This paper contains a brief survey of a series of instruments which have been developed for use in sailing yachts. It then outlines the principles of two computers; one navigational, the other for sailing performance indication. It does not cover radio navigational aids, echo sounding or radar. The data requirements are: 1. Navigational, comprising distance run through the water, heading, leeway angle. 2. Functional, comprising speed through the water, direction of the apparent wind and speed of the apparent wind.

For measuring distance run, the classical towed rotator type of log, fitted with a mechanical or electro-mechanical revolution counter has been largely superseded in racing yachts by logs employing through-hull sensors on account of drag considerations and the inconvenience of having to shorten the line when sailing slowly in shallow water. It still finds favour in cruising yachts on account of its generally lower price and its simplicity.