the same, only different

In the last week two brand new AC75’s have been launched and they could not be more different.

The first to splash was the New York Yacht Club entry American Magic later christened Defiant while on the other side of the world in Auckland, New Zealand the Cup defenders Emirates Team New Zealand launched their yacht named Te Aihe. Other than them both being 75 feet in length and both being sloops they don’t have much in common. This is the beauty of a rule that encouraged innovation without imposing too many restrictions. 

The hull shape on the American yacht is almost scow-like with a flat bottom, very rounded bow and round topsides with the maximum beam halfway down the topsides and not where the deck intersects as is custom.

The New Zealand yacht also carries max beam down the rounded topsides but the underwater hull shape is not flat but instead a series of hollows and chines. The yacht also has a distinct “bustle” or bulge where the foils are attached. This kind of deformity was prevalent in the old IOR days when designers placed bulges to beat the measurement rule. In the case of Te Aihe it’s not about beating any measurement rule.

Instead, as Team New Zealand’s chief designer Dan Bernasconi stated, “the hull deformity comes into play when the boat starts to transition between sailing in displacement mode and  sailing in foiling mode. As the boat lifts onto the foils the only part of the hull left in the water is the bustle. We’re trying to minimize the amount of wetted area and get as much of the boat as we can up and out of the water as quickly as possible. Every square meter of wetted area is friction, and that will delay the acceleration.” 

It’s clear that at least in terms of hull shape that Emirates Team New Zealand have opted for a much more radical design and that may account for the fact that the US boat was up and foiling right away while the New Zealand boat had some trouble to begin with. Both boats are now foiling just fine and are being tested in a variety of conditions.

One of the complaints from the previous America’s Cup was the cost of having the fixed wing removed at the end of each day’s sailing and so the new boats have more conventional sails that can be raised and lowered by the crew. The top half of both mainsails look similar and quite conventional and this was to be expected given that the rule stipulates girths.

Both sails have a massive square head, but the Kiwi’s have opted to carry their mainsail all the way down to the deck to avoid the “end plate” effect which occurs when wind from one side of the sail flows under the boom and disturbs the wind on the other side. The main is a deck sweeper similar to the mainsails seen on an F18 catamarans and while I am sure that this will improve performance it begs the question, “how are the crew going to get from side to side?”

One of the features of the New Zealand boat are two cockpits on either side deep enough presumably to allow the crew to keep their heads low to avoid windage. Perhaps there is a way for the to scamper around the front of the mast. I am sure this will all be worked out in the sail trials.

One striking similarity between the two boats is the size of the foretriangle. All the photos I have seen so far have the boats sailing with a main and headsail but they will also be carrying a Code 0 that will be set from the bow sprit. It’s going to be a massive sail that should get the boats up on their foils quite quickly, but I can see a scenario where  the boats can rapidly become overpowered with all that sail area and sudden increase in apparent wind. Again there is still plenty of time for them to work this out with the next Americas Cup still a year and a half away.

I have to admit that I was quite skeptical when this new design was announced but the smaller prototypes and now the full scale boats have won me over. There has been some incredible footage of the boats flying on the foils and I can only wonder how the crews are not getting blown off the decks. It’s anticipated that the boats will be able to sail at 50 knots. In 25 knots of wind that’s more than 70 knots of apparent wind, enough to blow a dog off its chain. – Brian Hancock.