What are the lightweight limits, and how does weigh-in work?

Under Rules of Rowing 4-104.3, no rower in a lightweight crew (including a single) can weigh more than:

The coxswain isn't counted, and any coxswain who otherwise meets the rules can cox a lightweight crew. Any event that isn't lightweight is "open" weight.

Weigh-in (4-106):

• You're weighed in racing uniform (covering the torso), without shoes.
• Lightweights weigh in once a day, in the weigh-in window for their first lightweight race that day. If racing is rescheduled and an adjusted window is created, you can use either window, but not both.
• You get one attempt. If you're within 1.0 lb of the limit, you get one re-weigh in the same window. More than 1.0 lb over on the first weigh means you can't race the event.
• Miss the window and you can't race.

Skill class doesn't depend on weight: Intermediate, Senior or Elite status applies to both open and lightweight events.