Watching an old WWII documentary I noticed that U-Boats normally had a large gun mounted on their upper “deck”. It was explained that in order to save on torpedos a U-Boat might engage at close range with a ship, for example a lightly armed supply ship, with their deck gun instead of firing a torpedo at it. While I struggle to see how a deck gun can be as effective as a torpedo, the question in my mind is how reliable can a deck gun that spends most of it’s time in salt water be? Knowing how easily most guns rust and jam, how did they keep these guns operational given the harsh marine conditions? Wouldn’t they become encrusted with barnacles after a while?
When a U-Boot sank an allied ship, whether it was military or merchant marine, was there usually an attempt to rescue any sailors that had survived the attack? I imagine that the U-Boot had very little room or supplies to take on additional personnel, but I could be wrong about that.
They needed a lot of grease. Keep in mind that WWII boats spent most of their time at the surface-even then of course during layovers the hull was cleaned and scrubbed by dock workers.
The U-boats were forbidden from rescuing anyone after an incident early in the war where one skipper tried to tow some survivors along on their life rafts, only to have Allied planes strafe the now unable to dive submarine.
Deck guns weren’t as effective as torpedoes, but you could carry a lot more 88mm shells than you can torpedoes.
They tended to waterproof them in various ways- there was a screw-in tompion in the muzzle, and some way of waterproofing the breech as well. I’m pretty sure the bore wasn’t constantly awash in salt spray.
It depends on when in the war, as to the feasibility of picking up survivors, etc… After September 1942, there was no attempt.
It was much more common for the U-boat crews to give survivors food and water and point them to the nearest land than to bring them on board. Even after the Laconia order, these sorts of things still went on because there was a loophole in the order that still allowed U-boats to make contact with survivors for intelligence purposes. So occasionally the crews would still check on the well-being of survivors under the pretext of “questioning” them, even though this was usually only possible for lone merchants whose crews almost certainly had no useful intelligence.
Torpedos are expensive, complex, hard to use, limited in supply, and at times unreliable. You use them if you have to, but a deck gun is a steady, reliable way to sink a ship. You can carry hundreds of rounds onboard and don’t have to worry about making a tricky submerged approach and hoping your torpedo calculations work and that the unit doesn’t misfire. A U-boat captain would always use the gun if it was practical.
As far as POWs - during the early war the u-boats would fire warning shots and allow time for the crew to get to their lifeboats before sinking their vessel, but after the British started using q-ships to ambush u-boats in that situation, it was impractical to continue that policy.
Offhand I don’t know. Later in the war submarines were less and less able to make surface attacks - there was a ton of air patrols, any convoys were impervious to gun attacks (there could still be night time surface attacks, but they had to be quick strikes with torpedos) and there were roaming sub hunting groups. Early in the war deck guns accounted for the majority of kills, but later on the circumstances in which a deck gun attack would work became more and more rare.
US subs also used deck guns. From Wiki (USS Wahoo):
*The next day, Wahoo sighted freighter Satsuki Maru. She launched two torpedoes; when both exploded prematurely, Wahoo “battle surfaced” to use her guns. She closed the target, raked her with 20 millimeter shells and holed her with almost 90 rounds of four-inch (102 mm) . The target caught fire in several places and sank in about one hour.
Wahoo left the following morning to investigate a ship on the horizon; the target proved to be a small diesel-driven freighter. The submarine commenced firing with her 20 mm and four-inch (102 mm) guns. The freighter tried to ram, but Wahoo kept clear, and continued firing at the target, setting it ablaze from stem to stern and leaving her dead in the water. The crew took turns looking through the periscope as the freighter sank.
Later that day, Wahoo sighted a 100 ton trawler and again attacked with her deck guns. When all three 20 millimeter guns jammed, Wahoo went alongside the riddled trawler and the Wahoo sailors hurled homemade Molotov cocktails (gifts from the Marines at Midway) onto the trawler. Wahoo departed, leaving the ship wrecked, spouting flame and smoke. On 28 March, while on the surface astride the Shimonoseki-Formosa shipping route, Wahoo opened fire with two 20 mm guns on two motor sampans. The targets did not sink but were also left wrecked.*
Given the unreliability of early WW2 torpedoes, the deck gun was essential as a back-up weapon.
They were used in both world wars by the British. Once air coverage became common, they’d also have radio frequencies on which they could report an attack - which meant that if the uboat gave too much warning or took too long to attack, an aircraft could respond and sink the u-boat, another disincentive for giving the ship warning and time to evacuate.
Partly because US subs were a long ways away from their base – if you used up all your torpedoes, you had to go all the way back to get more.
And partly because later in the war, there were very few ‘larger’ cargo ships around Japan – most had been sunk or damaged already. What was left was smaller ships, many wooden ones, and often under sail (due to shortages of oil) – not ‘worth’ a torpedo, and could be damaged or sunk with the deck guns (especially as later in the war subs were given bigger deck guns).
Both. It was more effective in WWI, given that it wasn’t really effective at all against WWII subs. (Did they kill a single U-Boat?)
(Interestingly, disguised armed merchant raiders did much better for the Germans than Q-ships did against them). I will have to dig up the cites, but I recall that WWII’s experience with Q-Ships ended up with more than a few of them being treated like merchants and ignobly sunk with torpedoes. The fact that the convoy system proved a much superior method of dealing with U-boats, ended further experimentation with Q-ships.
The following sites lists data from JANAC, the Joint Army-Navy Assessment Committee, detailing who blew up whom.
Annoyingly, neither site separates torpedo kills from deck gun kills. They also do not mention ships under 500 tons—sampans, trawlers, etc—which were a majority of the kills from the deck gun or AAA.
Granted, this is just me screwing around with things like Aces of the Deep and assorted members of the Silent Hunter series, but there’s no way I would try to duke it out with anything larger than a sampan on the surface, unless I knew there’d be no way for help to arrive in the 30 or so minutes it’d take to kill the ship. It’s just too easy to poke a hole in your sub, and you’ve little enough reserve buoyancy to weather damage. Given how slow submarines were then, even surfaced, I’m surprised that commanders took the time that many did to help survivors.