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

Then a pivot and a hinge mechanism could provide some adaptability, as you said earlier. Not an easy task. I wonder how many NASA's engineers would it take to make the thing work ? And if the final result would be more aesthetics than the lamp post head of the Gougeon Brothers ...

 

They do have designers with formula 1 links working on Ben Ainslie's team.

 

Yes, this is great. I guess some high end numerical simulations could then be made. One can clearly see some important contribution in their last design, regarding the parasitic drag of the flying hull. On the other side, American Magic made a partnership with AIRBUS, which also makes sense, as it's a natural approach to optimize the shape of these high speed flying boats and to fetch telemetry & real-time data management, in which both F1 and Airplane world are good at.

 

Take a look at American Magic- is the bustle deeper in front, less in back, and the jib endplated? More of a surfboard nose on top of the bustle bow too, methinks.

 

IIRR, Herbulot’s balloon? (Spelling?)

 

I hope I haven’t posted this before, but are there some general (streamlined?) similarities between your optimized db and Rodney Marches Dart keel series?

 

Indeed, there are some inspiration to be taken here. I've seen your thread, also featuring these picture. Makes my head spins....Brain food, as you said.... I've started drawing a final hull, based on what we've learned till now, from these calculations. I've also reviewed the hull shape of other catamarans, so your post comes at the right moment. Many thanks.

 

Please note that I'm a big fan of the Gougeon Brothers. It's just that, at the sailing school, some catamaran were equipped with these floats, on top of the mast. I've always find them very ugly, and as I was able to recover my small boat in every situation, I really doubted about their utility. Although I recognize that is the judgement of a fool, since I know some people who will forever be thanksfull to this device, as it saved their life.

 

My last attempt of getting rid of the parasitic drag on a an AC75. An hull with a bustle, with no gap between the sails (main + jib) and the deck. Same volume, same LCG as the hulls already tested. I've worked on the air entries at the fore part of the hull, drawing a very extreme bow that is, by itself, the entier bustle. Absolutely unforgivable. Should be used in conjuction with a compatible flight control system, and, if possible by the rules, an appropriate flight dynamics design. Sails feet are on the deck, for an optimal sail efficiency. The bustle vortex is completely destroyed by accelerating the flow against the hull as much as possible. The flow separation caused by the bustle is greatly removed, and the ground effect on the hull is minimized, allowing the boat to sail higher, lighter, and safer, on the water.

There are some known design issues :
- The outline of the boat may not be compliant in its fore part. Could be corrected without loosing benefits, since most of the work rely on the volume repartition in the bottom of the hull's shape. But at the cost of a fine tuning.
- There is still a margin of improvements -or degradation-of these benefits, depending on the volume distribution of the bustle. I've drawn a very basic shape that may not be realistic, as I didn't take into account its role in the boat take off and landing.

And one graphical output issue :
- I've put too much transparency in the stream line depicted in the bottom part of the picture above... That's why the line colors, in this region, appear more faded than in the upper region... Not very easy representation for a direct comparison, but I hope that everyone will overcome this issue, and observe, nevertheless, the obvious differences.

But anyway, and without taking out my calculator, one can imagine that this hull shape could be interessant. Since it's a pure aerodynamic work, there should be at least one team out of four, that may have already made incredible progress in this direction.

 

Thanks for this, Alan- engrossing and enlightening. Good stuff!

 

Once more we have a post that gives us a chance to ponder matters and give the brain cells a workout,always a good thing.I notice a mention of reducing the ground effect of the hull in the latest revisions and this makes me question whether it is an entirely good thing to lose the ground effect.I realise that it would make a difference to the drag but wouldn't it also reduce the downforce generated by the hull's proximity to the water and hence the righting moment?My instinct tells me that there is probably an optimum air gap,but gives no hint as to what it ought to be.
Which leads to the design of the foil support arm and how perhaps it too is a compromise between adding drag by immersion to achieve the ideal ride height and maximising righting moment when out to windward.

 

Here’s a vid of New Zealand, and what your work has done for me, Alan, is show how much can be learned by the spray pattern (and I’m thinking, airflow and drag) coming off the foils at different hull heights off the water, as well as how the boats seem to be concentrating on closing the gap of hull to the gravity interface. Thanks again!

 

It is a pleasure for me to share with you this work, as your comments, as well are those of everyone, given me the opportunité to discover some amazing insights about these boats and how they work.

It's really nice to see the vidéo that you've posted. We can almost see the airflow into the spray drag. I must say that it also gives me the idea to think deeper about what the foils, revising what I've done so far, following the intuition of WetFeet.

And maybe plays a little more with my tools.

In the meantime, here is another vidéo, where the british team can be seen testing a very deep bustle.

37th America's Cup on LinkedIn: Big Bustle Advantage https://www.linkedin.com/posts/americascup_big-bustle-advantage-ugcPost-7055033632117194752-EUY9

 

Yes, i share the same feeling. In the previous calculations here AC75 : How to get rid of the additionnal parasitic drag when foiling ? https://www.boatdesign.net/threads/ac75-how-to-get-rid-of-the-additionnal-parasitic-drag-when-foiling.67929/page-3#post-944170,
One can sée that there is indeed a 20% discount to get in the hull drag, with maintaining the average air clearance,of the tested shapes, at less than 1m above the waterplane. Too low or too high gives an aerodyamic penalty.
As you said, these conclusions could not be definitive, as the role of the foils has not been, at all, taken into account.
Having them into the water should :
- increase the righting moment
- decrease the spray drag of the foil
- decrease the induced drag of the foil.
- create other effects i'm not aware of

Cannot let you alone with your intuition, i am preparing a new set of calculations to investigate this, using AVL, a software of the MIT, for which M. Drela has also make some contributions. It is used to determine flight dynamics.

Stay tuned...

 

Would hull ground effect take enough weight off the foils to reduce their drag?

Edit- Watching the New Zealand boat above, it seems that there is less (and a different type of) foil spray when the hull is closer to the water. Air/water drag trade offs?