Camping and outdoor catering - a barrel of hot washing water in about 10 minutes!
Those blue plastic barrels you see on farms are about 150L (40 gals.). It's very easy to drill two holes, screw in by hand a couple of standard 50mm plumbing tank fittings, and screw the standard Kiwitub hoses into those.
At the New Zealand Alpine Club climbing camp up the Rees River we provided washing water for over 100 climbers for a week. It took about 10 minutes on gas for the cold mountain stream water to become steaming hot. A bucket of that tipped into the bucket with holes in slung under some willow branches gave a six minute hot shower. Luxury!
The plastic tank fittings cost about $30 each; the used barrel was $10 (new ones cost about $80); the bucket with holes drilled in the bottom was $2. Useful things, buckets.