Where an entrance you fancy is encumbered by shoals extending from one or both sides, it’s surprising how accurately you can judge them with nothing more than ‘Mk1 eyeball’. Approaching square-on, it’s easy to place an imaginary mark halfway across. Now divide each half into two and you’ve got pretty reasonable quarters. Chopping the gap into three parts can also be achieved with surprising accuracy. Check the chart, and so long as things aren’t too tight you may well find that one of these imaginary lines will lead safely in, or clear a rock shelf. If in doubt, though, give it a miss.