schedule for an ROH for a given ship varies between 2 and  5  years  according  to  an  established  cycle.  An overhaul can take as little as 2 months for small ships and  as  much  as  18  months  for  larger  ships.  ROH planning begins about 18 months before the scheduled overhaul. . A voyage repair availability is used for repairs while the ship is underway. These are emergency repairs that are necessary if the ship is to continue on its mission, and  they  can  be  done  without  changing  the  ship’s operating  schedule.  These  repairs  will  be  done  by  the ship’s force if possible, or if necessary, by personnel from an IMA, SIMA, or SRF. . A regular IMA availability is used for general repairs   and   authorized   alterations   that   are   not emergencies.  This  work  is  usually  beyond  the  capability of the ship’s force and is normally scheduled in advance. l An emergency IMA availability is used to repair specific  casualties  and  generally  takes  first  priority  at  a fleet  IMA. . A concurrent availability is used for ship-to-shop work by the shore IMA, tender, or repair ship. These availabilities  are  usually  scheduled  to  take  place  just before   a   regular   shipyard   overhaul   or   restricted availability. Before we get into the personnel and procedures related  to  repair  activities,  it  will  be  helpful  to  define repairs and alterations. REPAIRS AND ALTERATIONS Corrective maintenance and repairs to ships may be divided into the general categories of repairs, alterations equivalent to repairs, and alterations. REPAIRS A repair is defined as the work necessary to restore a ship or an article to serviceable condition without change  in  design,  in  materials,  or  in  the  number, location, or relationship of parts. Repairs may be done by ship’s force, by repair ships and tenders, by SRFs, or by naval or civilian shipyards. ALTERATIONS EQUIVALENT TO REPAIRS Before   we   discuss   alterations,   we   need   to understand  that  NAVSEASYSCOM  may  determine  that some work requested as an alteration may be better defined as an alteration equivalent to repair. In that case, NAVSEASYSCOM  forwards  the  request  to  the appropriate  type  commander  (TYCOM)  to  be  handled as a repair. An alteration is considered to be an alteration equivalent to a repair if it meets one or more of the following  conditions: l l l l Materials that have previously been approved for similar use and that are available from standard stock are substituted without other change in design. Worn  out  or  damaged  parts,  assemblies,  or equipment  requiring  renewal  will  be  replaced  by those of a later and more efficient design that has been  previously  approved. Parts  that  require  repair  or  replacement  to improve reliability of the parts and of the unit will be strengthened, provided no other change in design is involved. Equipment that requires no significant changes in  design  or  functioning  but  is  considered essential  to  prevent  recurrence  of  unsatisfactory conditions  will  be  given  minor  modifications. ALTERATIONS This  chapter  deals  only  with  ship  alterations (SHIPALTs)   as   opposed   to   ordnance   alterations (ORDALTs).  These  are  alterations  to  the  hull, machinery, equipment, or fittings that includes a change in design, materials, number, location, or relationship of the component parts. This is true regardless of whether the  SHIPALT  is  undertaken  separately  from,  incidental to,  or  in  conjunction  with  repairs.  NAVSEASYSCOM, the forces afloat, or CNO may originate requests for SHIPALTs. One  of  NAVSEASYSCOM’s  prime  responsibilities for  ship  maintenance  is  to  administer  SHIPALTs  under its   technical   control.   NAVSEASYSCOM   keeps informed of technical developments in its day-to-day relations with the forces afloat, the naval shipyards, private industry, and research centers. NAVSEASYSCOM may determine that a particular ship or class of ships should be altered to bring them to a more efficient and modern state of readiness. These alterations  may  include  changes  such  as  those  in  the following    examples: Changes  to  the  hull  may strengthen bulkheads or change deck arrangements to provide  space  for  machinery.  New  machinery  may  be added or old machinery made more efficient. Equipment may  be  replaced  with  more  efficient  types  or  new designs. 9-2