This boat is heading at ‘three-two-five’. If the navigator asks the helm to steer ‘three-twenty-five’ he’ll get what he wants just the same. But if the course had been 315 (‘three-one-five’) and he’d said ‘three-fifteen’, he could have been mis-heard on a windy night. The boat would have gone barrelling off on ‘three-fifty’, and Lord knows where she’d have ended up. Three single-digit figures including an ‘Oh’ where appropriate (or a ‘zero’ if you prefer) leaves no room for doubt.