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

"Very" is perhaps a little too much, looking at numbers, but there may be, indeed, something that is not definitively settled here, at least in theory. As you demonstrate with the example of a windsurf, the vortex shedding by the gap between the board and the foot of the sail is well known. A run in "slalom" mode has nothing to do with a run in "heavy weather", with a pocket-handkerchief in you arms, as you really endure these vortices shaking the sail. My guess is that the aspect-ratio of the float - or the hull- should be taken into account, because it seems that, for some given "magic" proportions, these vortices could be either enhanced or diminished.

Above 35kts, I had to take into account the lift force due to the hulls, in order for my VPP to produce results comparable with reality. And everytime there is lift, we see the nose of induced drag. When navigating, this become obvious, because of the sound of these vortices. When focusing on this sound, you become aware of the origin, the extend, and of the strenght of these vortices. Regarding AC75, my guess is that everything has not been said, by now, about these boat. VPP compute equilibrium based on a modelisation of a limited amount of forces, but there is quite a few VPP, taking into account the lift of a flying hull, the aerodynamics of out of water foils, and the interactions among all of these guys.

Should we make, instead, A musical theory of fluid dynamics ?

 

Luckily, no one has an answer to the question of what the theory in musical theory is?

Although the Baroque rules worked pretty well for a hundred years or so, until they didn’t. But were they rules, or theory?

It’s heartening to look at surfboard design- a board for every mood!

 

Here’s a board that was pretty good in waves, although I thought my CRIT d2 was better avoiding bow stuffing in waves.

 

so much fun with the Lechner ! Then the no-nose (with no volume at all)... And finally... the big nose. Amazing evolution !



Anders Bringdal has been one of the first windsurfer to use these type of board... And here goes the IMOCA's and Class 40's, some years later...

 

exactly! Although the formula type boards are almost, if not little airboats, so I think adding ground effect lift to your modeling at speed is to the point.

 

Second serie of calculations. Same outline, same volume, same sails, same foils, keel line modified to integrate a "bustle".



I'm preparing a precise comparison analysis between the two set of datas, and for that, some detailed post-processing of the results are still to be done. But for now, and with regards to the global table given down below, I anticipate the following conclusions, with regards to the aerodynamic behaviour of the AC75 plateform :

- The AC75 hulls do produce a significant lift, and, consequently, an induced drag.
- "Bustles" increase the ground effect on the plateform.
- Without "bustles", the higher the better, from an aerodynamic point of view.
- With "bustles", the lift produces by the sails is increased at low altitude, but the drag of the plateform is increased. The net result is approximatively neutral, in term of driving force left to equilibrate the hydrodynamic forces
- Hulls equipped with bustles should fly low on the water to be better that hulls without bustle, at the same altitude. When fly height increase, the hull without bustle will tend to be more efficient than a hull with bustle.


How can we conciliate the view of the AC75 designers with this study ? "The Lower, the better", from Benjamin Muyl, could be verified by this study, from an aerodynamic point of view. As pointed out by Mikko, the foils and the rudder do produce a high amount of wave drag and spray. We can then assume that the increase of these hydrodynamic drags with the fly height has a negative effect on the AC75 performances, by bringing the lifting surfaces close to the waterplane. Re-enforcing the principle of "the lower fly height".

However, this study demonstrate also that the ground effect on the plateform create an additionnal parasitic drag, in order of magnitude equal to the drag generated by the sailplane itself. Without "bustle", at an optimal fly height, nearly half of the drag of the hull can be spared. Since "bustles" do also play a significant role in the take off and landing behaviour of the boat, their use seems "mandatory". In that case, can a good management of the flow around a "bustled" hull, brings the better of the two world ? My feeling is "yes", and I will show it in the next post. Stay tuned... Cheers,

EDIT : In the file Results.PNG, second chart, in abscisse, it is not the "minimum" height, but the "average" height that should be read...My bad..

 

Air-boards are even weider... Have a look to what was on display, last week, at the "Foil Journées", at our National Sailing School, in Brittany.

Agree. There seems to be something here. That said, it poses questions about the commonly admitted forces breakdown. The thing is that foilers are more "dynamic" boats as non-foilers. Classical forces models used in simplified models are used to compute forces & moment equilibrium in Performance Predictor Software, models which validity could be questionned in case of their application to a non-static problem.

 

Kind of begs the question about wetted surface (air, spray, & water) vs endplate efficiency- I suppose one could argue that this is a more elegant solution, below, albeit upside down compared to an AC foiler- the endplate is the lifting body. Interesting parallels, at any rate, at 100 + kilometers and different Reynolds number?

 

As I understand it, the lower the better to keep windward air pressure up against the hull and, more importantly, against the sail.

 

I agree with you. This point of view is also shared by Mikko. Calculations made tends to the same conclusion. The lower the better.



On the other hand, the bustled hull drag is, in average 35% higher than a un-bustled hull. This additionnal drag comes mainly from the bow region, in way of the stagnation point, leeward side - a red region among the others in the picture down below -. I wonder if there is a way to minimize the aerodynamics of this region, seeking a -35% in the aerodynamic drag.

 

Wow! That was unexpected . What does it look like without a jib? Or no bustle under the bow? And the bustle moved aft under the main? Or would the stagnation point just move aft? What happens if the jib isn’t endplated? This is way cool!

 

Absolutely very interessant parallel... In one hand, Die dicke Bertha, computors, actuators, cyclors. On the other hand, a slip, a pair of tongues, 2 or 3 three funny stickers...

 

Sponsorship where you can find it?

But maybe complete endplate is counterproductive speed sailing in the ditch? Might flow from a small parallel gap between sail foot and deck smooth global flow of a whole craft?

 

Anybody seen this study of Moth sails- non decksweepers vs decksweepers?

https://odr.chalmers.se/server/api/core/bitstreams/e4142b57-4126-41ce-ad32-92e30477aaec/content

 

Speed sailing advice in Windsurfer land, scroll down to ‘closing the gap’- from what I’m seeing here, it seems more important to keep foot of the sail parallel, close to the board, but not touching at any point- maybe a uniform-ish sheet of air escaping underneath the foot is beneficial? IIRR, the entire foot of the sail on the board was considered the ideal for a while, years ago, but perhaps that’s passé?

https://www.surfertoday.com/windsurfing/the-definitive-guide-to-speed-windsurfing