The wing ump is visible, but the primary ump would be directly behind Oracle (if they were positioned properly) and would be able to see the gap between the boats.
The only umpire out there (wing or primary) was 70 feet from the contact, off the starboard transom of NZ. Impossible for the umpire in that boat to see contact. There was not another umpire boat anywhere in sight. Probably why they didn't rule was they couldn't see it, if it happened.
