Nefs were a sort of table-ornament shaped like a boat or a ship. Some were used either purely for decoration; to store table-linens or eating-utensils; or to hold some sort of condiment (the V&A's Burghley nef is a saltcellar) or beverage (one 16th century drawing of a nef, now at the Kunstbibliotek in Berlin, says that "When you remove the upper part, the lower section becomes a drinking vessel that holds two measures of drink," the Gilbert nef has a spout projecting from the figurehead); there's even a few clockwork nefs.
Although Arts and Crafts in the Middle Ages describes nefs only as a type of saltcellar, it does provide some interesting (if uncited) details:
The salt cellar was always a piece de resistance, and stood in the centre of the table. It was often in the form of a ship in silver ... The silver ship was commonly an immense piece of plate, containing the napkin, goblet, and knife and spoon of the host, besides being the receptacle for the spices and salt. Through fear of poison, the precaution was taken of keeping it covered. This ship was often known as the "nef," and frequently had a name, as if it were the family yacht! One is recorded as having been named the "Tyger," while a nef belonging to the Duke of Orleans was called the "Porquepy," meaning porcupine.
Worth Their Salt has a history of salts and nefs, and includes a photo of an early 20th century wheeled nef shaped like a 15th century ship. See also Stefan's Florileigum: Nefs.
A king at table, De secretis secretorum (British Library MS Add. 47680, fol. 60v), c. 1326-1327
Banquet for King Charles V, The Grandes Chroniques de France (BNF Fr. 2813, fol. 473v), c. 1375-1380; note that the entremet (entertainment) during the banquet includes a naval battle and an attack on a tower
A comment in a 1380 inventory of King Charles V (see Talbot's article) indicates that his small nef contains an "essay [serpents' tongue], his spoon, his little knife and his little fork."
Arthur and his knights in De casibus (BNF Fr. 226, fol. 240v), first quarter of the 15th century
January (detail) from the Tres Riches Heures du Duc de Berry, 1412-16
Olivier de la Marche's 1474 description of a nef owned by the Duke of Burgundy (see Talbot's article) indicates that it is so large that, if it were placed in front of him, "any prince or ambassador who came to speak with him at table would not be able to see him for it."
Reliquary of St. Ursula, 1574 While this is a reliquary (originally at Rheims Cathedral), it has a lot in common with the other nefs on this page. Talbot notes that it "is 28 cm. in length and was described in 1573 as a small nef," but, since nef is also a French word meaning a boat, I think it is more a description of the form than the function of this piece, rather than a nef for the table that "has been converted into a reliquary for St. Ursula." Talbot further describes the reliquary: "It bears a Tour's [sic] makers mark and was probably made about the year 1500. The hull is made from cornelian and is mounted in silver, gold and enameled copper. Approximately 12 realistic figures adorn the deck, some of whom are passengers while other, smaller figures, are the sailors on the ship. These sailors are doing such things as climbing the rigging and looking off the ship's prow."
Another automaton-nef with a clock, called the "nef of [Emperor] Charles V" and attributed to Hans Schlottheim; made in Augsburg at the end of the 16th century
Pictures of the nef: 1, 2, 3, 4, 5, 6, 7, and 8
Pictures of the clockwork and mechanisms:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, and 12
Details of the various figures:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18
Pictures of the ship's hull:
1, 2, 3, 4, 5, 6, 7, 8, 9, and 10