The problem with this captcha is presentation. It's all down to marketing really.
Just think, why does it have to be a boring, undescifrable text-based captcha?
We could have a random ship image captcha that requires you to input the AOB. This would also be good practice. Other ideas: ship identification, something similar to those old pc games that asked you stuff from the manual or a TDC solution captcha for the pros
Oh and I can do these in PHP for you if you like the idea.