Set in the 15th century CE. Using the resources and technology available during that period of time, would a medieval age manned sub guarantees victory in a sea battle? How would such a sub be able to obliterate(sink) the likes of Ship of the line? Accidents and sucidal missions do not qualify as winning.
No, it's been tried several times before their successful deployment in the 20th century and they didn't work.
The primary examples are the Turtle (1776) and the Hunley (1864). It wasn't until WWI that the proper set of technologies became available for U-Boats to be a major factor in warfare. Even with the primitive ASW defenses of the time the Germans lost over 60% of their fleet.
A submarine's sole advantage is to hide underwater. Thus a successful submarine must be able to do the following...
- Stay submerged.
- See submerged.
- Move (rapidly) submerged.
- Attack submerged.
Staying submerged is a matter of keeping the air breathable. Medieval technology would have no way to do this: no CO2 scrubbers, no compressed air storage, no oxygen candles. They could only submerge for a very short period of time before the crew would pass out. Alternatively, they could do as the Turtle did and not fully submerge. Either way this greatly limits their ability to approach a vessel undetected.
Next, how do you see without being seen? Telescopes and microscopes did not exist in the 15th century, nor did quality lenses. Even if they did, the scope housing would have to be sealed somehow. Early submarines used viewing blocks mounted in a shallow cupola which would peek just above the surface. This has the obvious disadvantage of being visible to the enemy.
Prior to the electric motor, moving submerged was performed by manpower. The US Civil War era H. L. Hunley of the Confederate States Navy was the epitome of this. Hand cranked by seven men, it could manage only 4 knots, much slower than most sailing or steam ships, and barely enough to go against a strong current. All those men doing all that work would reduce the time it could spend underwater. Such a speed would make it only effective against anchored vessels, as was the Hunley's only success against the USS Housatonic (the Hunley did not survive the attack). It would also have a pitifully short range and only able to operate near the coast or be towed by a surface vessel.
And finally, how do you attack? The motor torpedo wasn't invented until 1866 using compressed air to turn a propeller and a gyroscope to keep it on course. Early submarines attacked by attacking mines to the hulls of ships. The Turtle featured a drill and was thwarted by copper sheeting. The Hunley used a spar torpedo which it would have to ram into the hull of a ship and release. Neither of these were particularly effective.
In short, prior to WWI the submarine was only marginally effective at attacking anchored vessels in calm water close to a shoreline and at night.
Even in WWI and WWII submarines were not a factor in fleet battles. They were simply too slow to keep up with the battle fleet. Instead, they would be set up in long lines along the anticipated course the enemy fleet would take. The could then act as scouts and possibly take a few shots at the enemy. This rarely worked; it failed in the Battle Of Jutland and it failed in the Battle of Midway. The US had some success with this tactic in the Pacific, but it wasn't until the advent of the nuclear submarine, which could go fast enough to keep up with a battle fleet, and good radios for communication that submarines could work effectively with a surface fleet.
How would 15th century ships defend against these vessels? The same way early WWI vessels did...
- Stay on the move.
- Take a zig-zag course.
- Travel in convoys.
- Post look outs.
- Lay mines.
- Use anti-submarine nets.
- Ram them.
This may seem ludicrous to you, but this is how the majority of submarines were destroyed in WWI. ASDIC/SONAR was only a prototype, and depth charges weren't available until 1916. You had to spot a submarine with your eyes and ram it, or catch it on the surface and shoot it. 19 U-boats were destroyed by ramming, 20 by gunfire, and 58 by mines.
It's surprisingly easy to spot a submarine underwater, especially one creeping along at 4 knots and just 10 feet below the surface. Once spotted, they were helpless.
See this answer of mine. A plausible medeval sub would be very different from what you have in mine. In particular, pressure vessels never worked until industrial times. Saying yours did is not plausible.
My idea, detailed in the other post, is a mobile cason with divers in individual breathing gear. Casons were developed early for underwater harbor and tunneling work, and I explain how this work might lead tomthe discovery the your cement makes the air last longer, to then find the active ingredient and optimize it.
So it could be built.
So what? This question asks what good that might do for war.
Without explosives, what could a sub do?
They probably can't just drill holes in the ship: the occupents would notice and fill them. It's not enough damage fast enough.
defense against ships in a harbor
A company of heavily armed men can sneak up on a ship, and swam over the sides in a surprise boarding operation. Or, sneak on board ninja-style.
The invader's weapons and treasure can be tossed over. Cannons over the side to sink! And the locals can pick them up for their own use.
Boarding is what Leonardo had in mind, it appears. He was a renounced weapons designer, so you might look into his notes.
offense against a harbor
A ship "safely" some distance from shore, or disguised as regular commercial traffic, could drop a work crew and equipment they use back home for maintaining and improving their own harbors, to...
Mess up this foriegn harbor! Instread of clearing silt and removing hazards to new huge deep-draw vessels, stack up rocks and plant submerged spikes where they think the channel is clear!
Undermine docks and tunnel into warehouse facilities. That's the kind of work the technology was developed for.
fodder for stories
So can it turn the tine of war? A story could be crafted where that is the case. The enemy's flagship is found in the morning to be empty of life with no alarm during the night. An invasion force finds its provisions are poisoned. things keep happening.
Yes a medieval sub could theoretically turn the tides of war. Can it guarantee it? Well, that depends on its effectiveness and how much the war relied on a naval fleet.
Although the first submarine was built in 1620, there is no reason one couldnt have been built earlier. But how would it destroy large ships? With torpedoes, or at least an explosive device. Gunpowder was first developed by the Chinese before 1000 BC. It was used to propel rockets and may have reached Europe via the Mongol invasions of the mid-13th century. So the ability to create weaponized explosives had matured long before your submarine. There are numerous ways someone could create in order to deliver the explosive to the enemy ship. That just takes some imagination.
FYI - The first combat sub was the H. L. Hunley was used during the American Civil War. It was human powered and used torpedoes is armaments.
You can build a submarine during medieval times with medieval technology, that would obliterate the enemy navy, but for it, you would need modern knowledge. So it's sort of a paradoxical situation. You would have all the required craftsmanship that would enable a submarine to stay submerged and be effective, but in those times nobody knew how to utilize that technology and come up with some ingenius underwater beast.
What it means is, if somebody well versed in mechanics and hydraulics travels back in time to those days (15th century CE), they could incorporate their knowledge to build an extremely primitive, but nevertheless devastating sub (according to the standards of those times). But nobody born in that time knew how exactly to do that. I hope my point is clear.
This how you are going to build such a submarine:
Building The Submarine
You are going to build something that can carry 3-5 people and some good amounts of naphtha (crude petroleum. ~30 liters should be enough).
The sub needs to be rocket shaped. As in, the general shape should be like a tower and the front side needs be pointed. The material needs to be heavy and strong, stainless steel, most preferably. Of course people of those times didn't know how to make stainless steel but you do.
You would need to keep two holes in the sub. One in the front section (~3 ft diameter) and the other in the rear (~1 ft diameter). Both holes should be in the roof of the sub. From these holes, you would attach two metallic tubes. Rear tube is 10 feet long while the forward tube is 12 ft long. Both tubes need to be fixed (melt and merge). The forward tube has 4 mini-tubes in it. They all start together inside the main tube, but at the end, each turns at 90° angle away from the other. Since the tube is vertical to the sub, the mini-tubes would turn at 4 angels (front, right, left, back). Now you would need to fix thick mirrors at all the joints of these 4 tubes. When you are done, you would have an effective periscope that would enable you to look in all directions simultaneously. There's a very long (~200 meters), strong but relatively thin rope attached to the inner side of the front end of the sub (interleaved silk strings).
Put the sub in the water with 5 men and 50 liters of refines naphtha (which should be practically equal to a mixture of gasoline, kerosine oil and diesel oil). Add/remove weight to/from the sub until it stabilizes at a depth of 8 ft + the height of the sub. That is, the sub should stabilize at a depth where both tubes are 2 feet above water.
You are good to go now.
There is no steering mechanism in the sub. You are going to move it using a very primitive way. One of the sub-operators is a diver. He would have to dive underwater and reach an enemy ship. Once there, he would attach the rope end (remember it is ~200 meters!) to the ship. Now he would tug the rope a couple of times to signal his comrades.
Within the sub, the operators would simply pull on the rope to move the sub towards the enemy ship. Once there. A couple of men would disembark with the naphtha canister and pour ~5-10 liters of it on the side. Then get back in the sub. The diver would detach the rope from this enemy ship and dive in to attach it to another.
Now once naphtha has been applied to the ship's side, your archers would only need to shoot a couple flaming arrows there and that side would burst aflame. It would take ~10-15 minutes for the fire to spread and take the whole ship ablaze.
No need to risk the lives and try the extremely risky boarding method.
Sub operates underwater and very silently. Nobody can see it moving. Even if it is detected, it cannot be destroyed by arrows as it is made of metal.
The men slap soaked clothes to the ship's side to coat it with naphtha. Takes only half a minute to deliver ~10 liters of naphtha.
Extremely slow, clumsy and primitive design.
The diver's job is highly dangerous. If detected, it would only take an arrow to kill him, rendering all the sub ineffective (unless you have more divers inside and have a larger sub).
If a flaming torch is thrown at the operatives while they are coating the ship's side with naphtha ...