Page 14 - NAVAL FORCES 03/2017

Technology Spotlight

At Euronaval 2016, DCNS unveiled
its new “BELH@RRA” 4,000t wave-
piercing frigate design, for both the
French Navy and for export.
Note the bulwark to protect the gun
and missile launchers from green seas
coming aboard in rough weather.

for the usual hull. Their buoyancy supports a It is usually described as a semi-planing hull, elements block access to the main hull (which
platform. The struts connecting the underwa- a type used earlier by some high-speed yachts. is also much narrower than in “Freedom”).
ter bodies with the flat platform above water In theory, a planing hull achieves high speed
have very limited waterplanes. SWATH offers by riding atop the water. Instead of the waves Wave-Piercing Hull
outstanding stability for its displacement (as in produced by a typical displacement hull, a
US and other sonar surveillance platforms) but planing hull is supported dynamically and Another variation on displacement hulls
is generally not associated with high speed. If resistance is largely limited to the friction is the wave-piercing hull pioneered by the
the waterplane is the problem, why are trima- of the hull surface riding the surface of the “Zumwalt” class, conceived as very stealthy
rans and SWATHs so rare in the world’s na- water. warships. A ship’s side and the sea form a
vies? Besides linking a ship to the motion of Many small fast attack craft (and most corner reflector and thus create strong radar
the sea, the waterplane also determines how racing craft) have planing hulls, achieving echoes. The designers of the “Zumwalt” tipped
the ship reacts to changes in weight. The larger speeds far beyond what might be expected the ship’s side (tumblehome) back to eliminate
the area of the waterplane, the less the ship with a more conventional hull. That works the corner reflector when the ship was riding
sinks into the water as weight is added. A ship well in calm water, but planing is difficult or on an even keel. Their problem was that ships
with a very small waterplane area sinks hea- impossible in the more disturbed conditions to move with the sea. They had to maintain the
vily if its load changes; conversely, for such a be found in the open ocean, so it is difficult to ship’s low radar cross-section under various
ship weight control is essential. Large variable see the virtues of a planing approach. sea conditions. To do that they took two steps.
loads, such as large amounts of fuel, can be Some years ago, this type of hull was pro- One was to fix the ship’s position in the water
a significant problem, as can major modifi- posed to the Royal Navy as a preferable alter- by using submarine-like ballast tanks, inten-
cation during the ship’s lifetime. native to the long slender hulls typically used ded to compensate for weights consumed, to
The trimaran (and its cousin the catamaran) for frigates. Its main virtue was much greater keep the ship’s draft fixed. The ship was sta-
is also sensitive to some seas. It behaves badly steadiness for a given displacement (hence, in bilised against roll, which might have created
in a quartering sea, because waves hit first theory, cost). The cost was a need for far more corner reflectors as the ship tipped towards a
one side and then the other at an angle to the power to achieve a typical frigate speed. This radar. That left the usual ship motion created
direction in which the ship is traveling. Most trade-off is now more acceptable because gas as the bow crested waves. Normally it is consi-
of the time, however, a trimaran offers a broad turbine powerplants are so light for the power dered very useful for a ship to ride over waves;
stable platform, e.g. for a helicopter, without they deliver. A “Freedom” class LCS produces it keeps the deck dry. On the other hand, parti-
the resistance of a beamier conventional hull. about as much power as a destroyer three cularly at high speed, periodic pitching causes
The combination of high speed (for a reason- times her displacement (at least in theory), to discomfort on the bridge. Typically the rate at
able power output) and steadiness presum ably achieve nearly 40% higher speed. which a ship pitches determines the speed she
sold the USN on the trimaran hull adopted Compared to a trimaran, the more conven- can maintain in a rough sea. The main factor in
for the “Independence” class LCS. Thus far, tional hull of the “Freedom” makes it much such motion is the shape of the bow, including
there have been no other naval takers, although easier to handle unmanned waterborne flare above the waterline.
Austal, which designed the ship has proposed vehicles, which are essential elements of her In the case of the “Zumwalt,” the designers
a variety of trimaran frigates. modular combat system. Such operations are accepted that seas would break over the ship’s
apparently far more difficult for the trimaran deck. In a very stealthy ship, any activity on
Semi-Planing Hulls “Independence.” “Freedom” also offers easy deck is a threat to the ship’s relative invisibility
sea-level servicing for boats and unmanned – one reason the ship is so large is that every-
The “Freedom” class represents a very surface or submerged vehicles, which is thing must be internal. Allowing seas to break
different approach to an unconventional hull. not possible in a trimaran, because the side over the deck was considered entirely accept-
12 NAVAL FORCES III/2017