Eli5: Why did early Military/Naval submarines have deck guns?

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Eli5: Why did early Military/Naval submarines have deck guns?

In: 2

Because they had limited underwater range (like, a couple of days at most) before they had to go to the surface again, and they had to have some sort of protection while they were on the surface – some guns were used to protect against ships, some were used against aircraft. Also, they often targeted relatively unprotected merchant ships (cargo ships), so they could do some damage from the surface without relying solely on torpedoes.

Early submarines couldn’t stay underwater for long, they actually spent most of their time on the surface so they could run their diesels

A deck gun provides some mild anti air protection especially if the sub absolutely can’t dive for some reason

The deck gun also gives a much cheaper way to engage a target. The sub might carry 20 torpedos but those won’t last terribly long especially with early war accuracy and detonator problems. Against an unarmed and unarmored merchant ship a 3″ gun is good enough and won’t use a valuable torpedo that could be used on an armed escort

Torpedoes are pretty large, so small early submarines had a limited supply. U-Boats at the beginning of WW1 only had a half dozen torpedoes on board, and the workhorse of the German WW2 fleet could only carry 14. Shells for a deck gun are much more compact, so using that whenever possible allowed for longer cruises and more sunken ships before heading back for resupply.

To add to the other answers, submarines weren’t always just used for blowing up other ships and subs. Imagine if you want to inspect or capture a merchant vessel – you need to be surfaced to communicate with them and to board them, so it’s good to have a gun to threaten them with.

Deck guns could also be used to attack targets on shore, which are notoriously hard to hit with torpedoes.

Now that you have a better understanding of submarines, it’s time for a story from WWII; the tale of [U-123](https://en.m.wikipedia.org/wiki/German_submarine_U-123_(1940)) and a German [Submarine Captain](https://en.m.wikipedia.org/wiki/Reinhard_Hardegen) who was welcomed in the United States decades after the war.

The submarine was on its eighth patrol, stalking off the coast of Jacksonville, Florida, targeting shipping on 11 April 1942.

>>From Wikipedia:
After hitting [the tanker, SS Gulfamerica] with a torpedo, Korvettenkapitän (Corvette Captain) Reinhard Hardegen closed in for the kill with his deck gun. Noting the already large crowds gathering on the beach to watch the spectacle, as well as all the beach houses just beyond the Gulfamerica, Hardegen decided to manoeuver around the tanker and attack from the land side. **The move was quite hazardous, as the U-boat was clearly illuminated to any onshore weapons, and the shallow waters forced it to take up station only 250 metres (820 ft) from the tanker, which risked return fire from the tanker as well as getting caught in a blaze if the oil spilling out caught fire.** The highways leading from Jacksonville were soon thronged with curious people trying to get to the beach to look at the spectacle. After firing for some time with the deck gun, the tanker was ablaze and Hardegen decided to leave. Already planes were overhead trying to locate the submarine with parachute flares, while a destroyer and several smaller patrol boats were closing in.

>>Hardegen survived the war, and returned to Jacksonville in 1990, where he was received as an honoured guest. He would say of the occasion that “The town was very friendly to me.”

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Eli5: Why did early Military/Naval submarines have deck guns?

In: 2

Because they had limited underwater range (like, a couple of days at most) before they had to go to the surface again, and they had to have some sort of protection while they were on the surface – some guns were used to protect against ships, some were used against aircraft. Also, they often targeted relatively unprotected merchant ships (cargo ships), so they could do some damage from the surface without relying solely on torpedoes.

Early submarines couldn’t stay underwater for long, they actually spent most of their time on the surface so they could run their diesels

A deck gun provides some mild anti air protection especially if the sub absolutely can’t dive for some reason

The deck gun also gives a much cheaper way to engage a target. The sub might carry 20 torpedos but those won’t last terribly long especially with early war accuracy and detonator problems. Against an unarmed and unarmored merchant ship a 3″ gun is good enough and won’t use a valuable torpedo that could be used on an armed escort

Torpedoes are pretty large, so small early submarines had a limited supply. U-Boats at the beginning of WW1 only had a half dozen torpedoes on board, and the workhorse of the German WW2 fleet could only carry 14. Shells for a deck gun are much more compact, so using that whenever possible allowed for longer cruises and more sunken ships before heading back for resupply.

To add to the other answers, submarines weren’t always just used for blowing up other ships and subs. Imagine if you want to inspect or capture a merchant vessel – you need to be surfaced to communicate with them and to board them, so it’s good to have a gun to threaten them with.

Deck guns could also be used to attack targets on shore, which are notoriously hard to hit with torpedoes.

Now that you have a better understanding of submarines, it’s time for a story from WWII; the tale of [U-123](https://en.m.wikipedia.org/wiki/German_submarine_U-123_(1940)) and a German [Submarine Captain](https://en.m.wikipedia.org/wiki/Reinhard_Hardegen) who was welcomed in the United States decades after the war.

The submarine was on its eighth patrol, stalking off the coast of Jacksonville, Florida, targeting shipping on 11 April 1942.

>>From Wikipedia:
After hitting [the tanker, SS Gulfamerica] with a torpedo, Korvettenkapitän (Corvette Captain) Reinhard Hardegen closed in for the kill with his deck gun. Noting the already large crowds gathering on the beach to watch the spectacle, as well as all the beach houses just beyond the Gulfamerica, Hardegen decided to manoeuver around the tanker and attack from the land side. **The move was quite hazardous, as the U-boat was clearly illuminated to any onshore weapons, and the shallow waters forced it to take up station only 250 metres (820 ft) from the tanker, which risked return fire from the tanker as well as getting caught in a blaze if the oil spilling out caught fire.** The highways leading from Jacksonville were soon thronged with curious people trying to get to the beach to look at the spectacle. After firing for some time with the deck gun, the tanker was ablaze and Hardegen decided to leave. Already planes were overhead trying to locate the submarine with parachute flares, while a destroyer and several smaller patrol boats were closing in.

>>Hardegen survived the war, and returned to Jacksonville in 1990, where he was received as an honoured guest. He would say of the occasion that “The town was very friendly to me.”