Some people use Pex, coiled up in a big coil in a cold frame (to trap heat) as their solar heater.
The volume of water inside a 3/4-inch hose to be 1.01 gallons. The volume inside a 5/8-inch hose is 0.57 gallon, and inside a 1/2-inch hose, there are 0.25 gallon of water.
The mathematical formula is (3.14)•d•d/4, where "d" is the inside diameter. Using this formula, the inside area of a 3/4-inch hose -- with a 1/4-inch wall thickness -- is 0.1962 square inches. Corresponding values for a 5/8-inch and 1/2-inch hose are 0.1104 and 0.0491 square inches, respectively.
First convert the inside area from square inches to square feet, using the conversion 1 square inch = 0.0069 square feet. You can then multiply the result by 100 feet to arrive at the total volume of water inside the 100-foot hose, in cubic feet. Multiply by 7.48 to convert to gallons.
If you used an Aqua Helix shower nozzle (0.5 gal per min at 40psi) you can get, at a 50-50 mix of hot and cold water, about 4 min of warm water for a 100 foot hose.
Not quite enough for a 6 minute 'Navy Shower'.
This is dependent on water temperatures and heat losses to the shower nozzle.
If hot water was only 140 deg F and cold water only 60 deg F, your shower time will be shorter.
A good 18 min shower (females - average) would take about 350 feet of heated hose.
A good 12 min shower (males - average) would take 300 feet of heated hose.
Rubber hose will take longer to allow heat transfer to the internal water too.
You could coil up some copper tubing (3/4" diameter, about 5 feet long) and place it into a rocket stove for hotter water, longer showers.
Cold inlet at the top and hot outlet at the bottom in a vertical configuration.
Coiling that size tubing will not be very easy.
3/4" refrigeration tubing is actually 3/4" O.D. whereas 3/4" plumbing pipe is 3/4" I.D.
The use of loose sand or perlite in the tube can resist kinks.
NON ASSUMPSIT. I am by no means an expert at anything. Just a lucky guesser.
We used a setup like this for showers at family reunions and it worked quite well. And there is something exhilarating about showering outside. I learned in a recent renewable energyclass about the the bacterial issues with this simple setup so it's not part of my long term plan. But yes, another vote for outdoor solar showers!
"Have nothing in your house that you do not know to be useful or believe to be beautiful." - William Morris