That's a great question, and something I've paid close attention to in my SP bounty hunting. I believe not only does it require hull damage, but a certain percentage or amount. I have failed to get bounty credit on ships where I shot away the shield, but did no hull damage. And I've also shot away small amounts of hull and received no credit.

So my conclusion is you need to damage the hull, and more than just a little bit smile

Animals flee this hell, the hardest stones cannot bear it for long. Only men endure