Density as a ship design factor

A report on 24 January 2012 from the US Government Accountability Office (GAO) to the House Appropriations Subcommittee on Defense on the US Navy’s future surface combatant plans raises the issue of ship density; the extent to which ships have equipment, piping, and other hardware tightly packed within the ship spaces (usually measured in pounds of weight per cubic foot).

The USN in July 2011 contracted to restart the DDG-51 Arleigh Burke-class aegis destroyer production, with plans to build nine upgraded Flight IIA ships.  Plans then call for building 22 DDG-51s in a new Flight III configuration starting in FY 2016. This new Flight III configuration would include the new Air and Missile Defense Radar (AMDR) to replace the aegis radar.  The addition of AMDR and required additional electrical power and cooling equipment within the fixed DDG-51 hullform size would increase the density of the ship, which was found by a 2005 DOD study to already be about 50 percent more dense than modern international destroyers.

Why does this matter?

  • First, density will complicate redesign of the ship to place the new radars and associated cooling and electrical power equipment.
  • Second, as submarine builders know well, greater density complicates ship construction, increasing costs and often delaying schedules.
  • Third, added equipment in an already dense ship design will also reduce the ship's centre of gravity, with adverse impact possible on ship performance and safety.

Are there lessons for other navies in this?  Perhaps there are, along the lines of: “be very careful about the hullform you choose: you will have to live within its constraints thereafter.”