34th America's Cup: multihulls!

I think the case for twist-or not- has got to be taken on a case for case basis but for the AC I don't see the disadvantage. Can you explain to me what that might be? I fail to see how a twisting platform could exert any force on a single main lifting foil-the main foil and rudder t-foil would keep the lee hull going along in great trim unless you're thinking the rig could move forward enough to cause a pitchpole-but there is no evidence of a pitchpole in any of the Oracle video at the time of the failure.
I'll keep thinking about it ,James-hope you have a great weekend!
PS-only my back is stiff, damnit!

Gooday & Ha ha to you Doug. Thanks for the info 'down-hill boy' he he - good chart Gosh - I though I'd get totally blown out of the forum by now.

Still I do keep looking for one single MOD 70' that is 'twisting' - can't seem to find one but I'll keep looking.

In the meantime I'll try to see how to contact Nigel Irens & see what he thinks about the subject of 'twist'. Now I know he isn't the only designer in the world but few would not value his opinion. I M H O ciao, james
OOOoops - I put all this in the wrong place - darn - been there most of my life - so - nothing new - sorry guys - - jj

 

How would four side stays effect a plat form in twist.

I understand that Malcom Tennant had a stay going out to the ends of the cross beams and a tempory fore stay on the wild thing trimaran design.

 

DOGzilla trimaran with ama foil assist and no T rudders is an incorrect comparison with the AC72 catamarans with foils front and rear. The monster tri twisted but the windward foil and conventional rudder had no effect on the extremely long leeward float - which ran level and true with foil assist (no true flying hydrofoil here) and slight flexing through waves. What counted was the lee hull and main hull (which mostly was lifted clear) - the bow down windward float and foil had no effect on the performance - which is untrue in the situation with Batzilla cat.
NZilla looks to hardly twist whereas Batzilla twists so her windward T rudder hooks the water at a negative angle of attack.
Also, from posted images, there is a proportionately longer cantilever on Batboat hulls because the main beam (looks to me) to be further aft than NZilla's - and that may also be contributing to twisting of the platform, even though they have the wide chord after beam to reduce this, plus the bat box between the two which maybe is not wide enough to make a stiffening brace.

 

Platform Twist is the result of the shroud pulling up at the stern of the windward hull. TNZ has an inner shroud that lands amidships. The top shrouds have a bridle that land at the stern and 75% of the hull. Oracles rig is more conventiuonal with the top shrouds and inner shrouds landing near each other at the stern. When the shrouds pull up this load twists the platform. Perhaps the TNZ configuration allows the load to be shifted around a bit reducing the torsion ie the rigs load vector is more mid hull then at ther stern. I can't see any runners yet. They have been a bit clever with black makes it hard to see the rigging. Peter

 

i think the biggest reason the NZ boat can resist the twist is the triple Y dolphin striker arrangement. USA`s boat seems to only rely on higher section modulus beams instead, which obviously isnt working nearly as well from a stiffness point of view...

 

Silver R,

For a lot of racing multi's, you are right that stiff is better.

However, think about it. For a flying hull the difference in drag as air flows around it is just plain insignificant.

For the very special cases where you never intend to run with more than one hull in the water, the question comes down to how it will handle in turns and how it behaves in the transitions (hull skimming the surface, hull lifting out of the water or splashing back into the water). As long as it does OK in the turns and transitions, a platform prone to twist can be tolerated.

Stiff would still be preferred by most, but if you are pushing light weight as a priority, you may be better off learning to deal with twist.

These boats have one VERY SPECIAL design feature that you will rarley find on other boats. They are built for a very specific venue and they are generally discarded after competing once. The disposable nature and sheltered waters competion of an AC boat lets them tolerate stuff that no one would put up with on a more "normal" boat.

 

34th AC on Foils!

I've seen this picture several times but never picked up on the most significant thing about it. I initially thought the most important thing shown here was how deep TNZ was sailing but just tonight I noticed that there are four foils in the water-at least that appears to be the case:

#1 the original picture; #2 the four foils circled-

click on the image-

 

Not really, Doug, check the starboard foil length above deck; with that much protrusion there can only me a short measurement below hull, in fact it is not even touching water surface.

 

34th AC on Foils!

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I saw that Gary but what is the object that appears to be an extension of the stb foil?

 

Doug,

To me, it looks like the windward foil "L" section is just skimming the surface clipping wave crests.

 

34th AC on Foils!

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That must be it. It just looks lower down than it does in other pictures-see the one below-click on it to enlarge:

 

I would think that "foil up" on one of these boats means all the way up, but then again remember that the crew is only human.

I wonder how many "we did not mean to do that" events get seen and misinterpreted as "these guys are supposed to be good, surely they did (whatever) for a reason"?

 

Im going to go out on a limb here, i think these boats will run upwind like an orma tri and then downwind like hydroptere. So keep a hull immersed when beating uphill and speeds are moderate, then round the top of the course they will unfurl all their sail power and reach very deep with all their apparent wind and high speeds to lift the boat clear of the water for minimum drag and outright speed. in a decent blow, i dont see why these boat could not reach downwind as fast, or even faster than hydroptere. I guess it depends on the ultimate available power of the rig compared to hydropteres` rig... i cant wait to see if ETNZ will test this thing in 30kts of wind.... the down wind speed will be phenominal...

 

Get some tools & build a little one & play with it - you'll soon know the answer & be able to come back in here & tell us all about it. It's an interesting simple trial to do & you'll learn a whole lot about many subjects as well as twist. ciao, jj

 

34th AC on Foils!---twist and shout

Here is Oracle sailing and very twisted. Still no foiling pix.....

click-