what windspeed makes waves unsurfable?

This is a bit of a 'How long is a piece of string?' question. It depends on many different factors.

1) The break - there are breaks local to me that with 10 knot on/cross shore winds will be unridable they can handle stronger offshore winds because they are at the base of cliffs. Then on the other end of the spectrum there is a beach that only breaks in the largest of storms and as long as the wind doesn't have an eastery component it will be ridable.

2) The board you ride - In stiff offshore winds I can find it harder to get into a wave early enough to get a good ride, the waves normally break faster and hollower so you need to get in earlier. A larger board will allow you to paddle quicker and get onto a wave earlier.

3) The waves themselves - a lined up swell will handle more localised wind than a choppy swell, a choppy blown out swell can be a disaster but a good lined up swell with some wind can be alot of fun, and something like a cross shore wind can help with things like airs by keeping the board blown onto your feet.

Well hope it's a help.

Back in the day, I surfed at a secret spot on the North Shore called "Iron Boats" during the early stages of Hurricane Iniki when it wasn't yet a category 4 or 5 hurricane. The winds were, i would guess around 40 to 50 mph straight off shore and it felt like you were riding the waves in slow motion. That's the first and last time I did anything like that.

you have to factor tide into it too,and if there is any swell running,i've seen calm days with a swell running quite high,and a bit of wind against the tide and things can get dangerous add a swell and that's no fun in a boat let alone on a board

The one dat causes tsunamis or whirl pools. Don't try to ride that wave.