Ellway Aero-Hydrodynamic Designs
For all the technical sailors out there and everyone who is geeking out on the how behind the America's Cup, here is some detailed thinking on why Emirates Team New Zealand looks so good.
Why have ENZ been dominant so far - Are the kinks in ENZ foils significant?
In the light winds of the 1st weekend’s races of the AC finals, ENZ seemed to be able to sail at the same speed at Oracle, but at a higher angle upwind, and lower downwind giving considerably superior vmg.
Commentator Ken Read has often pointed to the noticeable difference in the front view shapes of the ENZ and Oracle foils – Here I have made a quick analysis of the likely effects. Please note, however, that because I do not have access to any foil geometry details, this has necessarily involved some significant assumptions and guesswork.
Let’s just look at upwind and assume both boats are travelling at the same speed, but with ENZ pointing around 3 deg higher. As a first approximation, ENZ will see an apparent wind angle (AWA) of 3 deg less than Oracle
The AWA is the ‘glide angle’ of the boat. It is the sum of the aerodynamic and hydrodynamic glide angles. The aero glide angle is determined by the ratio of drag to lift force from the rig. It also includes the parasitic drag of the hulls rigging and crew etc.
The hydro drag angle is the ratio of water drag to side force from the rig (Fy) – see fig 1..
So for ENZ to point higher at the same speed implies that either the air L/D is better (eg less drag for same lift) or that ENZ can generate more side force from the available righting moment, or that the foil drag is lower.
a) Foil drag
The foils have to support the total load of the boat (~29 kN inc crew) and also resist the side force Fy from the rig (~11.4kN).
Foil drag can be divided into 2 key components. Induced drag which is produced as a direct consequence of producing lift, and profile drag which is due to the friction of the water acting on the foils.
At low foiling speeds, induced drag dominates. At high speed, profile drag is dominant. At minimum drag, the induced and profile drags are equal – see Fig 2.
ENZ seems to have the greatest advantage in the lower speed range. This suggests that the foil design may generate less induced drag.
I have done some rudimentary CFD based on the observed foil geometries of the 2 boats – see Fig 3. Both foils have the same wetted area and have been designed to have the same lateral CofP (centre of pressure) at 23 kts such that both boats would have similar available righting moments.
As suspected, ENZ foil produces less induced drag. It is also indicated to need less side slip (yaw angle) to generate the required side force. At speeds of 18-20kts, the predicted difference in drag reduces the ENZ hydro drag angle by a about 1 deg maximum. At around 30kts, there’s negligible difference in the predicted drags.
It appears that the distinctive foil shape does provide some gain, especially at lower foil speeds. But this gain does not appear to be large enough to explain all of ENZ’s vmg advantage.
b) Wing trim
The side force Fy is determined by the righting moment (Mx) and the heel arm length – this is the distance between the lateral CofP of the foils and the vertical CofE of the rig.
The CofE can be moved from about 40% rig height down to around 10% by adding more and more twist.
So, by adding more twist, the heel arm is reduced, which increases Fy. This reduces the hydro drag angle. But more twist will usually mean more induced drag from the wing. This increases the aero drag angle. It’s a game of trade-offs and marginal gains – getting it right can make significant differences to upwind vmg.
The base of the wing is at about 2.5m above the water and the tip is about 26m above the water. Vertical wind gradient causes the true wind speed (TWS) at the base of the wing to be lower than that at the tip. Typically, the wind at the tip would be about 25% more than at the wing base. If the boat is sailing in 8kts wind and at a speed of 21kts at 55deg TWA, the effect of the vertical wind gradient is that the apparent wind speed at the tip is now some 35% greater than the base. In addition, the AWA at the tip will also be around 4 deg more than at the base. The increasing AWS and AWA from base to tip require that the wing needs significantly more twist and camber change along its span to obtain optimum loading when compared with the no wind gradient case.
All teams will have computer models to predict the optimum settings for their wing elements for different wind conditions. But the assumptions made in the models may not be the same for one team to another. Further, depending upon the wind direction, the vertical gradient can change markedly due to turbulent mixing. And then there’s the man-machine interface. It is possible that ENZ can control their wing more accurately and rapidly to match varying conditions.
So, my suspicion is that ENZ may be able to trim their wing more effectively to date.
c) Windward heel
I’ve noticed ENZ heel to windward. This will bring the vertical centre of gravity to windward and could add yet more righting moment and thus Fy (see figure 4)
d) Other
The ENZ foil design should be better at high yaw rates during tacks. This, combined with the faster and more accurate control facilitated by having a dedicated foil controller, should be a big advantage.
The foils also enter the water in a more gradual way when deployed, disturbing the boat less.
Summary
My analysis suggests that ENZ dominance to date does not stem from one specific design attribute. The distinctive kinked foils do appear to provide less drag at lower foiling speeds. I suspect that the bikes have provided more power availability allowing wing and foils to be more frequently trimmed. Importantly, the bikes have also freed the hands of the sailors. This has allowed complex tasks to be designated to specific crew members.
The wing trimmer and foil controller are looking at displays. I suspect that ENZ have a better man-machine interface which allows finer control of both wing and foils.
Is it all over for Oracle? Well not necessarily if the wing trim is the main issue. And I really hope for more of a competitive sporting contest in the upcoming races. But ENZ have both Ray Davies and the foil Kinks. I fear that if the weekend forecast proves correct, they may well preside over a Waterloo Sunset for Oracle. ... See more
