Charlton:
Excellent start, nice to see a live one on-line!
Although, it appears you've forgotten to take into account the surface area added by the thread "grooves".
eg, take a cylinder of any arbitrary length & diameter, cut 60ยบ threads all the way down the length, and it almost doubles the surface area.
I've been using my own calculations in a spreadsheet, and for a particular bolt I'm currently working with, I come up w/ 3.39 sq-in. Plugging the same numbers into your calculator gives 1.92 sq-in. If I double the shank length (my bolt is fully threaded) to accomodate full threads, it gives 3.12 sq-in., more in line w/my calculator.
I'm not convinced that taking ALL the threaded area into account is necessary, but considering how good zinc is at penetrating crevices, it would seem to be a significant factor.
Sean
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