Removing Hull Stains???

Can someone tell me a good procedure to get the yellowish brown stains off of the bottom half of my hull??
The boat is an 82 Nautique that I bought. I am sure they have been there a while. Any ideas??

I tried a search but I suck at the internet.

Some people have suggested Sno Bowl toilet cleaner. I tried it but it did not remove the stains on my boat hull.

A lot of guys will suggest a toilet bowl cleaner/perioxide solution, but I've tried that and it didn't work all that well. I'd recommend ON&OFF. There's a couple different manufacturers, but they are pretty much the same stuff. It's pretty much spray on & hose off!!

http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&classNum=34&subdeptNum=11201&sto reNum=4&productId=72467

Use muratic acid. I use an all-plastic spray bottle (garden type, cheap, Wal-Mart) to spray it on the hull. Let it soak in for a few minutes. Use a long pole car wash brush. Rinse well. This will corrode your trailer if there are any rust spots or paint chips, so rinse the trailer well. This will release chlorine gas. DO NOT do this indoors. Stay upwind. Do half the boat at a time and move the boat so you are upwind. Work from the bottom of the hull and work back toward you. This will also damage your skin, so use long gloves and safety glasses.

For small spots, use FSR gel.

Where do you get muratic acid??

Pool Supply. If you're using it on Concrete use plenty of water as it will eat that up if not diluted and left to sit.