The FAQ and Site Standards link will explain. Particularly the addtional site standards for sailing ships which quotes:
"A. Remember that site standards require full ships only - sailing vessels require masts (complete). Don't crop off top of masts"
Personally, I have to agree with the site rules on this one, sailing ships with masts cut off do nothing for the overall quality of the photo, but that's just my opinion.
Brgds
Phil