Hi DIYman. First step is to check what rpm is needed to get the voltage you want. Say you want to run a 12V system, then it means you need 14V from the alternator. So spin it until it gets 14V, and measure the rpm. How to do this depends on your instruments and ingenuity!
Lets say for the sake of calculation you find its 2000rpm. (please note the calculation below changes if this changes)
Now, you want 50 watts in a 5.5m/s wind. Max Power P = 0.3 A m^3, so
50 = 0.3 A 166
so Area = 1m^2
Working at say 50 efficiency of conversion, you need 2 square metres which means a radius of about 800cm.
That makes a circumference of about 5m, so 2000rpm means a tip speed of 2000 x 5 / 60 = 166m/s.
That would be way too fast, for reasons of noise, blade abrasion and safety.
My guess is that you will either need to reconfigure the coils in the alternator, or use gearing. From what I have read, most people who have tried car alternators have not had much success.
I look forward to reading what you do next!