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Tech Talk: Foredeck Bumps

The bumps on the bow of Alinghi Red Bull Racing’s Boat One are likely features to locally straighten flow leading into the tack point of the jib.

Ugo Fonollá / America’s Cup

Along the perimeter line of the Boat One’s bow there is a hard chine which is typically located at a stagnation point of the incoming flow. This is a locally high pressure zone where flow is divided between that heading underneath the yacht, and flow heading over the deck and across the jib and mainsail. The high pressure zone, along with the rounded upper surface of the bow results in flow accelerating from the windward to leeward sides which dramatically skews the local apparent wind angle (by around 15 degrees).  

Ugo Fonollá / America’s Cup

This skewed flow is forced onto the foot of the jib, which causes a local zone of flow separation, the creation of a vortex over the leeward side of the sail, and ultimately causes the drag of the sail to increase.  The bumps are arranged in a manner which would locally alter the pressure distribution and encourage flow to be redirected between the bumps (aft). They would therefore locally ease the angle of attack on the jib, and consequently slightly reduce the drag on the boat.

Alinghi Red Bull Racing, Day 1 of AC75 (B2), Launch of the new AC75, ‘’BOAT ONE’’’ in Barcelona. - 16April2024. 37th America’s Cup Recon.
Ugo Fonollá / America’s Cup
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