America's Cup Speed Difference, US vs New Zealand

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Bill Roberts
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America's Cup Speed Difference, US vs New Zealand

Post by Bill Roberts »

Why is the New Zealand boat faster than the US boat???

The reason can be seen by watching the two boats sail/fly.

OK, Here's a hint: The two boats are flying straight at you, front view.
What do you see as a difference? How much would the American AC Team be willing to pay for this answer?
Answer: It is too late now. They are already committed. The boat is already designed and built. Can't change now.

gahamby
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Re: America's Cup Speed Difference, US vs New Zealand

Post by gahamby »

It's that extra little kick in NZ's foils.

Bill Roberts
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Re: America's Cup Speed Difference, US vs New Zealand

Post by Bill Roberts »

Thanks for your input. The kink is not the trick. A smooth curve is actually closer to the ideal shape.

Here is the NZ advantage: The NZ boat is WIDER by a couple of feet. Therefore the righting moment is greater on the NZ boat. Therefore the wing pressure and thrust are greater on the NZ boat and it goes faster. Pretty simple isn't it.
How did they accomplish this advantage??? The hulls on both boats are the same overall width.

Let's look at Oracle first. On Oracle the leeward daggerboard goes straight down from the CB trunk. The foil is about 6ft long/span. The boats are 30ft wide and the hulls 4ft wide for discussion purposes. I don't know the exact measurements, but the exact correct numbers will not change the answer because these numbers are in the right ball park. The effective boat width that determines max righting moment is the distance from the center of pressure on the foil to the center of gravity on the windward boat parts. Lets call that windward CG point, the centerline of the windward hull. So, on Qracle that measurement is 30ft minus 2ft for half hull leeward hull width, minus 3ft for the distance to center of pressure on foil from CB trunk minus 2ft from the windward side of the windward hull to the windward center of gravity of the windward boat parts. So this is 30 -2 -3 -2 = 23ft. This is the lever arm length on Qracle that determines Max righting moment and wing thrust, BY The Way, these boats operate at Max righting moment as long as they are flying which is usually the whole race.

Now lets go to the NZ boat: This boat is 30ft overall width with 4ft wide hulls. When the daggerboards on the NZ boat come out of the CB trunk, they do NOT go straight down. On the NZ boat the boards bend outward to leeward and continue out and down until they reach the max boat width limit, outside surface of leeward hull. Then they bend quickly and head across the boat and become the lifting foil 6ft long also. Therefore the center of pressure on the NZ foil is further to leeward, 2ft further to leeward than on the Qracle. So the max righting moment lever arm on the NZ boat is 30ft -3ft -2ft = 25ft. Because the NZ boat takes the CB to max boat width and then brings it across as the lifting foil, this boat enjoys a 25ft max righting moment lever arm. The advantage is 25/23 = 1.087 or 8.7% more max righting moment. This becomes 8.7% more max wing thrust. Boat velocity varies as the square root of wing thrust so this max righting moment advantage becomes a 4.3% boat speed advantage. For example, 4.3% of 20 knots equals 0.86 knots or almost 1 knot. The NZ boat has this advantage all the time it is flying.
Watch the NZ boat fly this weekend. When it is coming straight at you, you can see the CB as it extends down from the trunk, it is also moving outward toward the max boat width limit. It looks like something is wrong, the board extension is not lined up with the CB trunk as it is on most boats.

Bill Roberts
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Re: America's Cup Speed Difference, US vs New Zealand

Post by Bill Roberts »

Well, what do you think?
In the last race Oracle was the first boat to the first mark. Tacking downwind near the end of the second leg, NZ drove over the top of Oracle and ended up ahead and to leeward. Then NZ was in position to jibe for the gate first and rounded at the gate first and NZ opened up the lead on every leg thereafter and won by 55 seconds or 5oo meters at the finish. NZ was a faster boat. WHY??? The second leg really showed it when NZ came from behind and drove over Oracle and ended up ahead and to leeward and closer to the gate at the end of the leg. What do you guys think????? Why did NZ accelerate at the end of the second leg and Oracle didn't ???

Hey, Did you guys notice that both boats used small, short luff jibs, in most of the races. I think they only used full size jibs when the wind was single digits. Ten knots or more, the small jibs were used. Remember the relative wind speed across these boats is always higher than the boat speed. These boats can outrun the true wind speed but they cannot out run the relative wind speed.
Last edited by Bill Roberts on July 3rd, 2017, 5:15 pm, edited 1 time in total.

gahamby
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Re: America's Cup Speed Difference, US vs New Zealand

Post by gahamby »

NZ just plain out sailed 'em. Their on foil tacks and jibes were flawless.The discipline was remarkable! They had the edge technically. NZ trimmed their wing hydraulically which seemed to give them a better shape. Using pedal power to pressurize the hydraulic system gave them, if you will, a leg up. Their pre start maneuvers were spot on. Spithill was sailing the last cup and hoping for another comeback miracle. It looked like USA got their boat speed up but to no avail. USA made too many unforced errors in a contest that allows for none. NZ got in a lot of good practice in the challenger series and put it to good use. They didn't peak too early this time. There's no training substitute for racing against a worthy competitor.
I can't see NZ changing boat class in the next Cup. The technology race should bring out some really amazing boats.

I see what you mean about NZ's foil entry being wider. What effect did the more angular shape of the foils have, if any? Did NZ ever switch out their foils?

Bill Roberts
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Re: America's Cup Speed Difference, US vs New Zealand

Post by Bill Roberts »

Thanks for your comments gahamby. You are right on as to the superior sailing of the NZ team. Their % flying time was always better than Oracle's.
Spithill had two over early starts and more penalties on the race course than NZ.
In the last race Oracle took the lead at the start and held the lead past the first mark. After the first downwind jibe Oracle was to leeward of NZ which could have put them between NZ and the gate at the bottom of the leg with boats even but then something happened. Near the end of the leg the wind increased maybe, it looked like it did because NZ accelerated relative to Oracle and pulled ahead of Oracle and then was able to bare off slightly and cross in front of Oracle and position themselves ahead and to leeward of Oracle. Then NZ was the first boat to jibe for the gate. Without that little NZ acceleration and re position ahead and to leeward of Oracle, Oracle would have blocked NZ from jibbing for the mark and Oracle would have been the first boat through the gate.
How did NZ put on that little sprint and get ahead of Oracle at the end of the second leg? In that maybe puff, NZ accelerated more than Oracle and then NZ crossed in front of Oracle and to leeward and then NZ jibed first for the gate. At the beginning of leg two, NZ was behind but at the end of leg two NZ pulled ahead. That little sprint at the end of the second leg that NZ put on could only have been done because their boat had more wing thrust and therefore more righting moment was required and the NZ skipper put it to work generated a little more speed and got ahead and in the best position for the gate. After rounding through the gate and heading to windward, NZ opened up steadly on every leg finishing with a 500+ meter lead. NZ did not do this in any other race. What's going on?

What was the sailing background of the NZ three prime sailing people? The skipper was the A Cat World Champion. The tactician and foil trimmer were the 4.9er World Champion Team. These people were highly skilled in sailing boats that are very lively and quick and fast and physically and mentally demanding. In the US sailors of boats that demand/require these sailing qualities are not admired for the above mentioned very reasons and these boats will never be the "most popular". Trapezing and being agile and quick and strong and smart are not admired by the sailing authorities in the US. The high performance high demand very physical boats are the very best TRAINER BOATS there are. We have to change our attitude and respect in the US for demanding excellent trainer boats and then maybe we can have some winning US Sailors on our AC Team.

In answer to your last question on the NZ foil: The kink is of no performance benefit. Just outboard of the kink the NZ foil comes apart. This allows NZ to change the so called "foil tip". NZ had eight different tips. The tip could change plan form shape and longer or shorter ones could change total foil area. It is important that foil area be matched to load or weight and speed and density of the medium in which the foil operates, same as an airplane wing.

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