It uses state-of-the art membrane technology, but works on the well-known principle of osmosis. If two solutions are separated by a membrane with ‘holes’ so tiny that water molecules can pass through but bigger molecules and ions such as the sodium and chlorine components of dissolved salt cannot, the water migrates to the side with the stronger solution.

To use the Sea-Pack kit, you put sea water in one compartment and concentrated dextrose syrup in the other. Each charge of syrup produces half a litre of liquid similar to dilute grape juice in about five hours, and the kit comes with 5 syrup charges.

That means one kit can produce 2.5 litres of drink in 25 hours, which should suffice for one person even when exposed to blazing sun all day. However, since you must wait five hours for your first drink, remember to carry sufficient bottled water to cover the waiting period.

Water, not cola or other sodas. They all contain so much sugar that digesting it leaves you more dehydrated than when you started – if you don’t believe me, talk to one of the Colorado river-running boatmen. Beer is even worse – not only does it dehydrate you as you digest it, but it’s a diuretic as well. Alcohol and survival do not go together.

A recharge kit of five syrup charges costs US$38 – just over seven bucks a pint for you drink, but if the alternative is to die of thirst…

To me, this seemed a little like the inkjet printer business – most of the profit in the consumables – so I asked Rick Feineis of Manta Ventures whether I could use any concentrated juice or rehydrate syrup as an alternative if I ran out and couldn’t get genuine Sea-Pack recharges in a hurry. I give full marks to Rick, who told me that they use a dextrose syrup with an additive that prolongs its shelf life to five years. It would be possible to experiment with your own formula, but you need to match the concentration to the task in hand.

My advice, therefore, is to buy the real thing, but know that a pack of grape juice concentrate might be a useful addition to your survival kit in case you are unlucky enough to be adrift for a long time. If you don’t use it on your voyage, you can drink it when you get back to port.

Those who get involved in such things should take a look at the rest of the Sea-Pack range, designed for things like hurricane relief and fighting AIDS in the Third World.

Quebec in five days’ time

Yesterday 27 June 2008 at 09:45 local time, the French sail training ship Belém left Halifax, Nova Scotia and the statue of Samuel Cunard. Low visibility and head winds dictated the choice of the shorter Canso canal route rather than sailing via the 60 mile longer Cabot Strait to reach Quebec by her scheduled date of 2 July.

Belem took on the necessary pilot at 18:00hrs. I wonder how many square-rigged sailing ships he encounters each year.

Reverse Osmosis – gone are the days of fresh water shortages.

Belém carries 20 tons of fresh water, and produces 3 tons from seawater each day, using a reverse osmosis plant. I don’t know the particular brand or model, but I found details of a typical marine system by Stromme (the smallest they make) with the same capacity.

It’s amazingly compact (1200 X 650 x 900mm – about the size of a domestic kitchen sink & cupboard unit), and requires a 3Kw electrical supply. That’s about 5 or 6 horsepower, allowing for losses – a tiny fraction of what her three generatotrs will supply for other purposes.

What’s reverse osmosis?

A process essential to all living things, for a start. If you separate a container full of a solvent such as water from another full of a solution such as salt water by a membrane with holes so tiny that the water molecules can pass through but the salt ions can’t, pure water will pass into the salt solution.

If the tanks are closed, the pressure in the salt water will increase until it is sufficient to stop any more water passing through the membrane. If you use a pump to further increase the pressure in the salt water tank, you can force pure water out of the salt water tank into the pure water tank, thus extracting pure water from a tank full of salt water – that’s reverse osmosis.

Of course, doing this will increase the saltiness of the water in the salt water tank, so a fresh water production system has to have a way of extracting extra-salty brine and introducing new seawater. The new fresh water also needs to be aerated and acidified a bit. Pure water is yucky to drink – have you ever tasted the stuff in battery top-up bottles?