Deflatable Dinghy

When you pull the air out of a foil pack of coffee it goes hard, as the irregularly shaped coffee grounds pack tightly and lock against each other.

Applying this phenomenon to a boat, you would construct a dinghy out of panels made of two sealed thick sheets of rubber. Between these two sheets would be several additional wafer thin membranes with tough, lightweight, irregularly shaped granules embedded in them*.

When folding/unfolding the dingy, you would allow air in between the layers: not enough to blow it up like a balloon, but enough to push the layers of granules apart. To use the boat unfold it, add the strengthening struts and, using the supplied vacuum pump, take out the air. This pulls the thin layers together, locking the granules against each other, making the what was a collapsable mass of rubber sheet into a rigid servicable boat.

The same technique can be used to make walking boots hard to protect ankles (vacuum pump in the heel?), to make instant zip-on plastercast/splints, or the 'soft'-top of a convertible car. Anything** that needs to be able to fold when stowed, but hard when used.

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*Embedding the granules in the sheets stops them pooling at one end of a panel. The granules should be embedded so that they would stick out of both the top and bottom surfaces of the sheet, i.e. two sheets laid on top of each other will touch granule to granule so that they can lock when the air is evacuated.

**No sex-toys please. It's just too obvious.