34th America's Cup: multihulls!

34th AC on Foils!

Spectacular racing on San Francisco Bay today-check it out again tomorrow.
Sure helps one understand the term "wave piercing bow"!

 

34th AC on Foils!

==========================
Regardless of how the actual match is sailed it has already been a revolution:
1) TNZ and Oracle are the largest(longest) sailing foilers in the history of mankind,
2) TNZ and Oracle are the first multihulls designed to be sailed on just two foils,
3) TNZ is the first multihull foiler in the history of mankind to use a single surface piercing foil in conjunction with a rudder t-foil,
4) Oracle is the first mulitihull foiler in the history of mankind to be flown on just two foils under MANUAL control.
And there is more we learn every day......

click:

 

34th AC on Foils!

Interesting(more or less) article in the NZ Herald: http://www.nzherald.co.nz/sport/news/article.cfm?c_id=4&objectid=10838714

 

The reason is to prevent using the windward foil to add to righting moment, ala the Rave. If this were allowed, then the structural design would be very difficult, as the loads would be almost unbounded. If there's no down-force on the windward hull (and no canting of the wing), then the maximum righting moment is approximately the weight times the half-beam, and, since the weight is constrained to a narrow range, the righting moment will be very similar between the boats.

The problem with this is, how to enforce it? There are times when both boards will be in the water, and it's quite possible that the windward board will have a downward force on it. This would be a violation of the rule if they simply said "no downforce". Instead, they have to have something that is observable and measurable.

So the windward board is required to be raised to within 0.5 m when going in a straight line. If the boat is tacking or jibing, when it would be going slower anyway and there's no real advantage to providing additional righting moment, the rule does not apply. This way, teams don't have to worry about someone protesting them because they thought there might have been a momentary downward force on their board during a tack.

You can think of it as an extension of the design rule. The design rule covers how the boat is built and is based on what you can measure at the dock. The racing rules cover how the design is operated.

 

34th AC on Foils!

Tom, given the twist seen in Oracle, what do you think is the potential of downforce from the rudder due to that twist. Seems like it might not violate the rule and be a way to increase RM? There are pictures showing the platform highly twisted with the windward rudder clear and one showning the platform untwisted with the windward rudder in. I'd like to hear your thoughts on this..

Pictures by Grenier- L= twisted, R=foiling untwisted, windward rudder in:

 

Oceancruiser - As the designers have full control over the design of the tee foil on the rudder and the designers are the best money can buy I'd expect that the induced drag will be minimal but the control given for flight will be fantastic. The rudder is used for trimming. As the daggerboard and the rudder cannot have any trim tabs, movable appendages or any triming devices at all, trimming (control ) of the 72 foot beasts will be very touchy. At 30kts you only need a horizontal surface of 1000x425mm to hold up 7.5 tonnes ie the entire boat. But to trim the boat you only need a very small area at the right place. The ideal place is the rudder. Trying to trim the boat via the daggerboards with one axis of control is a serious problem. TNZ have foiled in very flat water, Oracle have foiled in bumpy water hence the wobbly boat. The comments about not foiling in races is quite accurate. The teams will be generating velocity polar plots of the foils vs no foils and the control they have at speed etc etc and making decisions foiling vs being in water. They will be exploring all possible tools at their disposal figuring out their quiver of weapons to take into battle (on and off water ie Coutts trying to back out of the 72's... just stirring the pot) Tradition says that the Defender holds all the cards. Its too early to pick anyone yet. I have now seen images of the dropped section of the panel 1 onto the platform as an end plate. Maybe legal or not, I'm sure Oracle is data logging all variations and figuring out what is best. Peter

 

oceancruiser - Lunna Rossa (LR) probably have the smallest budget of the teams so I'd expect them to be a participant but not a threat. Having a deal with LR allows TNZ to recover development costs and increase operational budgets. LR won't build a second boat, you need to start now to get it by the Challengers Cup date. (anyone have info that they will?) Usually we require two boat testing to sort out boat differences, but these days we can GPS the boats and develop velocity plots very quickly. eg I have a speed puck and can go sailing and come home and print out a polar in seconds. So the teams don't need a two boat testing program for speed development. They do need two boats for sailors development however. Also since we are at a point of foiling development I notice sometimes they have had two different foils fitted. So they can sail port and measure it, then sail stdb and see the difference nearly immediately. Hope this helps. Peter

 

If Oracle is twisting, she is flying her windward rudder clear, therefore T rudder is doing nothing except fly through air. If the windward rudder is down. immersed and working, then Oracle is momentarily not twisting, therefore the rudder is running at zero angle, doing very little in terms of keeping the stern down because it is unnecessary, boat is level.
So we're back to the platform mostly being a twister ... and an unintentional twister.
All this fantasy nonsense about the twist adding down force through the T rudder, or controlling the boat's pitch, stability and angle of attack for the main lee foil ... is delusional. Also it must be playing hell with the rig.
And why try to hold the windward hull's *** down with twisted platform and T rudder anyway? - that's an attempt to be the same as windward main foil (at negative angle of attack) holding down windward hull along with leeward doing the main lifting work - but lee main foil has to work twice as hard fighting the negative windward one.
Understood that such a setup doesn't apply to AC72, illegal, but if it was so, that's twice the weight to lift and way increased drag. Don't care that a couple of foil designs like Rave do this; they're not fast boats anyway - and definitely dogs in lighter airs.

 

From using T foils on the rudders on my Stealth F16 cat, as soon as the T Foil is no longer neutral and starts to have any sort of AOA ( thus creating some sort righting moment ) speed of the boat drops off, its there more to stop pitch poling when you are pushing too hard with a spinny.

Mind you a big beast with tons of horsepower might over come the foil drag and the benefit of a smoother platform maybe the winner.

 

Hi Gary - I dont advocate that Oracle twists their boat to trim it, its a possibility and right now the teams are chasing possibilities. But the latest issue of Professional Boatbuilder has an article on Sam Bradfield and he has been building foilers for over 30 years. He states that "the wing on the rudder changes its angle of incidence with the boat, if the boat pitches up the rudder wing follows it and lifts the stern too. Even in real waves, pitch control can be inherently automatic" So I think this is the reality of the current designs, simple is best and "automatic" is simple, twisting boats is complicated. Peter

 

34th AC

Gary, there was an article on the Daily Sail interviewing Grant Simmer where he said the design tends to go toward the heavy side of the legal range because they need all the RM they can get. If thats true AND if Oracle can control twist to some degree ,it makes sense to me to use the windward rudder to pull down and keep the overall weight of the boat to the light side. The question is, in the foiling picture in my last post, with the windward rudder in, how come the twist is not evident?
The twist would allow them to develop downforce legally from the rudder.
-----
Peter, I've worked a bit with Dr. Bradfield when he bought one of my RC tri platforms to test ideas for the 40 ' Skat. The way he described the rudder foil then was that it starts off lifting(20% of the boats weight) and the load gradually reduces until at "X" speed it begins to pull down-all with no foil adjustment whatsoever-fully automatic.

Pictures-L=model,R=40' Skat

 

On the subject of Sam Bradfield, have any of the America's Cup groups signed him up? I ask because if I were in the shoes of groups other than TNZ and Oracle I'd be wanting to get airborn too, and I'd be looking to bring in people with hydrofoil experience like Bradfield, a Bradfield protoge, Andrew McDougall, Greg Ketterman, or someone from the Hydroptere team. Or I might be tempted to give a dynaplane hull a try. What are the other groups up to?

Lots of info at http://www.foils.org/
As a fun aside, here's a video of someone's hydrofoil prototype: http://www.youtube.com/watch?v=-4IUiIqz3Gw
and another: http://www.youtube.com/watch?v=lEqYTrzaHsc (http://www.c-fly.co.uk/)

 

I'm in full agreement that the inverted T rudders do an excellent job of reducing pitchpoling (I too, have been playing around with foils for too long) - if the nose goes down, T rudder goes to negative attack, does everything right. But on AC72 Oracle, the intention is, from reading the excuses, that twisted platform creates negative T rudder (if it's underwater) which will twist the main working lee foil to positive. It's an attempt to create Rave situation with rudder only ... but doesn't seem to be working very efficiently, imo.
The heavy side of platform weight makes sense - because the AC72's are not overly wide (in proportion to other extremist cats like big Alinghi, 18m2, M2 and the like) also they're carrying the most efficient and powerful rigs ever seen, creating a way overpowered boat.
Regarding the foiling pictures, Doug, maybe the big O doesn't foil very long and those selected shots are caught when the boat appears to be foiling perfectly. Other videos show erratic platform balance ... and horrible twist - when the rudder is doing zero, waving in midair.

 

=====================
Thanks for the videos, Stephen!

 

Surely "almost unbounded" is an exaggeration. If it were true wouldn't Raves and (especially) Kettermans be failing all over the place? I know Hydroptere had to be reinforced, but she seems to be holding together now...