34th America's Cup: multihulls!

Thanks Doug - good find. No real new info really but a couple of interesting things. For me the righting moment numbers are a key. USA-17 trimaran was 220T.m, an AC72 was 50t.m (with crew 60T.m by my estimate) and the new "AC62" will be around 25T.m. A Mod70 is 65T.m. So we have a very "small" boat by current standards ( to lower cost is the logic). The design speed will be the 40-50knt range. They say it will be slower then the AC72 but they never expected the AC72 to go as fast as it did. Development will just make it faster and faster. Cavitation will be the limiting factor plus aerodynamics. I don't think we will see the spider web under the boats anymore. The other thing is will they allow the windward board to pull down? In AC34 this was not allowed but this seems to be the easiest way to hold the boat down. Melvin talked about being able to create Rm by reverse cambering the wing at its head but this seems to be the hard way to do it especially as the manpower/horsepower is limited via less crew (unless they allow stored energy this time?) Plus do we have a genaker or jib? Its been demonstrated that we don't need it many times so a singular rigid wing maybe it? So many questions yet unanswered and so many people waiting.... Would have been good to hear question time!! Peter s

 

I went to see the battle of the boats talk live, very interesting! I kind of get the vibe that the Oracle boat at the end of the day had the higher speed potential, with less aerodynamic and maybe hydro drag at high speeds. I think NZ could have foiled upwind but for them it was not the fastest mode (maybe I read this somewhere, can't remember).

It was the whole sequence of events across the final 8 or whatever races that resulted in the Oracle win at the end of the day, some of it luck.

 

BTW it was definitely "NZ" versus "Oracle" here in NZ, which I'm sure would make an interesting psychology study.

 

Yes, you can hold the cat down with negative attack on the windward foil ... but it is slow ... and draggy ... and that windward hull will have to be kept flat, near level for it to work ... and I doubt that anyone will do it ... because one foil is fighting the other, plus the down force so resulting from the windward foil is making the hard working leeward one fight extra weight/down force.
If it was a fast mode, then with all the brilliant minds at work during AC34, you would have seen it (or heard about it) - but the only time both foils were down was in the few seconds of transition doing a tack or foiling gybe, and then the windward foil was promptly lifted.
I know there was an AC ruling against both foils being down for longer periods - but imo, even if it was legal, it wouldn't have occurred.
I mean, drag is drag and look at the lengths the designers went to airfoiling/streamlining their boats and crew - and water drag is far more damaging to performance than air drag; even though air drag at 50 knots AW over the boat is hugely significant.

 

34/35

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If I remember correctly, the rule prohibited downforce from the windward foil.
Greg Ketterman would disagree that having downforce from the windward foil is slow and draggy: his "Long Shot"(name?) version of the Hobie Trifoiler set a few speed records.
I doubt they'll allow it - costs would increase a lot because of the extra weight required to handle the enormous RM that would be possible.

 

I believe that for a given boat (wider is better for this topic), heading and wind speed, there may or may not be an advantage.

Longshot and the Hobie trifoiler were very wide and made good use of the concept.

From the C Class Rocker effort, those involved seem to feel that the narrow beam war a big part of why it did not work better.

For the AC, they would worry about safety concerns. If you use the concept, the whole structure would be more stressed normally, but then the stresses go through the roof with a big gust.

 

34/35

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The oversquare platform also reduces the loading on the foils on a boat using two independent altitude control systems. Dr. Sams Rave had a "never exceed" speed-I think it was thirty knots but I'm not positive. Anyway, the story is that a guy in Spain ignored that and broke the huge(5" ,I think) main crossbeam.
Based on Melvins video I'm fairly sure they've decided on a "scaled down" 72
beam for the specific purpose of reducing RM.
And as good as UptiP foils have gotten I doubt we'll see any wands.

 

Off Yer Rocker had four large T foils down all the time - and was draggy plus and slow; an experiment and the team found out it didn't work.
I also know the situation well.
To me it is stupid plus to have two main foils fighting each other; the platform has to be even stronger (and heavier) to take the contrasting loads, the rig has to be also.
The small Kettermans' trifoiler worked in flat canal waters - but large Hydroptere (one main fall working, the other, to windward, non negative attack foil flying) was considerably faster.
I wondered how Longshot would have behaved in waves if the windward foil (if was fixed negative; it wasn't because of wand) and popped clear - the capsize would have been very rapid.
Not knocking the brilliant Kettermans: the double low rig and wand actuated foils did their job extremely well ... but different story with larger craft. Just imo of course.
ps: Doug, read my post again: there was mention of the illegality of fighting foils.

 

As far as I know , Rocker was 14' wide to the tips of the foils(C Class rule) and that was too narrow to effectively use Bradfield's dual independent wand system .
I never liked the Ketterman "feeler" system but it sure was capable of high speed in the right conditions and he thinks(and proved a couple of times) that it was faster than a t-foil/wand system.
The new UptiP foils are such a breakthru when you consider what they are capable of: taking 80% of the load on a single main foil!

 

I think the windward foil idea on a trifoiler was to allow it to go around corners very quick and it sure does that.

 

Ketterman--DLA

Heres what Keterman says about his foil system:

HYDROFOIL SAILBOATS IN GENERAL
"Hydrofoil boats can be categorized into two categories; 1) Incidence controlled hydrofoils* and 2) surface piercing hydrofoils. The difference lies in the way the boat maintains the proper altitude above the water surface. A surface piercing hydrofoil boat maintains proper height by varying the amount of foil submerged. The boat raises up as the speed increases and reduces the amount of foil submerged and therefore the lift. The boat finds equilibrium at the proper altitude. An incidence controlled hydrofoil sailboat has a mechanism that controls the angle of attack of the foil to maintain the proper altitude. It is generally believed that surface piercing is simpler, but incidence control is more efficient. In reality, it is the method that works with fewer problems that is simpler.
From the beginning it was felt that incidence control was better suited for a sailboat even though most of the existing hydrofoil sailboats were of the surface piercing type. There are many advantages of the incidence controlled foils; however, the most important is what I call the DLA (dynamic leveling affect). This is the increase in righting moment or stability due to the ability of the windward foil to pull down. The DLA has little affect on the low wind performance, but it essentially makes the top speed of the boat limited to the strength of the boat. Conventional boats with a finite amount of righting moment can only extract so much power from the wind, but with the DLA, the righting moment is virtually unlimited.
Intuitively many people think that the added drag of the windward foil plus the increased induced drag of the leeward foil would offset the gain in righting moment, but calculations show and practice proves otherwise. The dynamic leveling affect not only produces a dramatic increase in top speed, but is also responsible for all the other key features that this stability provides.
The other major advantage of the incidence controlled foils is they are less affected by the waves and other surface affects. Drag and losses associated with the surface are the major reason incidence controlled foils are more efficient.
All hydrofoil sailboats have problems with ventilation; however, surface piercing foils have larger problems, because the foils are piercing the surface at a smaller dihedral angle which makes it easier to ventilate."
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* On the Trifoiler the entire foil was moved to control RM, lift and negative lift hence the term "incidence controlled foils". On the Rave the incidence was generally fixed at +2.5 degrees for the main foils though some owners found a way to decrease the incidence on the windward foil. Lift and negative lift on a Rave foiler is generated by the wand (designed by Dr. Sam Bradfield), a surface sensor(dragging in the water) and attached directly via linkage to a flap on each main foil. The wands are independent just like the trifoiler "incidence controlled" foil sensors.

 

Adding Righting Moment - Weight Versus Windward Foil Downforce

The fact is, both of these quotes are right, but they both neglect to specify under which conditions they apply.

When I was designing Broomstick (my 15' trimaran foiler), I went through a VPP analysis to try to determine how best to increase the righting moment when the boat became overpowered. If I needed moderate increases, adding weight was better than pulling down with the windward foil. But as the wind speed was increased, eventually the situation was reversed.

I'm quite sure that an AC 72 with downforce on the windward foils would not win very many races (or any), but that Longshot would not have set any speed records without it.

 

This is the basic issue. Weight is fixed, while the aero and hydro loads scale as Vrel^2 and Vboat^2. If you're using dynamic lift (foil download) to provide the righting moment, then theoretically you can trim the boat at any wind speed. In that case the only thing which limits the boat speed is cavitation, or structural loads which also scale as V^2.

You can look at this in terms of dimensional analysis. For a given boat shape (aero rig + hydro rig), we can say

Vboat/Vwind = f( Cw, Re, Fr, Ca )

Cw = weight / (0.5 rho Vref^2 Sref)
Re = Reynolds number
Fr = Froude number
Ca = cavitation number

Re >> 1, and for a high-speed hydrofoil Fr >> 1, and their precise values don't matter too much. In that case we have

Vboat/Vwind = f( Cw, Ca )

If the boat is much lighter than the possible aero and hydro loads, then Cw doesn't matter so much, and we have

Vboat/Vwind = f( Ca )

Assuming the boat is stable and doesn't break, then the only thing which limits boat speed is cavitation. Sailrocket is in this parameter corner, since its aero load is much greater than the weight. Not sure about the Trifoiler, but it's probably close to that corner.

 

Think of the local velocities around the vertical part of the board and the horizontal wing being superimposed on each other. If you have a sharp corner, then you get a substantial increase in velocity at the joint.

This is a problem for a couple of reasons. It means the peak speed is higher, which means there is a sharper deceleration of the flow towards the trailing edge, and a greater propensity for separation and increased drag. Just the higher speeds alone will increase the skin friction.

At high speeds, cavitation puts a limit on the local velocity, so you see cavitation starting in the junction when the boat starts going fast.

In addition, if there's an abrupt change in the lift between the two panels, there will be a strong vortex shed into the wake, which increases the induced drag.

When you increase the radius, you move the vertical panel and the wing farther apart, so they interfere less with each other. This reduces these adverse effects in the junction. The lift distribution can change more gradually and shed less vorticity into the wake.

 

34th AC

Innovations in AC 34:
Engineers, designers, and equipment specialists are essential components of a winning Americas Cup team. ORACLE TEAM USA Chief Engineer Dirk Kramers and Wing Designer Scott Ferguson will discuss their fascinating careers and their experiences on the 2013 Americas Cup winning team. They will also review recent innovations in America s Cup design and discuss how those changes have affected their work and the competition. A thirty-seven year veteran of the Americas Cup community and proponent of multihulls, Kramers has been part of five winning teams. Ferguson has participated in two winning Americas Cup teams. He is a Naval Architect and a specialist in the design of carbon fiber grand prix racing spars.


http://vimeo.com/100814145