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Comment by dogma1138 | Hacker News Reader

And only one class the Alfa fast attack submarine the titanium hull was used more to reduce weight (Half the displacement of a comparable Los Angeles class US attack submarine) to allow it to be the fastest submarine ever constructed (double the reported speed of US attack submarines and most likely at least 25% faster than their actual speed) rather than because titanium is non-magnetic. All Alfa subs were decommissioned in 1990 and sold as scrap.

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Yep, the Alfa was a strange design, Titanium hull, reduced to way below safe minimum shielding to save the weight, noisy as hell (like insanely noisy).

Basically designed as the submarine equivalent of an interceptor, get somewhere fast and open fire.

IIRC the top speed of the alfa was above the speed of the NATO torpedoes in use at the time.

Reported top speed. And it was seen as shocking enough to spur the investment into new torpedo technology.

https://en.wikipedia.org/wiki/Alfa-class_submarine#Impact

Like most fears over high-performance Soviet hardware though, the threat turned out to be overblown compared to the actual numbers build / operational state.

Yep, the Alfa also had a tendency to slowly kill it's crew from radiation exposure.

well thats what you get for putting a fast nuclear reactor with minimal shielding in a relatively tiny sub.

It was also used in the K-278 nuclear attack subs, but only in the inner hull so wouldn't have had so much effect on reducing magnetic signature. It seems I was somewhat miss-informed. Thanks for helping clear that up.

The K-278 was part of project 685/705 of which 2 subs were made the K-222 (PAPA) and K-278 (MIKE) both were prototypes PAPA used a full titanium hull and resulted in the Alfa class subs and the K-278/MIKE was lost with all hands with it being the only sub of it's class.

It was also used on the Sierra class (Project 945) of which at least two have been refurbished and remain in service.

Titanium allows an increase in dive depth but has no effect on speed.

The ship's total mass must be close to the mass of the water it displaces. Less mass spent on the hull means more mass spent on the reactor and steam plant.

The displacement is based on the volume of the sub not the mass. Basically the Alfa was a small and light sub with probably the "biggest" reactor in terms of power output for it's size. What you got is a sub with half the displacement of an LA class submarine with a reactor with equal power output which meant that you could strap on some really powerful engines. So half the displacement with about the same power capabilities means pretty much that you can double your speed.

mass = volume * density. Water has a (nearly) fixed density. So for any given displacement, less mass allocated to the hull means more mass allocated elsewhere.

Yes but the mass doesn't effect the volume of water displaced if you put in a hollow 10 cubic meter steel cube into water or a solid 10 cubic meter steel cube they'll both still displace the same amount of water which is 10 cubic meters.

BTW the density of the sub is also for the most part pretty much equal to the density of water.

The only times where it's going to be either greater or lesser than water is when it's doing a static dive or fast ascent.

When the sub does it's initial dive and takes on water into it's buoyancy tanks it becomes about the average density of salt sea water once that is achieved it can use it's diving planes (foreplanes) to adjust it's depth.

https://www.math.nyu.edu/~crorres/Archimedes/Crown/CrownIntr... was useful for me in correcting some myths about mass and density I'd learned.

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