The term
Daysailer was coined by the O'Day Company back in 1955. They built over 10,000 of their
Daysailer models over the years and the term has become generic to represent just about any "open" sailboat...usually under about 20 feet or so, although and "open" sailboat over 30+ feet might also be called a daysailer.
No overnight accommodations.
Even a 62 foot 12 Meter Yacht might also be refered to as a daysailer.
NB
http://sailboatdata.com/viewrecord.asp?class_id=3338
PS: I'd be willing to hazzard a guess that the O'Day Daysailer @ just under 17 feet long would NOT fit under the Weirs bridge whithout a crewmember hanging over the side to "heel" the boat over to reduce clearence required. You would need a "kicker" (~3 HP).. to transit the channel.