AC34...not over yet...

Iain C

Iain C

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What's this? Oracle doing some slightly borderline stuff to their catamaran? Surely not, they've got a squeaky clean record haven't they...ahem...

http://www.sail-world.com/index.cfm?nid=115070

And the rumour is that the AC is about to go back where it belongs...to the courtroom...as ETNZ challenge whether the system was legal or not...

And just when you thought that they'd found the hyperdrive switch by chopping the bowsprit off and putting Ben at the back...
 
Once the input of sir Ben was dismissed as secondary to suddenly having a fast boat it was only a matter of time before the details of the 'boat adjustments' that took place overnight and gave oracleUSA their turbo boost was questioned and eventually protested.
 
There's no doubt that the after guard were excellent...it was a pleasure to hear the information flow at the back of the boat. But when my SWMBO, who is admittedly a sailor but not a massive follower of technology or the cup, says whilst watching "I cannot believe how rock steady Oracle is on the foils compared to ETNZ or how they were sailing a week ago" it was obvious they'd made some pretty big changes to their foil package!
 
Iain C said:
There's no doubt that the after guard were excellent...it was a pleasure to hear the information flow at the back of the boat. But when my SWMBO, who is admittedly a sailor but not a massive follower of technology or the cup, says whilst watching "I cannot believe how rock steady Oracle is on the foils compared to ETNZ or how they were sailing a week ago" it was obvious they'd made some pretty big changes to their foil package!
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Perhaps the kingpost fairies snuck in and did some more wholly unauthorised modifications.
 
The author has made some mistakes. Dutch Roll is a lateral oscillation. Longitudinal oscillation is termed Phugoid.

He also says that .. would have allowed for lower drag foils, whilst previously acknowledging that the foils were not changed.

Oracle's early problem was that she was sailing bows down too much, so catching wave tops with the bows - not fast.

A simple tweak of the rudder horizontal foils might well have been enough to fix the problem. ETNZ had those too, and they are semi-automatic anyway, much like a decent aircraft (glider or airliner), fine trim is used to correct trim as speed or CofG changes over time, but the overall stability is designed in as a dynamic system.

The truly automatic foil system that allows pitch/ride and more importantly - wave contouring' only really works with fully immersed foils. Look at the Boeing tri-foil used commercially, and relatively successfully. One used to do Brighton/Dieppe in the 80s.
 
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boguing said:
The author has made some mistakes. Dutch Roll is a lateral oscillation. Longitudinal oscillation is termed Phugoid.
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I think what the author says is correct as far as it goes but I can't see the relevance. Dutch roll is an oscillation around the longitudinal (roll) axis caused by asymmetric lift resulting from oscillation around the vertical (yaw) axis.

The problem here must surely have been instability in pitch, which I think is what you mean by longitudinal oscillation although that isn't the normal term. Strictly speaking phugoid is an oscillation in pitch i.e. around the horizontal axis resulting from exchange of kinetic and potential energy of an aircraft i.e. airspeed and altitude. I don't think that's quite the case here. The altitude doesn't change enough!
 
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From the start of the regatta, much was made of the different foil control systems on the two boats, with the OTUSA boat known to have a digital controlled foil. This didn't stop the trimmers pressing the wrong button in one race and dumping the cat. If it's legal to make minor mods to the boats, and both teams did, then software mods must also be legal on a boat designed to be digitally controlled. I don't know what the trimmer interface on the ETNZ boat was; that level of detail has been kept highly secret as were other aspects of the designs.

As for Ben's contribution, See my post in the other AC34 thread.
 
boguing said:
Oracle's early problem was that she was sailing bows down too much, so catching wave tops with the bows - not fast.

A simple tweak of the rudder horizontal foils might well have been enough to fix the problem. ETNZ had those too, and they are semi-automatic anyway, much like a decent aircraft (glider or airliner), fine trim is used to correct trim as speed or CofG changes over time, but the overall stability is designed in as a dynamic system.
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One of the TV progs when they were doing rules and boat handling, they said the trim tabs on the rudder can only be adjusted prior to the start, after that they are fixed. The only adjustment allowed during racing was to move the forward or backwards to alter foil angle.

Brian
 
Troubadour, the author mentions Dutch Roll. Which is not happening on these boats. A Phugoid oscillation is a repeated, minor pitch up/down cycle, in an aircraft. The amplitude can be very small. If it's happening to the AC72s I think it would be almost impossible to quantify, just too many other variables. I don't believe that it suffered from any longitudinal instability, just now enough stern down trim to make the foils lift a bit more.

The simplest 'automatic' is the wand used by the foiling Moths. No sign of them, so if the rudder trim foils are adjustable on board, then this will be a part of the mode thing that we kept hearing about.
 
halcyon said:
One of the TV progs when they were doing rules and boat handling, they said the trim tabs on the rudder can only be adjusted prior to the start, after that they are fixed. The only adjustment allowed during racing was to move the forward or backwards to alter foil angle.

Brian
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Adjusting the foil "angle" changes it's angle of attack. This needs to change with boat speed to maintain ride height but more importantly pitching moment. I'm not sure if the cog is ahead or behind the CP of the main foil. I expect it's behind. In this case as boat speed increases if you don't adjust the foil the boat will rise and pitch up. This pitch could be trimmed by the rudder foil if that was allowed but as it isn't then the answer is to lower the angle of attack of the foil. This reduces the Cl (coefficient of lift) reducing the lift and pitching moment at a given speed. If you can't trim the foil you have to accept a pitch up followed by a reduction in speed followed by a pitch down followed by acceleration and pitch up. like the foil porpoising. This is phugoid oscillation. It wastes energy. Better to trim the foil as speed varies. Best is to keep it at best Cl/Cd but to do that you might need a little help ;-)

The comparison to Yaw SAS is valid as the mechanism is similar as is the solution. Just the axis has changed.
 
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Anyone know what they actually do to adjust the pitch if the foil? Are the daggerboard casings in cassets which can tilt? Or is there some very clever flap sort of thing on the foli itself?
 
boguing said:
Troubadour, the author mentions Dutch Roll. Which is not happening on these boats.

## Agreed, but it was described correctly, I thought you meant it wasn't.

A Phugoid oscillation is a repeated, minor pitch up/down cycle, in an aircraft. The amplitude can be very small. If it's happening to the AC72s I think it would be almost impossible to quantify, just too many other variables.

## Phugoid in my understanding is specifically long period stick fixed oscillation that will persist without any external influences i.e. even in perfectly still air, actually a limit cycle. I thought you were suggesting that was the problem here, but it appears I misunderstood what you meant, I think we can agree that it isn't.
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.
 
Tomahawk said:
Anyone know what they actually do to adjust the pitch if the foil? Are the daggerboard casings in cassets which can tilt? Or is there some very clever flap sort of thing on the foli itself?
Click to expand...

Not fully listening when they told you about it, but seem to remember the can adjust the top of the foil housing for and aft, but only when the foil is out of the water.


Brian
 
The commentators were quite definite, the rudder foils could not be adjusted while sailing, only the lifting foils. The lifting foils were adjusted by the trimmer directly in ETNZ but using a digital control interface on OTUSA.
 
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