90 x 90 Race Multi

I think you have incompressible flow confused with the compressibility of the fluid.

Link

The foils are not designed to lift the boat free of the surface like a Moth, no rudder foil is required.

Once the leeward ama is partially out of the water, the entire volume forward of the CG is reserve buoyancy to resist pitch.

The ama foils do not need a positive AoA, they are cambered. They work just like the t foils on a Moth. The effective AoA comes from the difference between the local flow and the zero lift angle of the aerofoil section.

 

It's an intelligent design but somewhat incomprehensible. I somewhat understand what you are trying to say. Said very blunt the curved dagger boards prevent the leeward hull from going under to a nosedive?

I saw Groupama 3 had the same devices. How are those curved daggerboards called?

 

The terms I've heard used are "Banana Board" and "Banana Foil"

The idea is that since water is 800 times denser than air, the drag of the ama in the air will be 1/800 th of its drag in the water. When you consider the drag caused (induced) by the force needed to lift the ama, the real gain is only about 50-75%

In an extreme case if the bow of the ama pitches down, or nose dives, the lift on the foil and go negative, and actually make a nose dive worse. During conditions an manoeuvres that make this likely, the boards should be retracted. The ability to raise the boards whit they are under load is a key safety feature to prevent the sort of forward diagonal capsize that Foncia suffered while Alinghi was using the boat for training. I am *not* saying that the crash was caused by the board being down in this case, only that the boards have been known to cause such problems.

Pitch stability will never be greater than the natural stability from the buoyancy of the ama, what the boards allow is drag reduction while maintaining pitch stability. Under some conditions 70-90% of the vessel weight on the foil is safe, in others only 20-30% might be safe.

The idea is that variable area is what makes these foils so attractive. Keeping the root of the foil (at the hull) in the water avoids the ventilation problems that surface piercing foils have. At speed these boats are not really 'foiling' in the moth sense, they are planing. Planing hulls do not have the magic 45-50 knot speed limit that surface piercing foils seem to have. This is one of the reasons that water speed records are not held by hydro foil vessels, but by planing hulls.

 

Planing Ama

Planing with 70% of the weight supported by the foil? Do you mean because so much weight is supported by the foil and the resultant displacement of the 90(or so) foot is so low that it must be planing?
I guess that could be possible since even with the ama's 30% being supported 100% by dynamic lift it would still resist pitching-maybe better than when in displ. mode? Is that what you're thinking?
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Somewhere some time ago I read that displacement multihulls with high L/B ratios should be able to reach a speed somewhere around 4 times the square root of the waterline length. I know some small multies have probably exceeded this. But if there is any truth to it (and the dog boats amas are 90') the boat should theoretically be able to reach 37.9 knots without planing......

 

Oh God, please spare us from the Manna From Heaven subject for multihullers; the ascendency to the Throne Room of the Gods; the sailor's twisted trail to Shang-ri-La.... the ever omnipotent claim of a planing multihull.

This topic has a toothless mouth suitable only for self-indulgent gumming of the poor thing until dead.

I get the discussion on the foil operation aspects and how they contribute and also detract from the function of this boat at different speeds, but to extend that, even so inconspicuously, into the dread planing zone is really not necessary.

And while we are on the topic of ama foils.... here's a little teaser from the Sodeb'O compound in La Trinite sur Mer.

Coville will not be equipping his boat with banana foils for his next attempt at the RTW record and.... he goes on standby for the next try as of mid-October-ish, depending on how things progress with the business of going over the boat in preparation.

Sodeb'O has likely been hauled by now and has her mast removed for a complete new set of fittings, halyards, electronics as necessary, etc. The rest of the boat will be totally cleaned with new ropes, rigging and a whole host of sundry parts prior to being put back into the water.

Chris

 

Okay ... I remember the planing multihull debate. It was a poor choice of words.

Try this:
The design point is when the majority of the boat's weight is carried by the submerged foil. The remainder of the boat's weight is supported by dynamic pressure on the hulls rather than by hydrostatic pressure.

Can we live with that description?

 

The amas are 105' long, the 90' LWL is the main hull. L/B ratio is around 30:1

 

Hey, just slightly off target on the thread going on, but related, nevertheless... I happened to see this poster in the window of a canoe and pedal boat rental shop in Nantes, France.

The good people of Nantes are going to have a party to christen the brand new, Banque Populaire V monster multihull. Mind you, this is a city of about 600,000 people, they happen to be French and they just go totally wacky about their big multihulls over there.

I asked a lot of people on the street in Nantes if they knew of Sodeb'O, IDEC and now, BPV and almost everyone of them had more than a passing idea of what all that was about.

Over here in the states, one can tend to feel like a scuttling new mammal that is hiding from the last of the reptiles.

 

Ernesto interview

...been away for a few days, so I hope this is not repetitive

Ernesto Bertarelli talks about the America's Cup.
http://www.valenciasailing.blogspot.com/

Our friends from the excellent Italian sailing website Zero Gradi Nord have published an exclusive video interview with Ernesto Bertarelli, Alinghi's owner, regarding the America's Cup.

The Swiss yachtsman was in Porto Cervo, Sardinia where he won Maxi Yacht Rolex Cup in the Mini Maxi division with a comfortable margin, scoring 4 bullets in 6 races. Bertarelli has chartered the 66ft yacht Numbers and raced with a crew almost entirely of Alinghi team members.

Bertarelli talked to the Italian media present in the regatta for about 30 minutes covering most aspects of the America's Cup. The interview was in Italian and for the convenience of our non-Italian readers we publish the translation.

Note: The translation is mine and hopefully as reliable as possible. Although I consider myself very fluent in Dante's language a few errors might have slipped in, especially when at moments Bertarelli's voice was silenced by the noises of forks and knives. Be patient, the article is long...

 

Ernesto and the AC are not really on topic ... unless you know that Ernesto has a 90x90 or bigger multihull.

At this point it looks like the BOR90 is going to be lonely for awhile ... all dressed up and no place to go.

 

The late Edmund Bruce nailed this old hoary myth way back in 1966.

Bruce had the huge Govt towing tank facilities at his disposal and during this time he was able to do physical (not theoretical) experiments with hull forms.
His findings were very specific. Hull planing in multihulls in a quest for speed is a fantasy. Poor hull shape and excessive wetted surface make these hull forms only advantagous in a small section of the average sailing spectrum , and that is --sailing downwind in ideal conditions.

His prescription for the best hull forms in a wide variety of wind and wave conditions were.
!. Light displacement.
2. Minimum wetted surface.
3. A waterline L/B ratio of min 12:1. Only more, if the displacement is drastically reduced to offset the increase in waterline length, with it's accompanyng increase in wetted surface.

In short ---- Long, Slim, Lightweight, semi circular section, displacement hulls, which ONLY the multihull configuration can provide.

Nigel Irens designs Sodebo and Idec are living proof of this, as was the stunning performance of the Trimaran "Toria" ---back in 1966.

 

There has been at least one fast planing multihull- though with poor light air performance:

 

Ama Foils

Below you will see a shot of the lower banana foil trunk opening on the port ama of Coville's ORMA60 tri, Sodeb'O. From this picture you can get an idea as to what kind of foil section is being used for these particular boats. There's no scale in the photo, but the chord is right around 14-15". The deepest end of the foil is equipped with a winglet tip that is set at about 90 degrees from the foil surface. They are loaded from below the ama.

I could not tell for sure, but it looks as if the foils were controlled by a hydraulic actuator as there were fluid lines running under the aft beam from the cockpit out to the foil trunks. It would take this kind of hydraulic force to move the foils in their trunks when loaded and I suppose that this allowed Coville to trim the boards while running on them. All this is just a guess on my part, as there was nobody around at the time who could answer questions on the subject.

I did not have access to any other ORMA 60's while in France, so I have no idea as to the section or chord being used by the other machines.

Chris

 

Thanks Chris, very interesting!

 

Cammas on sailing Oracle tri

Franck Cammas commenting on the BMW ORACLE Racing 90-foot trimaran, "It is the first ever boat of this size designed and built for in-port racing and match racing only. She is very impressive, and the size of the mast is just amazing. We haven't gone full on yet. We need to build up our confidence and then we will be able to sail more aggressively. The boat is designed for light winds. She reaches the (103-foot) Groupama's top speeds easily, however much quicker than G3.

Now it would be different with 25 knots of wind, and Groupama 3 would be quicker. But this boat is exceptional under 15 kts. We are still very careful. It's extreme, we must not take risks. There is a lot of pressure when bearing away, we could feel the pressure amongst the crewmembers when we did it for the first time." -- Full comments:
http://tinyurl.com/SButt0914e

Cammas has been working with the American team as a consultant. While competing for the Jules Verne Trophy to set a new non-stop around the world record, Cammas and his crew were rescued February 17, 2008 off New Zealand when Groupama 3 capsized following the breakage of her port float.
http://www.cammas-groupama.com/en/index.jsp

http://www.boatdesign.net/forums/showthread.php?t=21098
Another 'Round-the-World' Record Attempt, Groupama