Dry Bag for Rucksack: How to Pick and Pack One for Dinghy and Cruising Trips

Updated November 2026

A dry bag for rucksack use gives you a waterproof liner for the pack you already carry on the boat or ashore. This guide walks through how to match the bag to your trip, fit it inside or over your rucksack, and pack it so your gear stays dry even when spray or a capsize hits. Most sailors can get this right with a few measurements and a short checklist.

Step 1: Decide Between an Inner Liner and an External Dry Bag

You have two basic setups. An inner dry bag sits inside your rucksack and protects the contents from water seeping through the pack fabric or zippers. An external dry bag is a separate waterproof sack you strap to the outside of the rucksack. For dinghy sailing, where you are mostly keeping gear out of the bilge, an inner liner is usually the more practical choice. For cruising, where you may need quick access to a bag strapped to the deck or cockpit, an external bag is often easier to manage.

Step 2: Size the Bag to Your Rucksack

Measure the internal width and depth of your rucksack, then choose a dry bag that is about 2 to 3 inches smaller in each direction so it slides in without bunching. Leave enough height that the roll-top closure can fold at least three times. A bag that is too large will be difficult to seal, and one that is too small will leave you with wet corners where the fabric is stressed.

Step 3: Check the Closure and Material

Roll-top closures are the standard for dry bags because they seal by rolling the top down several times and clipping it. Look for a roll-top with at least three folds, and check that the seam is welded rather than stitched, since stitched seams leak. Most manufacturers specify PVC or TPU coated nylon or polyester for the body. Heavier denier fabrics resist abrasion better when the bag rubs against the rucksack frame or deck hardware.

Fitting Tips

If the bag will live inside a rucksack that has a back panel with a hydration sleeve or padding, test the fit with the pack fully loaded. Make sure the bag does not press against the zipper line, where pressure can push water through the pack seams.

Step 4: Pack for Water Exposure

Pack heavy items low and close to your back, and keep electronics and paperwork in a smaller inner dry sack within the main bag. Press out as much air as you can before rolling the top, since trapped air makes the bag float and can increase strain on the seal. Roll the top at least three times, fold the roll flat against the body of the bag, and clip the buckle firmly.

Checklist Before You Leave the Dock

1. Measure the rucksack interior and confirm the bag size.
2. Inspect the welded seams and roll-top for wear or pinholes.
3. Pack electronics and documents in smaller sub-bags.
4. Expel excess air and roll the top at least three times.
5. Test the seal by submerging the closed bag briefly in a sink or bucket.

Common Mistakes

• Choosing a bag that is too large, which makes sealing difficult.
• Leaving the roll-top loose or folding it only once or twice.
• Assuming a rucksack with a rain cover is waterproof on its own.
• Storing wet gear in the bag and sealing it, which traps moisture.
• Ignoring abrasion, which can wear through thin bag walls on hard edges.

Recommended Gear

• A roll-top dry bag sized to your rucksack interior, since a snug fit prevents folds from loosening during movement. View on Amazon
• A smaller dry sack for electronics and papers, which keeps sensitive items separate and easy to reach. View on Amazon
• A compact dry bag with a clip buckle for quick attachment to a deck line or rucksack strap. View on Amazon

For background on how different dry bag styles are used on the water, see What Is a Dive Bomb Dry Bag? Uses, Features and Buying Tips.

FAQ

Can a dry bag go inside a rucksack?

Yes. An inner dry bag protects gear from moisture that gets through the rucksack fabric or zippers. Size it so it fits without forcing the pack’s seams open.

How many times should I roll the top of a dry bag?

Most manufacturers recommend at least three folds before clipping. More folds generally give a tighter seal, but the bag must still have enough room to close.

Is a dry bag enough to keep gear dry if I capsize?

A properly sealed dry bag resists water well, but no bag is guaranteed fully waterproof when submerged for long periods. Check seals before each trip and consult the manufacturer’s specifications for submersion limits.

Conclusion

A dry bag for rucksack use works best when it fits the pack, seals with at least three roll-top folds, and holds gear packed to expel air. Check the seams and closure before each outing. Following these steps will help keep your gear dry on the water.

About the author