why do they call w double u when its a double v (v + v = vv?

Answer 1

'w' is pronounced 'double u' rather than 'double v:
English uses the Latin alphabet of the Romans. However, this had no letter suitable for representing the phoneme /w/ which was used in Old English, though phonetically the sound represented by /v/ was quite close. In the 7th century scribes wrote uu for /w/; later they used the runic symbols known as wynn. European scribes had continued to write uu, and this usage returned to England with the Norman Conquest in 1066. Early printers sometimes used vv for lack of a w in their type. The name double-u recalls the former identity of u and v, which is also evident in a number of cognate words (flour/flower, guard/ward, suede/Swede, etc.).

Answer 2

Probably something to do with the fact that in old Latin, there were no Vs. All U's were written as Vs. Look on old Roman inscriptions and you'll see what I mean. So in old Latin, two Us would look like VV.

http://www.omniglot.com/writing/latin.htm

Answer 3

Good question!! I've always wondered about that one myself! Hope someone can give you a good answer. =)