6.8.1 When a human surrogate such as the Hybrid
III is used to evaluate the potential for injury, the effects of occupant-seat
coupling as well as the effects of the water entry forces can be determined.
Because the data obtained from a dummy are predictive, it is possible
to identify the type of injury that would likely occur, if an injury
were to occur. However, because of the high acquisition cost of a
dummy (a dummy can cost in excess of $100,000 USD), such evaluations
tend to be quite expensive.
6.8.2 Because of the complex nature of human injury,
and continually evolving knowledge about injury thresholds, data obtained
from a dummy are best interpreted by medical practitioners with experience
in human impact. These data can be used as the basis for quantitative
and qualitative predictions about the type and severity of injury
that may result. Medical judgement also must be used to apply the
results obtained from a dummy to the broad range of individuals that
are likely to use particular free-fall lifeboat. The response of a
dummy is representative of the response of a particular class of people
(e.g., 50th percentile males). The opinions rendered by all medical
practitioners in these matters should be similar but may not be necessarily
identical.
6.8.3 The SRSS criteria and the dynamic response
criteria are two economical and quantitative methods which can be
used to infer the potential of an acceleration field in a free-fall
lifeboat to cause injury to the occupants. The results obtained from
these methods are easily interpreted; acceptability is based upon
not exceeding a prescribed value. Of these two methods, the dynamic
response criteria is preferred because the duration of the acceleration
impulse is explicitly considered in the evaluation.
6.8.4 For the dynamic response and the SRSS criteria
to provide a reasonable indication of occupant safety, good coupling
must exist between the occupant and the seat. Neither method can explicitly
evaluate the effects of occupant-seat coupling. Such an evaluation
can only be performed with a human surrogate. Good occupant-seat coupling
can be obtained, however, through proper use of a seat harness such
as that which has been described.
6.8.5 When using the dynamic response or the SRSS
criteria, it also must be remembered that neither method can predict
if an injury will occur nor what that injury will be. Rather, the
criteria merely provide an indication of the potential for an injury
to occur. If the criteria suggest that an unacceptable condition exists,
it may be prudent for the manufacturer to evaluate the acceleration
field using a human surrogate before making extensive design changes.
Also, use of a dummy would be prudent if harness arrangements significantly
different from those discussed are used and may be justified if the
lifeboat is to be certificated for free-fall launches much in excess
of 40 meters.