The Instability of the AC75s
- Thread starter AlastairLC
- Start date
I wouldn't call it instability. They had a situation that caused a slow capsize. If they were all unstable, they all would be doing it. These are new boats and they are figuring out the limits and adjusting to them. Just like anything else that is new.
Of course, not being there, we can't actually know, but it looks like she became slightly bow down which caused a downward tilt in the foil angle. The foil slowly dove and lost headway with the increase in drag, pulling the boat over. Does the foil have trim tabs?
-Will (Dragonfly)
-Will (Dragonfly)
These boats have limited ballast. They use foils for Righting Motion. That requires water moving over the foil to work. Stop moving abeam to the wind and they have issues. Notice to get it up they didn't pull it sideways (Like you would a dinghy), they pulled it FORWARD, and the leeward foil lifted the boat. They absolutely are not traditional monohull/keelboats... think 'catamaran' and you will be a lot closer. Without a weighted keel, when a bigger cat goes on its side its game over. At least here you can pull it forward!
EDIT - The rudder does not have trim tabs. The Ts do. I typed that too fast!
EDIT - The rudder does not have trim tabs. The Ts do. I typed that too fast!
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I don’t have much foiling experience other than recent learning on my windsurfer. At least one of my crashes was the result of a fast rate of climb that caused the foil to either breach the surface or “cavitate” I was too busy trying not to get hurt to really know. I was in a level controlled flight and weighted my back foot a bit more and boom. VERY fine line once your flying.
It looks like that happened very early in the video, no control surface then a whole bunch in a very short time frame.
I hadn’t noticed about the forward pull recovery.
It looks like that happened very early in the video, no control surface then a whole bunch in a very short time frame.
I hadn’t noticed about the forward pull recovery.
The rules allow flaps on the main foils which most feel are used to get the boat up on the foils. The rudder foil has to be fixed with no flaps or aircraft style elevator. The rake of the rudder is controllable which adjust the angle of attack of the rudder foil and the stern moving up and down adjust the angle of attack of the main foils. I haven't read anything (probably big secret) on how automated this is. The boat in the video main foil goes up and out of the water due to too angle of attack being to high then stalls due to no water flow. Then the foil goes back into the water at a negative angle of attack that was creating massive drag slowing the boat down and broaching. May have been a test that went wrong.
Short article
AC36: What's all the flap about "trailing edge flaps" — Sir Ben Ainslie and Mark Chisn
SAN FRANCISCO (#1224) — Sir Ben Ainslie (GBR) is the most decorated Olympic sailor of all time, and Team Principal of the British America’s Cup challenger. As you all know his INEOS Team UK is challenging for the 36th America’s Cup in New Zealand in 2021. Each month the UK's Yachting World is...
www.sailingillustrated.com
To save the teams millions in design and engineering, the AC75 tech committee has designed and will supply the wing arms and the cant actuators as One Design parts. The teams get to play in the gray box shown. The T foils. That is where the race will be won or lost.
Interesting about the post where the rudder cant have trim tabs.
All boats with active pressured sail want to pinwheel around a fixed point. The bows want to spin and nose dive into the water. With no more boyancy to counteract this force there needs to be a way to level the hull. Amazing if all they have is the central foil, and no "elevator" trim.
All boats with active pressured sail want to pinwheel around a fixed point. The bows want to spin and nose dive into the water. With no more boyancy to counteract this force there needs to be a way to level the hull. Amazing if all they have is the central foil, and no "elevator" trim.
I had read quite a bit about windfoiling before i tried it. Actual on water was trying as I had to overcome old habits to get it to fly.
First run I stood on the board like “normal” and although I had lots of boat speed I didn’t leave the surface, just a windsurfer with a really big skeg.
Next run I got some speed up and then “forced” myself to put more weight on my back foot. Suddenly flying. The trick is to change the angle of attack of the foil, get flying and then quickly level out, oh and STAY level. Basically what I’m doing with weight shift is what the AC-75 rudder rake is doing. Sort of “sink” the stern a little, climb, go to neutral, fly level, “raise” the stern, descend.
Having the front foils “rake” given their relatively large surface area to the rudder and the speed through the water would be pretty squirrelly I would think.
Fun to watch though!!!
First run I stood on the board like “normal” and although I had lots of boat speed I didn’t leave the surface, just a windsurfer with a really big skeg.
Next run I got some speed up and then “forced” myself to put more weight on my back foot. Suddenly flying. The trick is to change the angle of attack of the foil, get flying and then quickly level out, oh and STAY level. Basically what I’m doing with weight shift is what the AC-75 rudder rake is doing. Sort of “sink” the stern a little, climb, go to neutral, fly level, “raise” the stern, descend.
Having the front foils “rake” given their relatively large surface area to the rudder and the speed through the water would be pretty squirrelly I would think.
Fun to watch though!!!
Actually, this isn’t much different than the traditional monohull, where you try and find a balance between the sailplans center of effort and hull/foils center of lateral resistance. The only difference here is that the hull provides zero component to the CLR.
That understates the amount of secret sauce involved in making that happen.
Its like comparing the stability of a pencil on a desk versus resting level on top of another pencil. Impressive designs.
oh no, for sure it’s impressive. But the modeling allows that capability to be set as a fixed criteria. Then it always solves for that when creating design options. All of the boats will solid flyers right out of the box.