I’ve always thought a navy ship could have arms extending from each side, out say 20′ or so that holds some sort of draping system, like a chain or something, that extends below the bottom of the hull. Then, if a sub fired a torpedo at it, it would either explose on the chain or just get caught up in it.
In: 3375
Modern torpedos often don’t actually strike the hull of the ship. They well detonate a short distance below the hull to create a cavity/vacuum that will break the keel of the ship. The chains or curtain, which I think is a good idea, would have to extend pretty far down and they would have to be sufficiently heavy and secure as the torpedos might not detonate on impact
This is indeed what they used to do. In late 19th, early 20th century warship photos you can see [collapsed poles along the side of the ship](https://www.internetmodeler.com/2003/august/ships/HP_Dreadnought_02.JPG), and on top is bundled netting. When [deployed, it looked like this.](https://external-preview.redd.it/eX80qA3sNKrYpJmEISQweDV-pjQwJjD7XTjmHYqQTBM.jpg?auto=webp&s=77c34aa6495aeac8a5fcd98e8f9892ecc78b69b2)
The problem is, you can’t use torpedo nets while you’re underway, and typically, your ships don’t get attacked by torpedos when you’re in harbor – shallow enclosed waters are not fun to be a submariner in if you get discovered – you get attacked by torpedoes when you’re at sea and underway.
So torpedo nets were done away with largely by WWII and replaced with anti-torpedo bulges – these were a sacrificial fake outer hull, itself compartmentalized to minimize damage and water intake, that would force the torpedo to detonate outside of the main hull of the ship.
We’ve done away with even that because torpedos now are way more effective. Instead of trying to blast a hole in the side of the ship – which because of compartmentalization and damage control can be easily mitigated and would rarely immobilize a ship – they now are designed to detonate underneath the keel or bottom spine of the ship, breaking it in two essentially, buckling the hull and making so many tears and burst seams that saving the ship is impossible.
edit: keel not keep. damn autocorrect.
Its called a [torpedo net](https://en.wikipedia.org/wiki/Torpedo_net) and it was a thing, but only when ships are in port or otherwise not moving. You could either put a big thick net across the harbor or [around a specific ship](https://maritime.org/doc/netsandbooms/img/pg064.jpg)
The nets provide a significant increase in drag so they’re no good when underway, they’ll slow a ship wayyyy down and it’ll burn way more fuel attempting to drag them around and it makes the ship way more vulnerable to anything that isn’t a torpedo
The mobile version is a torpedo bulge which is a big blister on the side of the hull with layers of air/water/oil to dissipate the blast before it damages the core hull of the ship. This has a pretty minimal impact on overall speed and weight if designed in from the start, and could also be retrofitted onto existing ships
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