Here's how I made a dry bag to use up some of the leftover fabric after making my polytarp crab-claw sail. I probably should point out that I have never made one of these before, or even used one. Nevertheless, the final product seems fairly good.

This sort of bag is great to take boating or to the beach, or to put inside a non-waterproof backpack if you thought it might get wet. It keeps things dry most of the time. You could put some lunch, a towel and some warm clothes in it for winter sailing, or put your wet wetsuit in it to stop your car/bicycle trailer from rusting. It wouldn't survive immersion for long without letting water in, and a few drips could possibly get in anyway. If you need better protection for small, water-sensitive items use a plastic jar with improved gasket or a dedicated boat box.

How can a simple fabric bag with just a rope to close it be waterproof? Read on to find out.

Step 1: Get Ready

Right, you'll need some stuff.

Some fabric: I used Polyethylene tarp because i had it, but anything waterproof and fairly tough should do it.

Thread: Use polyester, heavier is better for a big bag. More expensive is often better too.

Stiffening material: On commercial bags, they often seem to use a strip of stiff plastic sewn into the hem around the opening. I couldn't find any so I guessed that some soft foam would work. What it lacks in actual stiffness it makes up for in squishness, helping create a reasonable seal.

Mouth closure: Use a webbing strap and a buckle if you want to get fancy, or some strips of non-brand-specific-velcro-like-fastener. I used a chunk of stiff cord, it works well.

Sealant: This depends on your fabric and intelligence. I'm currently trialling a silicone sealant but I don't hold out much hope. I reckon some kind of tape would work better.

- Double sided tape: I bought some stuff intended for laying carpet. It's super sticky and great for holding things in place while you sew, but not essential. I had to cut the roll widthwise with a knife as the stuff I bought was too wide.

To put this all together you need the following tools.

Sewing machine: - Sewing Machine: Someone you know has one they don't use. Older is better generally. When you find a machine spend lots of time sewing samples of your fabric with the thread you'll use. Fiddle with upper and bobbin thread tension until it works reliably. You may need to experiement with different sized meedles too. Things will be easier if you use fairly "normal" fabric, i.e. not to stiff, slippery or tough.

- Scissors: Some heavy shears and some thread nippers are a good combination, but whatever you have is fine.

- Small, sharp knife: Use this for everything.

- Measuring devices like a tape measure and ruler

- Permanent marker.

- I won't even mention an unpicker.

Step 2: Mark and Cut Fabric

Decide on the shape you want your bag to be. The simplest would be to cut a big rectange, fold and sew it along two edges like a pillow case. That's pretty crude though, and will be hard to fill efficiently unless you can pour the contents in. I went for a cylindrical shape, but anything remotely prism-like would work.

The photo below shows the two pieces which make up my bag. There is a circular piece and a rectangular piece with the base length equal to the circumference of the circle. I added a 25mm seam allowance around everything, and a double width allowance at the edge of the rectangle which would become the top edge of the bag. I think the circle was 34cm diameter and the rectangle was 75cm high. Remember to consider that the top edge needs to be folded over a few times to close the bag, so make it extra tall.

The second picture shows a simple way of marking a circle which I used for the base of the bag.

Step 3: Sew a Tube

Fold the rectangle in half so it forms a tube. Align the edges and sew a seam along the line you have marked. Use straight stitch, the longest you machine can do. You now have a tube with a long flap extending past the seam. If your bag was short and fat, you could zig-zag stitch the flap down onto the fabric on one side, making a very strong "flat-felled" seam. Since my bag was too tall and my fabric too stiff, this was impossible. I simply sewed a row of zig-zag stitching down the flap which probably doen't do much. The seam is on the outside, so we'll turn the bag inside out later to hide it.

A flat-felled seam would make lots more holes for water to get through anyway, so it's not all bad.

Step 4: Add the Base Panel

This part can be a bit tedious depending on the shape you choose for the shape of your bag. A rectangle with rounded corners might be about the easiest, but you can manage with whatever.

Cut small pieces of double-sided tape and stick them to your base panel all around the seam allowance. Remove the backing from one piece of tape and stick it to the corresponding seam allowance inside the tube. Remember the bag is inside-out at this stage, so all seams should be hanging out.

Work your way around the base panel, sticking the seam together. You may have to move and adjust it to get rid of any little wrinkles. When attaching a panel with curved edges like mine, it can help to make little cutts in from the edge, staying within the seam allowance. This alows parts of the excess fabric to overlap neatly.

When you're satisfied with the fit, go ahead and sew a seam just like you did to make the tube.

Step 5: Top Hem and Closure

Decide what you want to use to stiffen the top edge of the bag. Whatever you use has to hold the edges closely together when you roll and bend the edge of the bag to close it. Something stiff to apply pressure and some padding to ensure the fabric is pushed tightly together would seem ideal. I couldn't be bothered with that so I just cut a strip of some 5mm EVA foam I had.

You now need something to hold the top closed. I used some stiff cord, but you could use webbing, velcro or whatever else you have.

Stretch the mouth of the bag out wide and poke holes through two oppsite points, within your extra-wide seam allowance. Push the ends of your webbing or cord through these holes, so the middle bit can get incorporated into the hem. Fold the hem over and insert your stiffening material. I used double-sided tape to hold it in place for sewing.

Now sew a row of stitching right around to close the hem. For this job, it's easier if you can hang the end of the machine out over the edge of the table, letting the mouth of the bag hang freely. Once this is done you can turn the bag outside-out.

Step 6: Sealing the Seams

Clean the area around the seams with methylated spirits, or rinse it with hot water and detergent. This removes and contamination which might prevent your sealant from sticking. Apply a thin smear of sealant into the seam and let it cure for the required time.

Your bag is now ready to use.

Step 7: How to Use the Bag

Put stuff in it.

Fold the mouth of the bag flat, with the closure cord hanging out each end (photo 1).

Tightly fold the edge over 3 or 4 times (photo 2).

Bend the folded end around so the ends meet, and activate your closure. I tied a reef knot (photo 3).

Now go and indulge in potentially-stuff-wetting activities, smug in the knowledge that your bagged stuff will stay pretty dry. Enjoy.