After reading that sweet thread, I had another question in mind.

I read somewhere that at first, when countries started to design sailing ships with metallic armor, they were doing it with a mix of copper plates and teak wood. I wondered if the same kind of armor could be achieved with aluminium and if yes, how much resistant would it be compared to steel / iron armored ship and would it really be lighter than a warship with full steel armor ?

Bonus point : I'm not pretty sure when ships with metallic armor where first used. Did some ships like that exist before ships started using steam engines for propulsion ?

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    $\begingroup$ Copper has never been used for warship armour. Copper sheathing was applied to wooden ships to protect them from the life that otherwise grows on them, adding drag, and from shipworms that eat wooden hulls. Wooden steamships existed in the early nineteenth century, but were gradually replaced with ones built of iron, and then steel. "Ironclads" were the first ships with metal armour, and it was iron. $\endgroup$ – John Dallman Sep 17 '16 at 17:59
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    $\begingroup$ What era are you asking about? The process of making aluminum was extraordinarily expensive until the early 1900s, so much so that it was considered a precious metal. So before 1900, no aluminum armor. Modern warships don't have much armor, just around vital spaces and for splinter protection. The idea is modern anti-ship weapons are so potent the only defense is to not be hit. $\endgroup$ – Schwern Sep 17 '16 at 19:31
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    $\begingroup$ Pure metals as a whole are basically bad for armor. No metallurgist capable of smelting aluminium would make the rookie mistake of trying to use it for armor un-alloyed. I strongly recommend you tumble down a metallurgy-related wikihole (en.wikipedia.org/wiki/Metallurgy) for a bit if a novel ship armor is essential to your worldbuilding task, and handwavium won't do the trick. $\endgroup$ – SudoSedWinifred Sep 18 '16 at 3:29
  • $\begingroup$ Thanks for the comments, so basically there was no real armored ship before they used iron/steel hulls. Also against modern weapons it's totally pointless.... I was more thinking to ancient weapons, however CortAmon's answer convince me any weapon with enough piercing power would just shatter such armor; $\endgroup$ – Kaël Sep 18 '16 at 6:49
  • $\begingroup$ Exocet missile destroys Al/Mg alloy superstructure of British warship in Falklands war: youtube.com/watch?v=IUZu8bvxJs4 $\endgroup$ – Robert Smart Sep 18 '16 at 13:39

Aluminum is a poor armor. It's a lightweight material, but very heavy for its level of protection. The density of aluminum is 2700-2800 kg/m^3, while the density of steel is 7850 kg/m^3. The densities vary by alloy, of course, but in general steel is about 2.8 times more dense than aluminum.

This is useful because we can do comparisons to existing armor. We can't really compare against modern tanks, because they use highly advanced composite armors which are typically rated in terms of steel equivalency (the M1A1 Abrams is estimated to have an equivalent of over 30 inches of steel worth of stopping power!). However, we can compare against the M4 Sherman from WWII which had 3.7 inches of armor. If we built an aluminum tank with a similar weight, it would have 10.4 inches of aluminum armor. So how well would it fare?

It turns out that the internet has an answer for this question. Demolition ranch took shots at a 8-10inch block of aluminum with varying calibers of guns. It fared well against small arms, but when they moved up into powerful rifles, whole chunks of armor started to fail. When facing basic armor penetrating rounds, they managed to dig about half way through the block. They did not test any of the weapons designed to take on armored naval targets, like 20 mm and up.

The big issue for stopping armor penetrators is that you need a material that is hard and dense to stop them. Aluminum is neither, so it is very weak.

Of course, this is all against modern weapons. If you're talking about stopping ancient weapons it may work better. However, making aluminum is not easy until you could develop the large quantities of electricity used to refine the raw materials. Your nation would have a substantial advantage over other ancient societies simply by having this technology. Copper is easy to work with. Aluminum is not.

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    $\begingroup$ Yeah -- the M113's aluminum armor was fine against small arms, but sucked when hit by shaped charges as now you have a jet of molten aluminum on the inside of the thing, which made for a bad day for the folks inside. $\endgroup$ – Shalvenay Sep 18 '16 at 1:03

Per the other answers, aluminium can be used as armour. However, aluminium on ships ships does not really fare well with fires which makes it very unsuitable for actual naval combat where fire is a big, big problem.

  • $\begingroup$ Very interesting story. Another proof it is definitely a bad idea to use that. $\endgroup$ – Kaël Sep 18 '16 at 6:54
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    $\begingroup$ The Royal Navy had similar damage to 2 of their ships HMS Ardent and HMS Antelope hit by enemy bombing attacks during the Falklands Conflict. These also had aliminuim superstructure. $\endgroup$ – Sarriesfan Sep 18 '16 at 16:00

Unfortunately, Aluminum is currently used on Naval ships for armor plating. For a variety of reasons... weight savings, cloaking aids, and diminished threats of damage from an enemy. I'd prefer more than an inch or two of Aluminum in front of me during an attack.

One of the Allied battleships in WWII was struck by a Kamikaze. No injures and the damage was fixed with a couple gallons of paint. The 'dent' is still visible.

Give me steel.

[TRUE] A unique form of protection seriously considered was constructing the entire ship out of ice and some kind of Peat combination. Essentially carving it out of a chunk of tundra or iceberg.

So, YES is the answer, but not a good one.

  • $\begingroup$ Yup I knew that thing about a WW2 british aircraft carrier made of ice and somekind of wood. The idea was to use that to create a very cheap carrier. But definitely it was not needed and was never built (eyh who needs to build additionnal carriers when fighting alongside the world's largest navy's ?) $\endgroup$ – Kaël Sep 18 '16 at 6:53
  • $\begingroup$ Alongside? At the outbreak of war the Royal Navy was the largest in the world and was second only to the US by the end of the war. Agree on the main point though. $\endgroup$ – Joseph Rogers Sep 20 '16 at 10:28

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