The correct answer is the "Icicle".

Sixty-seven feet long, with over 1,000 square feet of canvas sail,
the "Icicle" still holds the Guinness World Record for largest ice yacht.
It was constructed for John Aspinwall Roosevelt in 1869 and
required a railway flatcar to be moved from location to location.

(This corresponds to the December 18, 2025 newsletter)