Page 15 - NAVAL FORCES 03/2017

T echnology Spotlight

able as a way of controlling the ship’s motion. have been the first to see wave-piercing as a the ship’s hull and the sea, a valuable quality.
In the case of the “Zumwalt,” the tipped-back positive feature of a more conventional ship. There are several ways to generate and
bow shape was apparently chosen to help keep maintain an air cushion under a ship, but in
the ship from riding over waves. It might be Norwegian Air-Cushion each case the idea is that the air in effect lubri-
argued that green seas over the foredeck may Benefits cates the boundary between hull and sea. Like
have unfortunate effects on exposed weapons a planing hull, an air cushion ship is subject
and equipment, in this case the tops of vertical Of earlier unconventional hull forms, the mainly to frictional rather than wave-making
launchers and the semi-enclosed 155mm guns. main survivor seems to be the air-cushion ship resistance, so it can move at high speed (for
At Euronaval 2016, DCNS showed a mo- typified by the Norwegian “Skjold.” At one its size) using less power than a conventional
del of a frigate the French Navy is to buy as time it seemed that by using an air cushion hull. The air-cushion craft does have to use po-
a successor to the current “FREMMs.” In a substantial ship, such as a frigate, could wer to maintain the air cushion, a factor being
place of the conventional sharply-raked bow achieve a speed as great as 70 knots. How- limiting leakage around the hull. Typically, the
of the “FREMM,” this frigate showed a rather ever, there is an effective limit on the art of sides of the hull extend into the water to cont-
blunt tipped-back structure, described as a the possible. The air cushion ship has to gene- ain the cushion and there are seals fore and aft
wave-piercing bow. The explanation given rate and maintain a sufficient cushion to carry to limit unavoidable leakage.
at the show was that pitching can damage the ship’s full weight, which requires a certain As in a planing hull, an air cushion craft
weapons and sensors; wave-piercing offers a specific pressure. Unfortunately, the ship’s riding the surface can suffer badly as the sea
more stable ride, which is preferable. weight grows as the cube of her dimensions, becomes rougher. Much clearly depends on
The new frigate was described as a less ex- but the available supporting surface grows as how long the surface effect craft is, since in
pensive follow-on to the “FREMM,” though the square. At 273t the “Skjold” works quite effect what matters is the average height of the
it is unclear just how wave-piercing makes well (she achieves 60kts), but it would be dif- sea under it. None of the unconventional hull
for a more economical design. It is possible ficult to scale her up to ten times that. forms seems likely to be adopted on a very lar-
that wave-piercing makes it possible to drive The Norwegian air-cushion mine counter- ge scale. The trimaran suffers from seakeeping
a smaller ship into a given sea, and speed in measures craft benefit from the air cushion problems; the semi-planing hull demands far
a set sea state may have been a key design re- design in two ways. One is in their large deck too much power (and hence has limited en-
quirement. area, far beyond what would be available in a durance); the wave-piercer must cope with
No one else at Euronaval was offering a conventional design. A second is in their abili- green water over the bow; and the air-cushion
wave-piercing bow. Earlier proposals for ships ty to run at high speed when transiting the long craft seems inherently limited in size. All are
with such bows have generally emphasised Norwegian coast. The air cushion prob ably likely to be adopted by navies, but probably on
stealth, as in the “Zumwalt;” DCNS seems to also offers effective sound isolation between a limited basis. NAFO