Stealth ship

A stealth ship is a ship which employs stealth technology construction techniques in an effort to ensure that it is harder to detect by one or more of radar, visual, sonar, and infrared methods.
These techniques borrow from stealth aircraft technology, although some aspects such as wake and acoustic signature reduction (acoustic quieting) are unique to stealth ships' design. Though radar cross-section (RCS) reduction is a fairly new concept many other forms of masking a ship have existed for centuries or even millennia.
Shaping.
In designing a ship with reduced radar signature, the main concerns are radar beams originating near or slightly above the horizon (as seen from the ship) coming from distant patrol aircraft, other ships or sea-skimming anti-ship missiles with active radar seekers. Therefore, the shape of the ship avoids vertical surfaces, which would perfectly reflect any such beams directly back to the emitter. Retro-reflective right angles are eliminated to avoid causing the "cat's eye" effect. A stealthy ship shape can be achieved by constructing the hull and superstructure with a series of slightly protruding and retruding surfaces. Furthermore, round shapes on the ship are eliminated or covered up, examples include smokestacks and gun turrets. Also, cavities that present a horizontal face are to be eliminated since they act like a trap and are very visible to radar. To get around these limitations many ships use features such as panels that cover reflective surfaces or use alternate designs of hardware. Also, every effort must be made to have the smallest gaps on the ship as possible. Hull shapes include tumblehome hull designs which slope inward from the waterline, and small-waterplane-area twin hulls (SWATH) which allow for better stability when using a tumblehome hull. These RCS design principles were developed by several navies independently in the 1980s using work done on aircraft RCS reduction as the starting point.
The currently developed U.S. "Zumwalt"-class destroyer — or DD(X) — is the US version of a stealth ship. Despite being 40% larger than an "Arleigh Burke"-class destroyer the radar signature is more akin to a fishing boat, according to a spokesman for Naval Sea Systems Command; sound levels are compared to the s. The tumblehome hull reduces radar return and the composite material deckhouse also has a low radar return. Water sleeting along the sides, along with passive cool air induction in the mack reduces infrared signature. Overall, the destroyer's angular build makes it "50 times harder to spot on radar than an ordinary destroyer.
The Swedish Navy's "Visby class" corvette is designed to elude visual detection, radar detection, acoustic detection, and infrared detection. The hull material is a sandwich construction comprising a PVC core with a carbon fibre and vinyl laminate. Avoidance of right angles in the design results in a smaller radar signature, reducing the ship's detection range.
Britain's Type 45 anti-air warfare destroyer has similarities to the "Visby" class, but is much more conventional, employing traditional steel instead of carbon fiber. Like "Visby", its design reduces the use of right angles.
Taiwan's' Tuo Chiang-class stealth corvette are a class of fast stealth multi-mission corvettes currently in service with the Republic of China (Taiwan) Navy. The ships is designed to have a low radar cross-section and evade radar detection making it difficult to detect the ship when operating closer to the coastline 
"Sea Shadow", which utilizes both tumblehome and SWATH features, was an early U.S. exploration of stealth ship technology.
Signature reduction.
Stealth technology represents more than just a low RCS; noise reduction plays a huge role in naval stealth because sound travels much better in water than air. Some of the techniques used include muffled exhaust systems, modified propeller shapes, and pump-jets. The shape of the hull can also have a great effect on the reduction of the noise from a ship. Another major element is signal emission control. Modern warships emit much electromagnetic radiation in the form of radar, radio, and even bleed-off from the ship's electrical systems. All of this can be used to track a ship and thus modern stealth ships often have a mode that switches off many of the electronic emissions, the downside of course is that the ship then has to rely on passive sensors and can't easily send messages further than line of sight.
Also of great importance are thermal emissions. A heat signature can make a ship stand out like a candle in a dark ocean making it easier to spot and because it is possible to see infrared emissions through features that would normally hide a ship such as fog, or a smoke screen, many detection platforms like patrol aircraft, UAV's, and satellites often have the ability to see multiple bands in the infrared spectrum including heat. This necessitates the control of these emissions. The most common way is to mix any hot gasses emitted by the main source of heat which is the engines exhaust with cold air to dilute the signature and make it harder to pick out the ship from the background warmth. Another method vents the exhaust into the water though this increases the ship's acoustic signature. For the hull water can be actively distributed across the hull of the ship cooling the ship. Another less crucial but still relevant part of a stealth ship is visual camouflage. This area is probably the oldest form of stealth, with records going back almost as far as the writing of ancient mariners using visual tricks to make their ships harder to spot. Though still relevant this area has taken on lesser importance with the advent of long-range radar.
Materials.
Just like choices in shaping, the choice of materials affects the RCS of a ship. Composites such as fiberglass and carbon fiber are great blockers of radar and give smaller vessels an advantage in further RCS reductions. However, composites are fragile and often unsuited to larger ships or ships that expect to take fire, though new laminates can partially negate some of the weaknesses. This restricts larger ships to metals such as steel and aluminum alloys. To compensate a ship may include a coating of a radar absorbing material though this can be quite expensive and may not stand up to the corrosive effects of salt water.