The single-letter signal is one that every skipper should know because it may be of some importance to him. If U (dot-dot-dash) is made to you by light, sound or flag, it means, ‘You are running into danger’. But be careful not to confuse it with V, which is only one dot different. The phonetic for U is Uniform.

The single-sailed type of rig used by the American cat-boat Una which aroused much enthusiasm (and copying) when she appeared at Cowes in 1854. Although that particular rig raised the Gaff by a single halyard and a complex system of blocks, the term Una rig is generally applied to the Cat-boat pattern, where the mast is stepped well forward and no jib is carried. (But not to the various kinds of lug which are used without a jib.)

In pilotage, a rock or other obstruction which is never covered by water at any state of the tide.

With no sail set. (But understood to be making way due to the wind pressure on hull and rigging.)

See: Not under command

For the purpose of the Collision Regulations a vessel being propelled by both sail and power is regarded as under power, and subject to the relevant rules.

On the leeward side of something, such as a headland, a harbour wall, a ship’s hull…

Making way through the water. Often confused with Under weigh, which is a term specific to an anchor, meaning that the weight of the anchor is off the bottom and on the cable. The foredeck hand may call out that the anchor is ‘under weigh’, whereupon the helmsman may feel free to get the vessel under way (though slowly). The cause of confusion is pretty obvious.

If you have a line out to a post and you work your boat along the line towards the post, then you under-run that line. The same expression applies even when the end of the line is being brought aboard and coiled on deck.

An anchor, a mooring buoy, or any other object is underfoot when it lies vertically beneath the vessel’s Forefoot, whether it be at the surface, or on the bottom.

The current which flows to seaward from a beach on which waves are breaking. The flow is below the water surface.

Obselete chart term meaning a light which is not continuously under human supervision, and could therefore be extinct for some time before being put to rights.

A sailor’s life is much influenced by wind and tide, and the terms Up and Down are much used in the sense of towards or away from wind or stream. Thus up-helm means that the tiller should be moved to the upwind side of the boat, which is usually topographically up as well, being towards the higher side of a heeled boat. Down-helm is the opposite. To sail Higher, or to Point higher is to sail nearer to the wind, but ‘Lower’ is rarely used in this sense. (See also: Bear away, to)

An exhortation to get on and weigh the thing, or just a verb as in, ‘We’ll up anchor as soon as we’ve heard the early forecast’.

Instruction to oarsmen requiring oars to be pulled equally, or more strongly on one side than the other.