Pendantic Question for Naval Architect types about BMW/Oracle for

[Paul Pless]

It seems like she's always sailing in this attitude or at the least should be sailing this way. So. . . why a tri instead of a cat?

http://www.movac.co.nz/images/3884/14625/bmw_oracle_2.jpg

[StevenBauer]

I think it's just so they don't have to run back and forth at each tack from one hull to the other. :D
Or maybe just more places to party when at the dock. :D



Steven

[cookie]

My guess would be that it has to do with the rudder length. In this setup a nine foot rudder in the middle is still in the water. On a cat setup they would probably need to double that.
Just guessing though ;)

[rbgarr]

My (non-naval-architect) thoughts are several:

1) The center hull makes for a stronger platform for the mast,

2) the amas can be spaced wider apart, be angled higher and outboard, and have less volume/drag as long as they are shaped and supported/lifted by the daggerboards so as not to 'submarine',

3) if the amas can be balanced out (or almost out) of the water during a tack, the boat can pivot faster on the shorter, rockered central hull, and thus turn tighter and regain speed sooner.

4) the crew managing the sails can stay in the central hull versus sprinting across the tramp so much of the time from one hull to another (as on the catamaran).

[johnw]

If you make a cat too wide, it will tend to pitchpole. Trimarans can be made wider before they encounter this problem, so they can be powered up more. Oracle was 90' wide, Alenghi was 80' wide.

I'm not sure why Trimarans don't pitchpole with the wider beam, but that's the logic I've been told.

[John B]

Way back when before the multis were in the water and still all secret, my mate at alinghi was saying the party line of a. the monster boat which was 20 or 30 ft 'longer' because it was a ketch or yawl rig and b. that a cat of that size wasn't viable because of the structural loadings without a central (tri) hull.
surprise surprise.;)

[Harbormaster]

I suspect that weight and balance also enter into it; the two amas have to be bigger on a cat to support the weight of the whole boat when at rest, and have to be bigger to balance the rig's torque when heeling. On the tri, the middle hull acts as a counterbalance weight on either tack. As small as the amas are, if they are bigger, they still have more wetted surface area.

[Daniel Noyes]

well I guess the questions answered!!!

I think ther's a good point about the center hull being "vestigial" the Alinghi had a big tube structure/backbone down the center of the boat, think of Oracles center hull as a bouyant backbone, also both boats only sailed on one hull but when the cat put it's other hull in the water it was dragging the other HALF of it's bouyancy in the water where when Oracle dipped it's center hull it was dragging less than 1/3rd of it's bouyancy.

Dan

[Chris Ostlind]

Superior righting moment

Superior forestay tension

Single set of controls and winches so no running from side to side

Faster tacking

The photo above is out of date compared to the boat that just won the AC. Wing sail now instead of soft sail rig. No vaka daggerboard anymore.

[Todd Bradshaw]

The middle hull is there to hold the beer because you can't fit much in those skinny amas.

[purri]

I thought amas were Malay nannies.

[Hwyl]

tri a little tenderness (http://www.youtube.com/watch?v=dael4sb42nI)

[Balanda]

Considering that the aim of the game appears to be, to have only one hull in contact with the water, the apparent wind constantly from way forward of the beam, and powered winches allowed, I wonder whether development will go down the line of single hulled craft with ballast slung to windward.

Advantages being a lot less wind-age and potentially lighter for the same righting moment.
Possibly a bit of a circus act to keep at optimum heel. But that's what computers are for.

Anyone?

[rbgarr]

I wonder whether development will go down the line of single hulled craft with ballast slung to windward.

Canting keels and rigs on monohulls could be a feature in the next AC.

[David Cockey]

Here's my guess of at least one advantage for the tri arrangement.

The center hull is used to reduce the roll moment of inertia. Most of the equipment including the engine, winches, etc can be put in the center hull which would significantly reduce the roll moment of inertia.

[Woxbox]

I wonder whether development will go down the line of single hulled craft with ballast slung to windward.


Been done: Crossbow (http://www.dcss.org/speedsl/crossbow.html)
http://www.dcss.org/speedsl/Crossbow_I.jpeg

And then there are the no hull boats that rely entirely on foils to stay up. How would the traditionalists like to see that in the AC?

[john welsford]

One consideration is that in order to tack reliably the load carrying hulls ( when upright) of a multihull need to be quite heavily rockered. You will note that Alinghi's two catamaran hulls are much more heavily rockered, especially aft, than the tris outside hulls. When tacking the tri pops back down onto the heavily rockered center hull which tacks and turns easily, goes through the wind, powers up and lifts the rockered hull back out of the water, running on the straighter hull with higher speed potential and lower drag.
The cat cant do that, it has to have both of its load carrying hulls rockered so has a lower potential speed and higher drag at high speeds.
I'd say that there is not a lot in it, but every little bit helps.
John Welsford




It seems like she's always sailing in this attitude or at the least should be sailing this way. So. . . why a tri instead of a cat?

http://www.movac.co.nz/images/3884/14625/bmw_oracle_2.jpg

[paladin]

Tri's will pitchpole, believe me......it was an early problem. And the lift generated can be used to a great advantage if the hull is asymmetrical and designed such that as the heel angle increases the reserve buoyancy shifts far forward driving the hul;l up and out of the water.

[RFNK]

Wasn't there some controversy about BMW/O's waterline length being based on the centre hull rather than the outer hulls, which were regarded as floats rather than hulls, so this boat could, in effect, cheat the system with this configuration? I recall something I read a month or two ago ... sorry I can't be more specific! Anything in this? Rick

[David Cockey]

Wasn't there some controversy about BMW/O's waterline length being based on the centre hull rather than the outer hulls, which were regarded as floats rather than hulls, so this boat could, in effect, cheat the system with this configuration? I recall something I read a month or two ago ... sorry I can't be more specific! Anything in this? Rick

Two controversies about waterline length. Waterline length is critical because these races were governed by the deed of gift which specifies 90' maximum waterline length.

1) Alinghi wanted waterline length measurements to include the rudder, presumably because they thought that would cause problems for BMW Oracle. That went to court, various experts provided depositions, etc. and the judge ruled that the rudders did not need to be included. My understanding is that BMW Oracle's amas and center hull were all under 90' WL if the rudders were not included.

2) Alinghi has water ballast which can added, moved, and dumped during the race. Question was how much water ballast and where it should be for measurements, and also wound up in court. The judge gave direction and sent it the the race committee (two of the three members of which were SNG/Alinghi representatives) to implement. They decided the boat had to carry water ballast and be at an even heel, when measured, but not necessarially in the same locations it would carried in while racing. Reports were Alinghi would considerably exceed the WL maximum with the ballast in normal position, but had tanks in the stern which pulled the stern down and bow up, and resulted in a legal WL for measurement purposes.

[luckystrike118]

I think this discussion can not be satisfactory for everyone nor present any representative results. The designs where perverted by the influence of the various court decisions.

It was obvious that James Spithill was clearly the better helmsman than Ernesto Bertarelli, even than Loick Peyron. He had much more training time on such a boat than Loick.

The Amas of Oracle and there waterline length resulting in higher base speeds, where much longer than Alinghy's, because the measure where taken from the main hull. But I think the biggest advantage of Oracle has to be seen in the wing rig with its driving force and angle to the wind during the first run.

Alinghys afterguard was very disappointing too, making two very important mistakes out of two decisions to be made.

For me, the 33th Americas Cup was a real diappointment. Both Teams together spent 60.000.000$ for 5 hrs of sailing. One boat was faster and the slower boat didn't even fight for it's victory. Sailing Perverted!!!!!!

I wish Americas Cup would go to where it's have been. Large, slim displacement hull keelboats without hydrodramatic gimmicks... designed to be matchrace-fighting-maschines with only slight speed differences. These boats would produce exiting races where crewwork, tactics and SAILING stands in the first line ... not Bertarellies and Ellisons million dollar jerk off

[luckystrike118]

Sorry I have forgotten......
...Greetings from the North Sea Coast, Michel

By the way. The question if a cat is better than a tri ... I think a tri is faster generally.