Weight is a popular topic of conversation at the INEOS Britannia base in Barcelona, but not because everyone’s interested in the outcome from the latest dietary fad, rather it’s the weight of the boat that’s the focus.
“The weight of the AC75 [the type of yacht that will contest the 37th America’s Cup] is defined by the AC75 Class Rules, and they say that the yacht needs to be between 6200kg and 6160kg. So there's a very small window for us to fit into, around half a percent of the overall weight. The centre of gravity also has a very small window, positioned between 9m and 9.35m from the back of the yacht.” The speaker is Laura Davies, Weights Engineer for the team, and the person responsible for monitoring the yacht’s mass, declarations of change, and management of the measurement program.
“The weight of the yacht includes the platform [or hull], all of the control systems and equipment that go inside it, the foil arms, and foils, wings and rudder, the mast and the sails, and not forgetting the crew and the equipment that they carry.
“The design process starts with going through the class rule and looking at all the one design or supplied equipment that has a pre-determined weight that we cannot change. Some of the crucial one design and supplied parts that we have to put on the yacht include the foil arms and the media equipment. Each foil arm weighs in at around half a tonne. Then we've got the media equipment, which is a bit over a hundred kilos. We also have the foil cant system that adjusts the foils in and out of the water, and that weighs in at 343kg.
“We add all that up, and what’s left is the mass of material from which we must build the rest of the yacht. It’s divided up, and a weight budget is defined for the different areas of the yacht. Each of the teams of engineers must respect the budget for the equipment that they are designing.
“It’s not just about hitting the overall weight though, the centre of gravity is important because there’s an optimum position. So, getting the weight right is super important, not just for compliance with the rule but for performance. We're constantly looking at the design and trying to figure out ways to alter the architecture, mainly with the systems, moving equipment around the yacht to try and hit the target for the weight and CoG.”
“Throughout the design process, some of the rules may be interpreted by the Rules Committee during a Rule Enquiry, after a lot of back and forth and advocacy by the different teams. Those interpretations might force us to change our weight budgets for an area of the yacht, or reassess our design.
“My role is to track all this and provide feedback for the designers and engineers. We use software to model where everything is positioned in the yacht, and it can calculate the overall mass and centre of gravity, so we know the impact each change will have. The yacht has a database that contains mass and centre of gravity data, and calculations for inertia that are fed to performance for simulations. This is updated each time the yacht configuration changes.
“Some of the parts in this model are easy – we know the weight of some hydraulic components from the suppliers for instance – but calculating the design weight is quite difficult with composites. We initially start with laminate calculations, predicting the final mass from the number of layers of carbon fibre laminate and resin. We only know how good a job we did when we weigh the part after its completion.
“During manufacture, parts are assembled to a certain stage where we can check the weight before they go in the yacht. This means we have an opportunity to update everything and can slowly move from design weights to finished weights. This allows us to keep updating the predictions, it's important that we catch any changes early, so we can compensate for it.
“We also need some redundancy, in the last America’s Cup critical repairs added a lot of weight. The sailors also have their own weight to manage, so they all have to fall under a certain mass. And if they don't hit their own targets, then we're constrained in crew configurations which could create problems if there are injuries. We need to prepare ourselves for all eventualities.
“Once we’ve launched the boat and get towards the racing, we are weighing the boat fairly regularly to make sure it still tracks our models. We have to hit the weight for the measurement period when it’s checked by the Measurement Committee. And then throughout racing, they will continue checking yacht and component masses around all the teams, making sure that we're all still complying.
“The rules are so bold because they're really quite constraining. Trying to build an AC75 that's fast, structurally sound and within a few kilos of its target weight of just over six tonnes is incredibly challenging for everyone in the team – but the aim is to win the America's Cup”