“OR can quite legitimately claim that they need this change to improve the safety of their boat to acceptable levels. ETNZ/LR can quite legitimately claim that the issue is created by design choices.”
When you’re foiling a catamaran, you’re generating lift just like an airplane. The Emirates Team New Zealand yacht generates more lift with the forward element than the rear. The Oracle Racing Larry Ellison yacht does so as well but it gets a bit more lift from the rear compared with ETNZ.
So, ETNZ is more like a regular airplane with a small tail and OR is more like this goofy thing:
Mandating a bigger tail for all comers in the name of safety is a way for the Larry Ellison America’s Cup people to gain an advantage for Larry Ellison and/or Artemis Racing, the team with the other failed design,
“ETNZ have a boat that is almost entirely supported on its single main foil. The rudder provides very little lift, just control forces, which are relatively small. As speed changes, lift changes. The main foil is correspondingly adjusted, as this is allowed, so the lift remains as required. The lift on the rudder changes, but since this force is relatively small, the change in attitude on the boat is not problematic, and the local effects of free surface and small size provide a natural limit to motions. When it goes wrong, the boat will pivot about its main foil, potentially creating a high bow down pitch angle, so they’ve included sufficient buoyancy in the bows to cope.
OR have a boat where the lift is shared between the main foil and the rudder. The amount of lift provided by the rudder is still a small proportion of the total, but the rudder lift force is large compared to the rudder control force. As speed changes, lift changes. The main foil is correspondingly adjusted, but the rudder isn’t. The change in force on the rudder is significant, and affects the attitude of the boat. A small rudder foil that is required to provide a lot of lift can only do so with a large angle of attack, so with a fixed angle, a large change in trim of the boat is required, hence poor control. When it goes wrong, the boat pivots about the rudder foil, and hence a reduced bow down pitch angle, and hence the boat can have lower volume bows to allow recovery.
I’m sure both teams have simulated both approaches. ETNZ decided that they would go for the former, at a price of bigger main foil, lower righting moment, and more aero drag, so they have better control over a wide range of speeds for a given rudder setting. OR decided on the latter as it provides a lower drag solution, but for a smaller range of speeds for a given rudder setting. Boat 1 was an extreme example of this, but boat 2 is less extreme.
However, OR have found that the range of speeds over which they have good control is too small using the maximum size of rudder foil allowed. Hence, when they are running in the narrow speed range, they look good, but as soon as this is not the case, they have large pitch angles. Using a larger rudder foil requires less boat trim to generate the change in force, and hence better control.
The problem OR face is if they were to move the main foils further aft and increase their size, they would then have a boat which, if it goes wrong, will not have enough buoyancy in the bow to recover from the large pitch angle that would occur with the bigger main foil. A potentially dangerous solution, and rebuilding the hulls is probably not feasible, since even if they had the time, the added weight is more than their program has in the bank. They aren’t allowed new hulls. Furthermore, they have made corresponding design choices with their wing that also suggest the expectation of a narrow speed range, and moving to a higher drag foil solution would present them with power issues.
I suspect that OR may have been using a larger rudder foil recently to achieve the improvements we’ve been seeing, and consequently they already know that operating with class legal rudder foils is not a safe option for them, since if they set up the small rudder foils for lighter winds, and the winds increase during a race, they will have an unacceptably high probability of pitchpoling.
As such, the move to increase allowed rudder foil size and control is a real issue for OR, as without it they will have to choose between pulling out of certain races when conditions change, or risking the boat and crew by continuing. ETNZ and LR, on the other hand, don’t have this issue, and in fact increasing the rudder foil size on their boats would not only increase drag, but also create control problems due to the size of the control force generated becoming too large.
Hence the current dichotomy OR can quite legitimately claim that they need this change to improve the safety of their boat to acceptable levels. ETNZ/LR can quite legitimately claim that the issue is created by design choices. Since the AC is not just a design and sailing competition, but a design, sailing, and legal competition, we’ll have to wait and see who has the best overall package.
And then, in response to a question about how the engineer knows all this:
“Because when the OR boat is in the water, its static waterline, combined with its visible hullform when on a crane, shows that the vessel CoG is well aft of the main foil location. On the ETNZ boat, this is not the case. Their main foil is about where the CoG appears to be (actually slightly forward, but not by much).
As such, the resulting moment generated by the offset between main lifting foil vector and sum of sailing force vector on OR requires the rudder foil to provide a significant lifting force, plus also to provide the dynamic positive and negative control force, whilst on ETNZ the rudder foil provide very little lift force, just the control force element.”