Learning to Fly- A test ride on Stratis, the foiling SL33

by Ben Gladwell, Boating on 23 Sep 2014 They told me to wear a wetsuit and booties. What they should have said was: 'Wear a full diving bell with chain mail gloves. And chest armour. You are going to be riding a raging bull that flies above - and sometimes under - the water at thirty-five knots. And all you have to hang on to is a bit of netting.'

Born from one of the best design labs ever assembled - in-house at Emirates Team New Zealand - the SL-33s were the prototype boats in the build-up to America's Cup 34 and are responsible for the team leading the charge in full foiling.

All the America's Cup syndicates were allowed to build two, 33-foot catamarans to test their design ideas; the boat developed by ETNZ would change the face of the Cup.

ETNZ needed to test its SL-33 concept and couldn't risk being observed by Oracle's spies. Rod Davis, ETNZ coach, devised a plan.

'We definitely did not want them to know that we had figured out how to fly at that point, but we needed to test, so we had to sail,' Davis wrote in a blog.

'The boat with the flying (foiling) package appeared to break down, slowly being towed home. The other boat, with lots of interest from the shadowing chase boat, started sailing in the other direction, to draw the Oracle flies away. Once separated, the boat with the foils could do their thing, without the spies knowing. The mistake Oracle made was not checking who was going with which boat - had they done that they might have seen through the smoke screen.'



Foiling in love

Getting wet on, Stratis, the SL-33 was no surprise, but I was not prepared for the frequency or volume of water that erupted through the trampoline, making it almost impossible at times to see or breathe. Nor was I ready for the G-forces as the boat writhed left and right as it foiled downwind, as though desperate to eject its crew.

Travelling in a straight line while foiling is virtually impossible. Constant and often violent adjustments to the foil orientation, helm and sail trim are required to keep the boat in the narrow sweet-spot of balance and speed which facilitates foiling. The sensation of foiling is quite surreal. The day I went for a ride saw 25 knots of sou'westerly breeze blowing down the Waitemata Harbour.

The crew said this was near the top end. It definitely felt like we were perilously close to the edge, though it felt distinctly different from travelling quickly on other boats. The SL-33 feels like it wants to go fast - there is no sense that the hulls are fighting the loads from the rig and sails.

It feels light. The world feels like it is in fast forward - that you are travelling at normal speed and the water, blurring past, has been sped up. Perhaps the most surprising quality of the SL-33 was its exaggerated response to the breeze. As it sailed into a gust of wind, the cat lurched forward as though it had a lit flare up its stern gland - and it continues to accelerate, like a powerful sports car with a jammed throttle.

Helmsman Will Tiller says the boat's rapid acceleration calls for extra safety measures. 'We can't really have the chase-boat going in front of us,' he says. 'It's got a 250 [horsepower outboard] on the back, so it will do 35 knots, no problem, but it won't get there as fast as we will.'

As the boat tears downwind, significantly faster than the wind, the apparent breeze can come so far forward that the sails stall. Meanwhile, the leeward hydrofoil is still generating colossal amounts of lift, so as the sails stall, the boat begins to heel to windward. The dolphin-striker and windward foil suddenly submerge, hurling chunks of ocean skywards.

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