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

Cool stuff. The spray coming off the leeward foil in the pic of Luna Rosa, and your flow lines at that area look very similar- and also the splashing at the aft part would seem to be in less wind pressure, again, as your simulation shows, unless I’m reading too much into it. Kudos. How high from the water is your simulation? I wonder if the LR has a sweet spot, as far as height from the water of the keel, and characteristics of airflow? Maybe a dynamic of boat speed, wind speed and sea state?

I wonder how much difference there is between Ground Effect and end plate (keel touching the water) - my hunch has been endplate might help during takeoff, but possibly detrimental at speed?

 

Fascinating thread and it gives much to think about.does the CFD study extend as far as calculating the air pressure beneath the hull on the windward side?I have a feeling that there may be an optimum gap between the lowest point of the keel and the surface of the water that will both keep the hull clear and maybe generate a modest amount of lift.I can only speculate that this may be the case and it may be too complex an exercise to calculate both aero and hydro results.

 

Does that beg the question of a flat bottom sealed with parallel-ish fences (keels?) on both sides-from the front, very crudely:

\—-/

Or

|—-|

Or

|__|

so kind of like an air boat (or bizzarro Hickman Sea Sled/Laser Vortex/Naish Pan Am Race Board/multi concave Windsurfer hull/hydrofoil scow moth) with side lifting foils partially supported under the hull by a cushion of air/foam that also smooths and messages beneficial air flow around the hull, deck, and sails?

we have entered the era of the scow, after all…. (The Medium is the Message?)

I think I’m getting beyond the scope of this. Apologies. But there is the issue of flying too high, and what that does during a crash.

 

Thanks, Paul. These are the heights that I used by now for the simulation :



I think that I may be build a test matrix, to at least have an idea of how bottom shape and fly height may affect this ground effect. Let's see... How about this :

└┬┘ └─┘ ├─┤ \_/ \--/

3 heights : Touching the water, medium, Maximum

3 apparent wind speed : 8kts, 25kts, 40kts​

ISO Longitudinal Center of Gravity, Total Volume, Displaced Volume, Frontal Area​

It may also brings additionnal informations to Wetfeet, if pressures are presented for both sides of the boat.

It is sure quite tedious to mix aero and hydro calculations. In this small exercice, I wish I could keep the feet of this problem out of the water. Do you think the test matrix could at least be of some meanings ?

 

I would suspect that the teams have run some simulations to investigate the forces.Equally,I don't suppose they would circulate the information,so any light that can be introduced would give us an insight if the amount of work isn't prohibitive.

 

I'm preparing the matrix calculations, and work estimate. In order to validate the speed matrix, I found the thesis - https://scholarshare.temple.edu/bit...riguezNunez_temple_0225M_14734.pdf?sequence=2-. which contains very valuable data analysis of foiling AC's - not very much background, though -. In consequence, I may reconsider the speed range for used for my calculations, especially the lower speed, which do not make sense in the scope of my intended static calculations.

I've also spent some times this week-end with the parametrized model of the hulls, and recreate INEOS Britannia I - "The trawley bus";


And could not be but amazed by the evolution that has been made in the design of INEOS Britannia II



Not to mention the test of a keel plate that has been performed between the two version :

Considering the simulations that I've made so far, and looking at the work of the british team INEOS, I agree very much with your comment, Wet Feet. The Ground Effect and the reduction of the Parasitic Drag may have been quite an important amount of work ; in particular around the keel plate and the hull shape. What is the exact role of this keel plate ? I wish we could have some hints by flow visualizations. Comparing the simulations results with the work of "the Brit" will certainly be of a help. Stay tuned...

 

Thanks for the link to that thesis which I will attempt to digest over a day or two.I am wondering whether the keel plate is performing the function of a "waterski" as the boat drops off the foils and as such helps to delay the dramatic increase in skin friction as the hull and foil arms descend.

 

These steps are called "bustle" and support the foils when taking off.
America's Cup: Two team designers analyse AC75 hull shapes - Tip & Shaft https://www.sail-world.com/news/233558/Americas-Cup-Two-designers-on-AC75-hull-shapes

 

Excellent article. There may be also some additionnal important infos that are not said here. As a multihull sailor, i recognize that having a linear variation of the displaced volume with sinking greatly helps flying the hull. The aerodynamic role seems also quite important, consider the great differences of this bustle between teams.
Benjamin Muyl "It's not completely objective, and you need to imagine things."
So is it possible that some teams have came to similar shapes based on completly different concerns ? Or that different shapes may address identical issues ?

 

If you look at the AC 75 drag breakdown attached, you will see that there is not much to be gained in the hull air drag. Foils are most important, even tha rudder twice as important as the hull. And of course, the "negative drag part", sails + mast.

 

These diagram are really handy. Are they from real data, or simulations ? I have a Rans simulation baking right now. I will put my numbers on this forum for comparison, because I'm very astonished by the windage contribution of the bare hull.
I sailed on multihulls above 35 kts, and the air noise on the boat was far lighter from what i've heard on the Acc. Besides the vortices from the mast and the rigging, not much sound come from the multihull plateform. That's my clue. At these speed, the main drag is generated by the daggerboard, and is like 10 times the hull drag (hydro + Aero). Still, given a span and a surface, there is not much to be gain between different profiles.

How much do you think can be gained from the sails and the foils of an Ac75 ?

Also, how do you interpret the remark given in the article given by WetFeet, regarding the flying height : "the lower the better" ? Isn't it a sign that the ground effect has a role in the overall navigation performance ?

 

The diagram is theoretical, from a VPP for a generic AC75 which corresponds fairly nicely to measured sailing data.

I don't think you could gain anything much at all from the sails, as I'm sure the sailmakers know their job ;-). Altough I'm not convinced that the double sided sail with the D-mast is necessarily better than a wing mast with fully battened normal sail would be. But that's in the rule. From the foils... that's where the gains to be done are.

I wouldn't perhaps call it ground effect, but yes, sealing to the surface will improve the efficiency of the sails.

 

Having had a bit of sailing experience (not design) in the early early days of foiling windsurfers, getting up to lift off speed, speed while staying up, crashing (whether fast or slow) and recovery seemed to me to be the main issues. (Crashing above all.) I thought a speed needle hull would be the best, since it was gentle at speed in chop or swell, vs a surfing hull, but IIRR, lighter, flat, and smaller hulls came out on top, although they really stuffed their nose in a crash.

And then there’s rocker and prismatic which the IMOCAs are discovering.

Endplate, control, and wetted surface all are at interplay, and all seem at odds, at least for now. Add to that bluff bodies eek!crew running around and messing with the aero. It looks like the hull/water endplate under the mainsail gets in the way of steering, so that’s being eliminated, and yet main/deck endplate seems to be important for some of the teams. But from your first run, Alan, it seems that there is a hull generated upsweep vortex aimed at the boom, so endplate there is important assuming that the speed, rotation, and rotational direction of the vortex at that area lead to very low pressure?

(Given the delightfully subjective aspects of design here, Gnarly stuff! Yay! (But the more I look at hydrofoil boats, the more it looks like Surfing & Windsurfing design & practice has been here already. I wonder if any of the teams have messed with human sailed long board windsurfer sailing hydrofoil models just to feel what’s going on- for example, sail/hull/ water endplate ( although without hydrofoils) has been solved in windsurfer longboard race designs- see pic above)

so, hull bottom set up for a bow crash shape (maybe similar to Milizia‘s IMOCA bow?) get rid of the jib, move hull/water endplate back under the main? Starts looking like an old school full keel boat or an Iren’s Rozinante? With a crash optimized bow, and a skinnier stern, could the prismatic be optimized lower than seems to be current practice to accelerate up to foiling speed more quickly, and have better sub foiling hull speed in light air?

how would all this change aero flow around the hull?

 

Here is where I am in my calculations, on type └─┘



Cannot directly compare with Mikko's calculations because the navigation conditions are not the same, and also I'm only calculating aerodynamics of parts above water, but still, I think that the hull drag is under-estimated in the VPP calculations given in 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-2#post-944027. Having speed outputs in close agreement with reality does not mean that the forces breakdown is correct ; errors can compensate. Only verification of each and every forces with actual load measurements can give a true validation. The same goes, unfortunately, for my calculations, because they should be taken with care. Nevertheless, I've made several computations with different meshing sizes, in order to come to a convergence in the calculated forces. Again, convergence does not mean that the result is good. It only means that there is no mathematical inconsistency in the results.







What is interessant, is that the Hull+Foil drag (Fx), is found to be of the same order as the drag generated by the sails. Also, we can see that there could be, indeed, an optimum fly height [WetFeet], with regards with the plateform drag (Hull+Foils+Sails). Finally, it is interessant to see that, indeed, the hull does seem to produce a non-negligible amount of lift, flying like a low aspect wing [Paul Scott], that is consistent with the parasitic vortex that i've previously highlighted. I will continue this study for other hull's shape and bustle type.