ssc-367 - Ship Structure Committee

H~ is the significantwave height, and
‘m
is the
frequency of maximum spectral energy.
The Bretschneider equation
may be written in terms of the peak period
instead of the peak frequency, by substituting urn=2iT/T.
P
S(u) = (5/16)*(ti)2* [(21r)4/(u5*(T)4)]*
exp[-l~25*(2~q/(~*T )4] p
A.3.2
Pierson-Moskowitz Spectrum
The Pierson-Moskowitz (Reference A.4) spectrum was created to fit
North Atlantic weather data. The P-M spectrum is the same as the
Bretschneider spectrum, but with the H* and Mm dependence merged
into a single parameter. The frequency used in the exponential has -
also been made a function of reported wind speed. The equation for
the Pierson-Moskowitz spectrum is as follows.
S(u) = a*g2/~S)*exp[-~*(~0/~)4]
where:
a
: 0.0081
B
= 0.74
&l
o
= g/u
and, U
is the wind speed reported by the weather ships.
The Pierson-Moskowitz spectrum equation may be obtained from the
Bretschneider equation by using one of the following relations
between H~ and Wm.
A-12
--;”,
:“,< L.