate forward thrust within the vortex field, like a sailboat tacking upwind. I’d like to see that applied to an AC hull- would they survive splashdown?

https://www.nasa.gov/pdf/89234main_TF-2004-15-DFRC.pdf

 

That is a trainer, not a plane that would go into combat.

 

I've also had a thought about the rear spoiler on the Porsche 911 Carrera RS, which greatly improved the aerodynamic efficiency of the car. Dr. Ing. h.c. F. Porsche AG https://presskit.porsche.de/specials/en/porsche-1mio-911/topic/the-innovations-of-the-911/id-1972-front-and-rear-spoilers.html

Hereby attached, an overview of the culprits and their misdeeds on the poor flying AC75. At lunch time, I've also prepared a T-shape hull and a post-processsing loop, so we will be able to see what is happening on a different shape, and get an idea of the incidence of this parasitic drag. I must say that my poor drawing of the jib may also count as a culprit in this tragic affair, ( Mikko, if you're here, you surely be of a great help ). So, as long as I've not corrected the jib shape, I won't be able to have a meaningfull drag breakdown, for gain or loss to be estimated. I work with CATIA, all the models are parametrized, and comply with the ACM rules. But I just cannot close the slot between the mast and the jib leech, with the given measurements. But maybe I'm mistaken somewhere....

 

This a plane going into combat, and you’re right, no wingtip junk-

 

Couldn't wait to test the T-shape... Very instructive. More comments to go. A very interessant shape, although I didn't use the true "Gate" shape that could be seen on LEQ12.