Diving Depth- how common do modern subs go really deep- and how useful is it?

[Kapitan]

Alfa's are rumored to be 900 m capable with some account of a captain of one of them taking his boat down to 3000 to regularly lose tailing 688I's and make them look like a extreme joke - but Alfa's seem to be actually in the 700 m range give or take..

Sierras... seem to crush at 900 m + - the 3000 ft regime- that's the limit of ADCAPs- and they're still active.

There was that one Mike class boat which could hit the 4000s ...and would crush at 5000- that had the accident..

Heres whats what with the golden fishes

Alfa class project 705 and 705K were built as intercept submarines much like the mig25 foxbat designed to get somewhere fast, the main mission for an alfa is ASUW not anti submarines although yes its capable in the area to a limited degree.
after long time of studying designs and the backgrounds ect the roomer of the Alfas hitting 900m is a spoof, most of the time an alfa would not go below 450m because this caused cracking in their hulls and being a very expensive hull to build the soviets required them to last (hence why the first test boat was launched in 69 and scrapped in 74), the crush depth of alfa would be around 750m maximum operational 550m still deep enough for its role.

The sierras are quite different however again not so much in deep diving maximum operational would be around 650m and crush at 850m again made of a titanium alloy and based of the victor III, most of these boats are experimental Alfa Papa Mike and Sierra classes didn't really take off into a mass production like what we see with the other classes of submarines.

Despite spending a huge amount on defence the soviets couldn't afford to build and maintain large numbers of these boats so they restricted diving depths to extend hull lives, one alfa was still knocking about in 1995 as a test platform and as mentioned the sierras are still about.

The mike class is probably the only true deep diving submarine she did reach 1020m during several tests before she sank, however the cost for diving deep meant that only one submarine was built and later lost to fire in the norwiegen sea.

papa is basically a fast titanium hulled submarine (that the later steel hulled Charlie SSGN are copied from) what most people don't know or realise is that papa is actually faster than alfa and holds the speed record for fastest submarine 44.7knots sustained (alfas can hit 45knots but only in bursts usual maximum cruise speed 41knots)

Only the sierras remain active with the B230 carp a sierra I to be reactivated by 2016.

In terms of the 688 captain following an alfa to put it mildly as captain of the alfa I would have just gunned the throttle its unlikely that the 688 would be able to keep pace.

[Kapitan]

To be honest and answer the second point i don't generally go below 450m in the akula and 200m in the kilo but that said it does depend on the layer depth.

Speed and depth are not too important when stalking the greater the depth the torpedo doesn't even have to hit you and your dead atlaest at 200/300m you have some chance.

In regards to seawolf SSN21 if i remember rightly she is made of HY130 steel much like the UK Astute class this is more cost effective and also easier to maintain and build than a titanium hull.