Legendary British yacht designer Hugh Welbourn has strongly criticised Team New Zealand over their America's Cup spill, describing it as "careless".

Welbourn also claimed Peter Burling and Team NZ has "shot themselves in the foot" with an over aggressive start that has cast their campaign into doubt.

"That was very careless," asserted Welbourn who is considered one of world yachting's most innovative designers and an expert on the speciality of using foils on monohulls.

Welbourn said Team NZ has been brought unstuck by the wake created by Ben Ainslie Racing after it beat the Kiwis in a pre-start game of cat-and-mouse.

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"They should know not to run into another set of foils' wake at a shallow angle," he said.

"Turbulence, aeration and the vorticity off the leading foil set will totally disrupt the normal flow patterns.

"Plenty of aircraft have been flipped 'A over T' by tip vortexes.

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"Sail any foiler and try and run it behind a prop-wash and you soon find that out.

"You can cross at a steep angle OK with a brief excursion but they shot themselves well and truly in the foot there."

Welbourn's company has been at the leading edge of innovation and yacht design for 30 years with his most recent work being around foil development.