America's Cup: A first look at the AC40, stress testing, and AC40 compared the AC75's
by Richard Gladwell/Sail-World.com/nz 13 Sep 2022 18:30 NZST
13 September 2022
Deck layout of AC40 - Emirates Team NZ's AC40 - September 2022 © Emirates Team NZ
Emirates Team New Zealand's first AC40 has arrived in Auckland, and is being commissioned in the team's new base in Auckland's Wynyard Point.
While some images of the AC40 have been revealed by McConaghy Boats, the video of the AC40 hull set up for stress testing gave Cup fans their first look at the half-scale AC75 to the Version 2 of the AC75 class rule.
The AC40 will be used by competitors in the Youth and Womens America's Cup Regattas in Barcelona, as well as by the America's Cup crews in three Preliminary Regattas. The final Preliminary Regatta will be sailed in AC75's the America's Cup raceboat, of which the teams are permitted to build only one new boat.
The AC40 will also be used by the teams for development and test sailing as well, training within the teams. Most teams will get two AC40's, from either McConaghy Boats, or the teams can construct boats themselves, which are strict one-designs.
Emirates Team New Zealand designers opted to make the AC40 a half-scale development of their America's Cup champion, Te Rehutai - even though this will give the Challengers some insight into how the team has extended its design thinking beyond Te Rehutai.
The AC40 has similar but softer lines than the AC75, but uses the same basic concepts to achieve the same characteristics as Te Rehutai. There is not a lot in the AC40 that is a recognisable import from one of the challengers in the 2021 America's Cup.
The AC40 is quite flat across the transom, and as can be seen from the early sailing shots of ETNZ's AC75 most of the draggy after section gets lost when the boat heels slightly running on its bulbous keel-line and corner chine.
The AC40 hasn't moved much towards to Luna Rossa's 2021 AC75 - which was generally reckoned to be a very good approach to the AC75 rule. Certainly the Italian challenger had softer and fuller hull lines, but with the keel bustle being more of a sharply defined underhull skeg, particularly in the after sections, to provide additional bouyancy and hull lift. Its rather severe skeg attachment to the hull was also designed to be a very efficient endplate between the hull rig and water surface when the AC75 was foil-borne.
One of the issues with the AC40 is that while it leans towards ETNZ's design thinking for 2021, if the other teams are going in another direction, the AC40 hull when used as a test platform may have little relevance. However its big positive from a reconnaissance point of view is that all teams will have a common shape - meaning that factor can be discounted when comparing reconnaissance - particularly foiling lift out speeds and other hull variable factors.
Foil Test Video from 2021 America's Cup - Destruction testing foil arms
Going into the 2021 America's Cup a flaw in the design of the foil arms resulted in a failure during stress test when the arms failed before reaching their designed load. In the video below two stress tests are shown - the first failing spectacularly before design load, and the second improved design only failing at approximately double the design load. The video also highlights the audible warning sign of imminent carbon structural failure when a cracking sound can be heard, just before the catastrophic failure occurs. In the second test small microphones can be seen taped onto the structure to give a preliminary warning of failure.
The principle of stress testing is the same for both the AC75 foils and AC40 hull - that the component is locked or tied to a frame/weights, by the same points as when it would be in use. Then load is applied gradually to replicate the forces anticipated when sailing. In both the AC40 video and the foil arm video the force is increased gradually, with the test team carefully monitoring load sensors at the key points with the read-outs being displayed on the computer screens. When designers are happy that the loads are being properly applied and the structure is behaving as designed, then the load is further increased.
Depending on the component, it will bend to a tolerable extent before there is catastrophic failure - which is presumably the function of the measurement tapes attached to the transom of the AC40.