I agree with Roger on GPS but my Garmin 38 which I bought for £80 sometimes does funny things. Like terrestrial bearings there is always an area of uncertainty caused, I suppose, by reception conditions and the position of the satellites. Quoted accuracy is within 100 metres 95% of the time which I take to be 2 standard deviations and would give 50 metres 60% of the time and 150 metres 99%. But these are radii since you don't know in which direction the error lies and you can be sure only that your position is within a 200 metres diameter circle - or 300 if you need to be extra sure. You can check this by not moving the set and watching the map page develop. Your position is updated about every second and the page will slowly fill with a random pattern of fixes, most within a 100 metre radius. But the position page will also show a speed and distance covered even though you haven't moved.

The problem is that it has to distinguish between apparent movement due to the random error of fixes and your own forward movement and if that is slow, perhaps about 2 mph (and remember this is speed over the ground) and your course fluctuating it can give false readings for speed and bearing (but not position which is unaffected). Once it thinks it knows your course and speed it seems to reject aberrant readings; from a cluster of readings a clear course will appear on the map page. If you stop the map page begins to show a random pattern again, so it may be that the most accurate positions are given when you are moving. I have also noticed that the waypoint page will go berserk pointing in all directions particularly as you near the waypoint. I think this must be because the closer the waypoint is the more random error will affect the bearing. An error of 100 metres when the waypoint is a kilometre away will have less effect on the bearing than it would at 400 metres.

My Garmin eats four AA batteries at a time. Other sets use less. I use rechargeables but they only give 5 to 6 hours of continuous use. Rechargeables vary in capacity between about 400 and 750 mAh so it pays to read the small print. I have worked out a way of charging from a 12 volt car battery but that is not much use in a dinghy. They would last longer of course if you only switch on occasionally. Another problem is where to put it. It likes a clear view of the sky overhead down to 45 degrees but it seems to be happy in a closed plastic box.

On the subject of loos, there is a product called Insta-Loo which consists of a polythene bag and a sachet of crystals which reduces the human waste to an odourless jelly such that you can throw the bag into a waste bin. They say the RNLI uses it. I have no experience of it myself. Insta-Loo, Unit 3 Scotland Road Estate, Dry Drayton, Cambridge CB3 8AT. tel: 01954 211955.

Peter Francis