Before you toss out last year's canvas hammock, consider cleaning it. There are two basic hammock types--those with spreader bars that can't be removed and those with spreader bars that can be removed. A hammock has spreader bars at each end to keep the hammock from rolling over. Before attempting to clean your canvas hammock, you need to determine if the bars can be removed. Then you can begin the cleaning process.
Machine washing is for a hammock that has removable spreader bars as you don't want to put a hammock with wood attached in the washing machine. You can wash your hammock in a large washing machine, possibly taking it to the laundromat if your machine is not big enough. Before washing the hammock, you need to use strings to tie the hammock together just before below the metal rings and then tie the two ends together. This will prevent tangling. Wash the hammock on the gentle cycle, or place it in a large pillow case (like a body pillow pillow case).
Fold your hammock in half and tie it just below the hanging rings and tie the strings together to prevent tangling. Fill the tub with mild detergent and water (no bleach) and submerge the hammock in the water. Use your hands to agitate the canvas, and use a small scrub brush to attack any stubborn stains. Do not use this method of your hammock has spreader bars as you don't want to submerge wood.
Lay Flat to Clean
This is the best method for cleaning a hammock with spreader bars as it allows you to not damage them. Lay tarps out on your driveway or other flat surface, and spread out the canvas hammock on top. Use a soft bristle scrub brush, and bucket of water and detergent to scrub the hammock clean. After cleaning the front, flip it over and clean the back side of it. Re-hang the hammock and allow it to dry in the sun.