The 3-meter foam buoy

A ring of closed-cell ionomer foam, 3.0 m across and about 1.0 m deep, around a central aluminum well. Its sides flare out above the water to a broad, rounded edge; below the waterline, the hull narrows in two cones to a flat bottom about 1.5 m across. It stands about 0.62 m out of the water, has a draft of 0.39 m, and weighs about 1,540 kg, some 180 kg less than the aluminum discus. Its buoyancy comes from the foam itself, not from watertight compartments like the discus's. It is yellow, painted red below the waterline.

NDBC began testing 3-meter hulls of ionomer foam around a standard aluminum well in the early 1990s, for more buoyancy, less damage and lower maintenance. It calls the design value-engineered. The structure is aluminum: the well, rolled from plate, 0.93 m across and 1.22 m tall; a plate 1.42 m across under the foam; and, on the deck, a triangle of square tubing 2.28 m on each side, with a spoke from each corner to a ring around the well. Six stainless-steel rods through the foam pull the triangle and the bottom plate together, clamping the foam between them. NDBC buys the foam and the structure separately. A hatch with latching dogs closes the well about 0.2 m above the foam.

A lifting eye stands at each corner of the triangle, 120° apart. Under the hull, a bridle of three steel pipes, each 1.72 m between its pins, hangs from brackets on the bottom plate beneath the eyes. The pipes meet at a head about 2.3 m below the waterline, where the mooring hangs from a swivel: chain in shallow water, nylon and polypropylene in deep water. Eight of the 38, all in the Pacific, also carry a seal cage around the frame (not drawn).

All 38 carry NDBC's SCOOP payload, described on the discus page: a weather tube with two masts (a propeller-vane anemometer and an all-in-one sensor), four solar panels around a lithium battery, and a wave tube holding the motion sensor. The tubes communicate with each other by short-range radio, and with shore by Iridium satellite.

On this hull, three square posts bolted beside the lifting eyes carry a triangular platform about 1.9 m above the water, with the battery and the weather tube on it. The wave tube stands on the hatch in the middle of the deck, under a lattice post that rises to the platform. The frame's shape is estimated from NDBC's photographs; no drawing of it is published.

As on the discus, nothing on the buoy measures the water's surface. The motion sensor in the wave tube records the hull's heave, pitch, roll and heading 1.7066 times a second for 20 minutes, twice an hour. NDBC treats the motion of a hull this size as the height of the water and its slope in two directions. The record is turned into spectra on board, then corrected and converted into wave height, periods and direction on shore, as the discus page describes. When NDBC first installed SCOOP on three foam hulls, it compared their wave measurements with nearby buoys already in service and found small differences.

Bluegraph's wave height, periods and direction at these stations, and the spectra that Wave Intelligence separates into wave systems, come from this record.

The anemometer is 4.1 m above the water on this hull, 0.3 m higher than on the discus. Wind is averaged over 10 minutes, six times an hour, and the gust is the highest 5-second running mean in the period. The vane reads the direction relative to the hull, and the buoy's compass converts it into a direction. NDBC publishes the wind as measured at that height, and so does Bluegraph.

Air temperature and humidity come from the all-in-one sensor 3.7 m up, averaged over 10 minutes; the dew point is calculated from them on shore. Pressure is measured 2.7 m above the water as a 1-minute mean, six times an hour, and corrected on shore for that height. The 3-hour change in pressure is reported on the hour.

Sea temperature comes from a probe clamped to the bridle 1.5 m below the waterline. Two of the 38, 46050 and 51101, list the discus's heights instead (wind 3.8 m, air 3.4 m, pressure 2.4 m).

Weather readings go to shore every 10 minutes and wave readings every 30 minutes, by Iridium satellite. NDBC checks each report automatically and publishes it as provisional data. Analysts review it within about a day.