AC 36 Foiling Monohulls

Ben Ainslie on the "bustle" or "blister": Road to the America's Cup ep5: AC75 hull design (from about 4:20). He thinks it will help the boat accelerate to start foiling, and has the potential for "really big aerodynamic performance gains" as an endplate between the hull and water when foiling.

The shape of the various boat 2s will be an indicator of which approach is best in practice based on actual sailing and extensive digital analysis.

 

Yes, the idea behind the bustle must be to minimize the drag before take-off in marginal foiling conditions. Foiling boats use to have the highest drag when partially supported by the foils and partially supported by the hull. So if this bustle has some 1/2 the static displacement of the boat and has a semi-circular cross-section, it will provide the lowest possible wetted surface in this phase, when the wide flat-bottom part of the hull is lifted out of the water. Creative thinking by the designers!

 

Funny how LR and ENTZ had prior knowledge of the design and came up with a similar philosophy (though very different implementation) whereas INEOS and AM had no prior knowledge and came up with much more conventional hulls below the waterline.

The flat hulls might come into their own in displacement mode if they can get say 20° windward heel when initiating foiling (Moths are now up around 27° and cracking 20kn going to windward[1]), but I haven't seen anything like that much yet on an AC75.

Comanche is a rocket upwind when heeled over riding on the chine, quite the opposite when sitting flat in light breeze.

1. Noting that the gain comes from an increase in RM from leaning over, there are minuses from less efficient rig, wasting sail drive as lift (counteracting the additional RM), extra drag from inclined foil, etc. but the overall effect is better VMG. It's actually really hard to find the sweet spot or "zone".

 

http://www.pressure-drop.us/forums/content.php?9785-Luna-Rossa-Dismasts

 

An interesting video on YouTube that includes Te Kāhu, the ENTZ test boat, going from displacement to foiling. If this is the required technique, it's nothing like what I expected.

E47 - Te Kāhu taking off

 

But it is pretty much what I expected. This video shows how important the boat-speed is for these boats to get enough stability from the active foil. The boat that can stay on her foils a full race will win.

 

Nice video, Under these conditions they have pretty good control. But look at the streak of ventilation on the leeward side of the foil-arm 0.5 m below the water-surface or so from 0.40 and on. When I design the surface-piercing parts of e.g. hydrofoil-struts I try to avoid such ventilation.

 

I'm sure they did too. Just shows that even with world class expertise and design tools and a huge budget things can still go not quite to plan. ;-)

By not having a bulb, ETNZ need to have thicker foils and maybe strut extension to reach the required mass, which likely affects optimisation of the overall design. But clearly they think it's better than a bulb as the test boat's foils are bulbless too.

 

Thanks for the video Doug. Well, this was one weakness of this configuration I was warning for in my post #285. When they fly the boat high on the leeward foil, just some 5° to 10° leeward heeling is enough to ventilate the rudder-foil. Hope no one was hurt.

 

From the onboard view, the boat is pretty much level when the rudder comes out. There is video of all the AC75s sailing with small amounts of leeward heel while foiling.

The transition from downwind to upwind is quite difficult. The boat accelerates as it turns up so the apparent wind goes way forward–turn too quickly an it backs the sails. So the capsize moment is reduced and the boat tends to float higher. It can be quite disorienting so very easy to move something the wrong way. The stern does a bit of a bobble just before they lose control so maybe it was just a momentary lapse of concentration.

 

Things are pretty quiet while boats are transported to Cagliari, here's a video of American Magic training in light conditions. A voice off–screen says "under 6 knots".

 

Looks like Cagliari is off with all sporting events in Italy cancelled: Coronavirus Italy: PM extends lockdown to entire country. I guess the boats will return to their home ports for training, so another month or so with no sailing news…