For really tough boat hull cleaning you will need to break out the acid. Muratic acid is the best for this. You'll want to have a 50/50 mix of muratic acid of 6-8% concentrated (12% is the highest concentration you can legally buy over the counter), water and a good strong sprayer. You can purchase muratic acid at Wal*Mart, K-Mart, Pool Supply Stores, Home Depot, Lowe's, Van Waters and Rogers Chemicals. Sears Craftsman makes a nice strong sprayer or try a local store for another brand of commercial pump-up sprayer.
Spray onto the boat directly after doing a presoak with a good strong blast of water. Let it sit for approximately 2 minutes. Just let the acid do its work breaking up and removing the algae. If it is still there try it again for longer and longer periods of time, but not more than 10-15 minuites. You'll be amazed how nice it looks even by the time you come back to rinse it off. Usually you can remove all this excess with a good strong brush after the acid shock treatment.
You may have to get with a smaller scrubbie into some areas around the drive system and other nooks and crannies underneath. If it's on a trailer and the bunks are on it, that's just the way it goes, you can't get around those - you can't move that boat on the trailer. On a hoist, you'll have full access to the boat.
Once you've scrubbed off one side of the boat, rinse, and repeat the procedure on the opposite side. Give it a very strong thorough rinse getting all the residual acid off the trailer as it can sometimes cause rust spots and corrode. Only use this muratic acid mix for boats that are extremely algae covered, heavy in barnacles, and other growth associated with long-term in-water moorage. Often times you can use Slimy grimy and hot water or Starbrite's buffered acid will work just fine for cleaning a mildly dirty boat bottom. For cleaning diesel soot found around the transom, aft deck ceiling and along the water line around exhausts, but not bottom paint, you can use Meguiar's #50 first, then proceed with compound Monster Agua-Blue 200, Maguire's #44 or #49 if this doesn't work.
If you are cleaning along the waterline and boat bottom that is not painted, you will want to reach for the bottle of On/Off by Maritime Products Company. You will remove all barnacles, moss growth and browning incredibly fast. Don't get this stuff on your hands or on any bottom paint as it will streak the paint and will burn your hands. This is corrosive, but will not harm gel coat and neutralizes with water. Don't get it on the trailer, it will streak and etch it. It is an oxcillic acid mix and is available at West Marine stores. If you are a contractor and doing this for a living the pricing usually goes from $3.00 to $4.50 a foot. Again, market conditions in your area may be a little more or a little less. You can adjust to fit those market conditions.
The Marina-Tec boat wash system is great. If you are contractor and have lots of work you may want to consider leasing a location to wash boat bottoms. GMBH & Co. has a hull-leaning machine that uses two brushes that rotate horizontally along the hull and two brushes that rotate vertically at the heel. It is an automated system and does not use toxic chemicals. It is similar to an automatic car wash only for boats in the water.
Do it right the first time, do it efficiently and think about it.
"Lance Winslow" - If you have innovative thoughts and unique perspectives, come think with Lance; www.WorldThinkTank.net/wttbbs