Frigate Design
Today, the maximum practical speed of displacement ships is about 32 to 35 knots. This can be achieved in a relatively small ship by making it long, narrow and light but also costly. There are major problems in delivering this power efficiently through conventional propellers due to cavitation problems and using conventional diesel or steam machinery which provide a very poor power/weight ratio. Modern large ships have traditionally been propeller driven with diesel power. Propellers are, however, inherently limited in size, and they also present cavitation and vibration problems. It is generally recognized that applying state-of-the-art technology, 60,000 horsepower is about the upper limit, per shaft, for conventional fixed pitch propellers. Moreover, diesel engines sized to produce the necessary power for higher speeds would be impractical because of weight, size, cost and fuel consumption considerations.
Another means to achieve high speed ships is the planing hull. This popular design is limited to a very short hull form, i.e. typically no more than 100 feet and 100 tons. If a 50 foot boat is scaled to the length of a frigate of 300 feet, the speed scales to the precise range of 12 to 60 knots. Thus scaled, the power required for a 300 foot planing frigate would be about half a million horsepower. Furthermore, the ensuing ride on this 300 foot ship would cause material fatigue as its large flat hull surface would be slammed at continuously high speed into the ocean waves inasmuch as it would be too slow to plane or "fly" across the waves as a much smaller planing ship would do.
Structural additions of various kinds and configurations have been implemented for various types of marine vessels in order to improve powering performance. Some very small pleasure craft and planing boats have been provided with adjustable trim flaps (trim tabs) for controlling the trim. As for combatant vessels, during World War II some small German ships were provided with stern wedges for the purpose of improving powering performance. Until the early 1980's, however, neither stern wedges nor stern flaps were known to be pursued by anyone for combatant ships of the frigate/destroyer size (approximately 3,000 to 10,000 Long Tons displacement). In the 1980's some navies began to successfully apply stern wedges to larger ships up to the frigate size.
A stern wedge design was initially attempted by the U.S. Navy for the FFG 7 frigate class; however, in the course of model testing it was discovered that a stern flap was more effective than a stern wedge on this class. The model tests demonstrated approximately a 5% decrease in delivered power at speeds of 20 knots and above. In 1989, a stern flap was designed and retrofitted by the U.S. Navy on the USS Copeland frigate (FFG 25). Analysis of the ship trials data for the USS Copeland frigate having a retrofitted stern flap indicated an 8% power saving, somewhat greater than the model test results, and increased top end speed.
Helicopters have now become entrenched as an integral part of the weapons system carried by destroyers and frigates in anti-submarine search and strike capacity. Invariably, landings and take-offs of these helicopters from vessels must be made in moderate to severe turbulence and once on the deck, the helicopter must be quickly secured and stored for protection from the environment.
Use of remotely piloted vehicles (RPV) or unmanned air vehicle (UAV) in the Naval environment [Shipboard Launch and Recovery system (SLAR) [also known as Launch, Recovery Securing, Handling (LRSH) Equipment of the RPV] adds a number of new challenges. For the proposed short range RPV in particular, operating from frigate sized and smaller ships means taking off from, and landing on, an unstable moving deck, with severe airwake turbulence from the superstructure and very tight space constraints both during operation and stowage. A strong trend is already emerging in favour of RPVs with a VTOL capability for the maritime role because of the demonstrated difficulties of landing a fixed wing air vehicle on even relatively large and stable ships' decks.
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