With reference to survey and certification of a ship, briefly discuss:

• The tail shaft (propeller shaft) is surveyed as part of the hull/class program. "Condition monitoring" here refers to the systematic monitoring of the shaft's condition (e.g. by non-destructive examination - ultrasonic thickness measurement, inspection of the shaft/bearings, alignment, and the use of "condition monitoring" where, instead of a fixed shaft withdrawal to bare the shaft at every intermediate/5-year survey, the shaft may be left installed and its condition monitored/maintained within the survey regime, using an approved through-bottom seal and a monitoring system).
• Types: (i) a full examination/withdrawal at the prescribed survey intervals

  (ii) condition monitoring: where the surveyor accepts leaving the shaft in place with the monitoring of the stern gland (e.g. a through-bottom seal, monitoring of water ingress/current) and periodically checking the condition, allowing longer intervals between withdrawals (a "condition monitoring" approach for the propeller shaft and its seal) as allowed by class when the shaft is otherwise maintained;

• The tail shaft is withdrawn and examined when required (new ships; and at intervals defined by the flag/class for shaft survey, often 5 or 10 years, depending on the ability for in-water inspection and the monitoring arrangement). Records of the shaft diameter/tüM, bearing condition and alignment are kept.
• The monitoring is important because a tail shaft failure is catastrophic; condition monitoring reduces unnecessary dockings while ensuring safety through informed inspection of the key component. The IOPC? (Not related; this is class survey requirement.)

• CAP (Condition Assessment Program, e.g. the classification "CAP" of the corrosion/warying condition under the IACS condition assessment scheme, or the CAP scheme in relation to bow/structural assessment). In the context of surveys, CAP refers to the Condition Assessment Program schemes (e.g. the "CAP" programme for tankers / the IACS condition assessment) which assesses the structural condition of the hull and provides a "CAP rating" - a voluntary but common survey/assessment used by oil majors/vetting (e.g. USCG and the "CAP" for tank vessel structural condition in the classification "Condition Assessment Program").
• CAS (Condition Assessment Scheme): CAS is a mandatory, enhanced survey scheme under MARPOL Annex I (for certain single-hull and to be phased-out tankers) and under the OPA/flag requirements for aged tankers, providing a rigorous structural condition assessment (involving close-up/thickness gauging) when an older single-hull tanker is to continue in service (e.g. within the special conditions of phase-out). CAS determines whether a tanker can remain in service beyond its normal limits; it is a higher-level combined programme that checks the hull's structural integrity and verifies maintenance and provides the continuing fitness to trade. Both CAP and CAS are used to assess and extend/limit the service of older tankers based on their structural condition, protecting safety and preventing catastrophic structural failure.