Submerged Performance of Submarines

                     
How fast are modern submarines?  Have you ever wondered about this secretive bit of submarine lore?  Here's what I did recently, with the disclaimer that I used only the Internet and the books I have in my house.  I have no classified or private information, I have not served in the US Navy and am a US citizen with just a bachelors degree in engineering.

OK, disclaimers out of the way, what sources of information do we have?  The US Navy always gives modestly low performance numbers, such as a top speed of 30 knots, or more generously, top speed in excess of 30 knots.  So how about US intelligence on fast Soviet subs like the Alpha?  Foreign commentators on US and other submarine performance?  Browsing the web gives some tantalizing numbers and from what I could see the top speed of the Soviet Alpha is the highest ever quoted.  The K-162 Pappa should be mentioned here.  A test boat which apparently hit 44.7 knots on trials in 1971 but which at 42kts also broadcast 100db of noise! 

The Alphas were small vessels designed in the late 1950s to act as carrier battle group interceptors.  They had a number of advanced features including a liquid metal cooled reactor, automation to reduce the number of crew and a titanium alloy hull to cut weight and increase operating depth.  The first of the Alphas didn't see service until 1965 and apparently four of the seven boats in this class suffered reactor failures.  So is this our boat; a fourty year old design that has been retired since 1995?  Maybe.  There's also another, later Soviet sub the K-162, NATO designation Pappa, which apparently hit 44.7 knots on trials in 1971 but which also broadcast 100db of noise at 42kts.

In this decade top speed may not be the important design feature that it once was.  Hot-rod submerged performance has been replaced with the more blended goal of undetectably quiet operation at the highest possible speed (silent speed), so we may have relatively few boats to consider when looking for top submerged speed.  But I'm not interested in top silent speed and just want to know how fast a submarine can go at full throttle making as much noise as needed.  Cavitation, turbulence, and reactor noise?  Not a concern.

Albacore
An interesting boat to consider is the USS Albacore.  This smaller diesel/electric boat was built in 1952 with the following specs.

• length: 62.2m
• beam: 8.4m
• displacement: 1850 ton
• 15,000shp
• speed: >25kts

She was built as a testbed at a time when the US Navy was transitioning to nuclear submarines that would mostly operate submerged.  This was a huge change from the usual surface operation of the earlier diesel boats and entirely different design choices were involved.  The Albacore project was managed by Admiral Momsen who  really pushed for advancement with this new prototype.  His orders to her designers where, "When in doubt, think speed." 

Albacore went through several modifications and eventually set a non-nuclear submarine speed of 33kts, using a 33,000shp motor with silver-zinc batteries running at a 15-minute discharge rate. 

Alpha
For now let's go back to the old Soviet Alpha, since I'm finding a little more published information on it and it seems to be accepted as to-date, one of the fastest nuclear military submarines.  Here is some anecdotal information that I've found which needs to be looked at critically:

• The USS Plunger clocked an Alpha at 50kts
• Military.com quotes the Alpha speed as 43kts

A little math should be adequate to crudely estimate how fast these boats run based on their powerplant performance, drivetrain efficiency and some basic hydrodynamic estimates.  Let's see what the numbers tell us.

• Power: 47,000 shaft hp
• Hull diameter: 31 ft
• Hull length: 261 ft
• Displacement: 4,320 tons

To be continued...

 

Photo credits: U.S. Navy

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