what's the name of the US spy drone sail?

The most likely name is Saildrone ; the specific model often mentioned for U.S. maritime surveillance is the Saildrone Voyager or Saildrone Explorer. In recent coverage, the U.S. Coast Guard has been using Saildrone Voyager craft, and older references mention Saildrone Explorer in U.S. naval contexts.

What people usually mean

If you saw a “spy drone sail,” that usually refers to a wind- and solar- powered autonomous surface vessel with a tall sail-like mast, not a flying drone. Those craft are commonly described as drones, unmanned surface vessels, or sail drones.

Names to know

• Saildrone : the company and general brand name.

• Voyager : a current Saildrone model used by the U.S. Coast Guard.
• Explorer : an earlier Saildrone model often referenced in reporting.

In one line

If you mean the U.S. “spy drone with a sail,” the safest answer is Saildrone , especially Saildrone Voyager for the newer Coast Guard deployments.