Stop one ship from either sailing into a storm or (later) being taken over by pirates.
In 1793, Thomas Jefferson ordered a set of instruments from France that would let the US calibrate to SI units, but the ship hit a storm at sea, blowing his ship into the Caribbean. There it was attacked by pirates (British privateers). The materials never arrived. By the time France sent a second set, US had a new Secretary of State who didn’t like metric system.
You either need to delay the ship departure (so it misses the storm) or help take down the pirates. Minimal intervention.
- Pirates — yes, pirates — may be why the U.S. doesn't use the metric system
- How Pirates Of The Caribbean Hijacked America's Metric System