The 2.1-meter ionomer foam buoy

A solid hull of ionomer foam, 2.1 m across at its widest and, according to NDBC's dimensioned elevation drawing, about 0.9 m deep. From a flat deck 0.53 m above the water, it widens in a cone to a short vertical band, then narrows to a chine just under the water and a shallow cone down to a flat bottom. Its draft is about 0.37 m. It weighs about 490 kg and stands about 3.2 m tall, compared with about 1,720 kg and 5 m for the 3-meter aluminum discus. NDBC developed it to carry SCOOP and put it into service in 2019: at 45001 in Lake Superior, it replaced a 3-meter aluminum hull that May, and at 46029 off the Columbia River in May 2020.

NDBC's specification for its ionomer foam hulls calls for closed-cell foam of Surlyn or Lotek ionomer resin, extruded in yellow sheets 3 to 10 mm thick. The sheets are wound around the hull's axis, layer on layer, and heat-sealed along the full length of each layer. The outside is then densified by heat and pressure into a skin 6 to 8 mm thick: the foam inside weighs about 64 kg per cubic meter, and the skin at least 480. The color runs all the way through the foam, and the first hull built must survive being dropped 3 m onto pavement. A 1996 guide to moored buoys sums up what such foam gives a hull: more buoyancy, less damage, and less maintenance.

The whole payload goes into a well down the hull's axis, which NDBC says makes handling quicker and safer: technicians spend less time on a buoy at sea replacing sensors one by one. Above the deck, three plate legs and a ring 0.6 m up hold the payload. Under the hull, three legs meet about 1.7 m below the water, where the mooring hangs from a swivel. NDBC's 2023 drawing of this buoy shows 25 mm chain, then nylon rope held up by syntactic foam floats, down to a 3,855 kg concrete sinker. One of the ten, 46029, is listed with a seal cage, a ring of bars around the frame (not drawn).

The payload is SCOOP, NDBC's current payload, as on the 3-meter discus buoy: the electronics are in sealed modules, stacked into one unit. The weather tube stands on top of the power module, 1.3 m above the deck, with two masts. The taller one holds a propeller-vane anemometer, its propeller 3.2 m above the water. The shorter one holds an all-in-one sensor, 2.8 m up, that measures wind with sound, along with air temperature, humidity and pressure. Below the weather tube, four solar panels lean around the electronics and batteries, 0.75 to 1.3 m above the deck. The modules communicate with shore by Iridium satellite.

Nothing on the buoy measures the water's surface. A motion sensor in the payload's wave module, low in the hull at its center near the waterline, records the hull's heave, pitch, roll and heading 1.7 times a second for 20 minutes, twice an hour. NDBC treats the hull as following the surface, so its motion stands for the height of the water and its slope in two directions. The processing is the same as the 3-meter discus buoy's: spectra in 46 bands from 0.0325 to 0.485 Hz (periods of about 31 s down to about 2 s), with direction from how the hull tilts as it rises and falls. The discus page explains how. Wave Intelligence separates the wave systems in these spectra.

Before the hull went into service, two of these hulls were deployed beside Datawell Waveriders for about a year, in 2017 and 2018: off Virginia Beach, 0.84 km apart, and off the Columbia River bar, about 13 km apart. On average, their significant wave height read 0.08 m and 0.10 m lower than the Waveriders', with a spread of 0.12 m. Their mean direction differed by less than 1° on average, with a spread of 11° and 21°.

A later study at 45001 and 46029 compared the new hull with the 3-meter hulls it replaced, each against a nearby Waverider. The smaller, lighter hull measured long, low waves with less noise and short waves above 0.25 Hz more closely, and measured direction about as well. Its authors conclude that the hull's size doesn't change NDBC's directional estimates.

Wind is averaged over 10 minutes, six times an hour, and the gust is the highest 5-second mean in that time. The vane reads the direction relative to the hull, which turns freely on its mooring, and the buoy's compass converts that into a direction from north. NDBC publishes the wind as measured: 3.2 m up on this hull, compared with 3.8 m on the 3-meter discus. That difference is worth knowing when comparing stations.

Air temperature and humidity are averaged over 10 minutes, and the dew point is calculated from them on shore. Pressure is measured in the weather tube, 1.8 m above the water, as a 1-minute mean, and corrected on shore for that height. Sea temperature comes from a probe on the bridle, 1.1 m below the waterline.

Weather readings go to shore every 10 minutes and wave readings every 30 minutes, by Iridium satellite. On Bluegraph, a station on this hull reports wind speed, gust and direction; air temperature, dew point, pressure and its 3-hour change; sea temperature; and wave height, periods and direction, with swell and wind sea shown separately, plus the directional spectra where the station sends them.

The two on the Great Lakes, 45001 in Lake Superior and 45002 in Lake Michigan, are taken out of the water every fall before the lakes freeze and put back in the spring, so they report only during the open-water season.