Page 324 - Introduction to Naval Architecture

SHIP DESIGN 309

All the cash flow elements must be brought to a common basis by
treating them as though they occurred simultaneously. This is because
cash has a time value in that it might be used more profitably in some
other way. It is usual to apply discounted cash flow methods to establish a
net present value for comparison of different design options. A
compound interest rate is used to determine the "present" value of
money to be spent in later years. The net present value must be positive
if it is to be acceptable. The higher it is for any option the better that
option is from an economic point of view. The process can be inverted
to give the freight rate needed to give a net present value of zero.

Availability
An owner wants a vessel to be available for use when needed. This is not
necessarily all the time. Many ships have a quiet season when time can
be found for refitting without risk to the planned schedules. Ferries are
often refitted in winter months for that reason. Availability can be
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regarded as a function of reliability and maintainability, '
Reliability can be defined as the probability of an artifact performing
adequately for the time intended under the operating conditions
encountered. This implies that components must have a certain mean
time between failures (MTBFs). If the MTBF is too low for a given
component that component will need to be duplicated so that its
failure does not jeopardize the overall operation.
Maintenance is preferrably planned. That is items are refurbished or
replaced before they fail. By carrying out planned maintenance in quiet
periods the availability of the ship is unaffected. The MTBF data can be
used to decide when action is needed. To plan the maintenance
requires knowledge of the mean time to repair (MTR) of components.
Both MTBF and MTR data are assessed from experience with the
components, or similar, in service. The other type of maintenance is
breakdown maintenance which is needed when an item fails in service.
Unless the item is duplicated the system is out of action until repair is
effected.
The time taken to maintain can be reduced by adopting a policy of
refit or repair by replacement Under this scheme complete units or sub-
units are replaced rather than being repaired in situ. Frigates with gas
turbine propulsion are designed so that the gas turbines can be
replaced as units, withdrawal being usually through the uptakes or
downtakes. The used or defective item can then be repaired as
convenient without affecting the ship's availability.
Another advantage is that repairs can be carried out under better
conditions, often at the manufacturer's plant. The disadvantage is that
stocks of components and units must be readily available at short
notice. To carry such stocks can be quite costly. But then an idle ship is