Self-propelled torpedoes dramatically increased effectiveness of submarine warships.
 Initial scouting patrols against surface warships sank several cruisers in the first month of World War I.
 Incidental encounters with merchant ships were handled by signalling the ship to stop and sinking the ship after removing the crew in accordance with international law.
 After unrestricted submarine warfare began in February 1915, any ship might unexpectedly sink rapidly from heavy underwater hull damage inflicted by torpedoes.
 Many large ships sank unknown to friendly forces, and the submarines which sank them were too small to rescue more than a few survivors.
 Heavy personnel casualties continued through World War II, and there have been a few later sinkings.
The Attack on Convoy AN 14 was a naval engagement during World War II between a British naval force defending a convoy of merchant ships, which had departed from Port Said and Alexandria to Piraeus and two Italian torpedo boats who intercepted them north of Crete on 31 January 1941.
 The Italian vessels, "Lupo" and "Libra", launched two torpedoes each.
 The torpedoes fired by "Libra" missed their target but one from "Lupo" hit and disabled for the rest of the war the 8120 LT British tanker "Desmoulea" which had to be towed to Suda Bay in Crete and beached.
 One other merchant ship turned back; the other eight vessels reached Piraeus.
An aerial torpedo, airborne torpedo or air-dropped torpedo is a naval weapon, a torpedo, that an aircraft—fixed-wing aircraft or helicopter—drops in the water, after which the weapon propels itself to the target.
 First used in World War I, air-dropped torpedoes were used extensively in World War II, and remain in limited use.
 Aerial torpedoes are generally smaller and lighter than submarine- and surface-launched torpedoes.
A submarine chaser is a small and fast naval vessel that is specifically intended for anti-submarine warfare.
 Although similar vessels were designed and used by many nations, this designation was most famously used by ships built by the US.
 Many of the US World War I sub-chasers found their way to friendly powers by way of Lend-Lease in World War II.
Yanxi class weapon trials ship is a class of little known naval auxiliary ship of the People's Liberation Army Navy (PLAN).
 The name of this class is after the first unit commissioned, with the exact type still remains unknown.
 Contrary to the usual naming conventions of Chinese military where the ship is designated by a combination of two Chinese characters followed by three-digit number, Yanxi class is designated by only a single Chinese character, which is Xun (训), short for Xun-Lian (训练), meaning training in Chinese, because these ships of this class is also used as training ships, but only a single unit of this class has been built.
 However, the pennant numbers may have changed due to the change of Chinese naval ships naming convention.
The "Grillo"-class was a class of torpedo armed motorboats in service with the "Regia Marina" (the Royal Navy of Italy) during the First World War.
 The notable feature of these vessels was that each was equipped with a pair of spiked continuous tracks, intended to allow them to climb over harbour booms and attack enemy shipping at anchor.
 In 1918, two attempts to use them to penetrate Austro-Hungarian harbour defences both ended in failure.
The Bliss-Leavitt Mark 9 torpedo was a Bliss-Leavitt torpedo developed and produced by the E. W. Bliss Company and the Naval Torpedo Station in Newport, Rhode Island in 1915.
 The Mark 9 was originally intended to be used on battleships.
 Before the Mark 9 could be issued, however, use of torpedoes on battleships was discontinued and Mark 9 torpedoes were placed in storage.
 These torpedoes were modified for deployment on R-class and S-class submarines, and used in early World War II to supplement the initial supply of Mark 14 torpedoes.
 Torpedo production for the U.S. Navy was terminated by the E.W. Bliss Company about 1920 after completion of the Mark 9 project.
The T1 tanker or T1 are a class of sea worthy small tanker ships used to transport fuel oil before and during World War II, Korean War and Vietnam War.
 The T1 tanker classification is still in use today.
 T1 tankers are about 200 to 250 feet in length and are able to sustain a top speed of about 12 knots.
 The hull designation "AO" is used by the US Navy to denote the ship is a T1 oil tanker and "AOG" that the T1 is a gasoline tanker.
 The small size allows the T1 to enter just about any sea port or to anchor around a small island, this was very useful during the Pacific War.
 The T1 tanker can carry about 48,000 to 280,000 BBLs.
 Some T1 tankers were used to transport goods other than oil, a few were used for black oil-crude oil, diesel, chemicals and rarely bulk cargo like grain.
 T1 tankers are also called liquid cargo carriers.
 The T1 tanker has about a 6,000 to 35,000 DWT of cargo.
 The small size also gives the ships short turn around time for repair, cleaning, loading and unloading.
 A T1 tanker carrying dirty cargo, like crude oil needs a few weeks of labor to clean before carrying clean cargo.
 Most T1 ships during World War II were named after major oil fields. T1 tanker are operated by the US Navy, War Shipping Administration and United States Maritime Commission.
 Some T1 were loaned to England in the Lend-Lease program for World War II, after the war most were returned to the USA.
 After World War II many of the T1 ships were sold to for civilian use.
 Each T1 had emergency life rafts on the boat deck.
 The ships had cargo booms and piping to load and unload fuel.
 During war time the T1 are armed for protection with deck guns.
 A typical ship may have one single 3"/50 dual purpose gun, two 40 mm guns and three single Oerlikon 20 mm cannon.
 A T1 at war time normally had a crew of 38 and up to 130.
 If operating as a United States Merchant Marine ship, the crew would be a mix of civilian Merchant Marines and United States Navy Armed Guards to man the guns.
Fireships served in the Royal Navy over a period of several centuries.
 The earliest fireships – ships filled with combustible and flammable materials and explosives and sent into lines of enemy ships to attempt to set them on fire – were small merchant vessels deployed in large fleet actions, such as by Sir Francis Drake against the Spanish Armada at the Battle of Gravelines in 1588.
 Fire was a major hazard on the wooden warships of the time, which carried large quantities of flammable and explosive materials into battle.
 Both sides used fireships in a number of engagements during the Anglo–Spanish War, with varying levels of effectiveness.
 Fireships reappeared in unconventional forms during the English Civil War, and were used in earnest during the Anglo-Dutch Wars, particularly to great effect in 1666 during Holmes's Bonfire.
 Successes such as the burning of the "Royal James" at the Battle of Solebay in 1672 caused considerable interest in the application of such vessels, eventually resulting in the construction of purpose-built ships.
 Interest in the fireship declined during the eighteenth century.
 Though new vessels continued to be taken into service, they did not play a significant role in either the Seven Years' War or the American War of Independence.
 There was a resurgence in the use of fireships during the 1790s during the wars with France, and they were deployed with some success by Thomas Cochrane at the Battle of the Basque Roads in 1809, but they were steadily supplanted by new methods of war, such as heated shot, torpedoes and mines.
A torpedo boat is a relatively small and fast naval ship designed to carry torpedoes into battle.
 The first designs rammed enemy ships with explosive spar torpedoes, and later designs launched self-propelled Whitehead torpedoes.
 They were created to counter battleships and other slow and heavily armed ships by using speed, agility, and the power of their torpedo weapons.
 A number of inexpensive torpedo boats attacking en masse could overwhelm a larger ship's ability to fight them off using its large but cumbersome guns.
 An inexpensive fleet of torpedo boats could pose a threat to much larger and more expensive fleets of capital ships, albeit only in the coastal areas to which their small size and limited fuel load restricted them.

Question: Which country used small and fast naval ships designated to carry torpedoes into battle during the First World War?
Answer: Italy