Mookie if you take your time and use proper technique you could be screaming forever as far as I'm concerned because it doesnt take as much push and strain as most people think. Hopefully you will practice enough to find this out with proper technique. I cant teach that over the internet. Learn to get raspy notes at a low amount of push. Check out the isolation thread if you havent.
http://www.musicianforums.com/forums/showthread.php?t=219911
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Burning water...knowing about your diaphragm is good but its not going to directly translate into singing...at least not from a swimming background. You'll have good volume and air supply maybe. But it is a bit different when you have to clamp your cords around this pillar of air...and to minimize clamping otherthings. You have to be able to keep the larynx relaxed as you go up. And you can learn to hold it open like a yawn position, as you activate the cords. If you can sing, then its just a matter of constant practice and trying to stay relaxed. Be sure to warm up good too.
http://www.musicianforums.com/forums/showthread.php?t=219911