On the Water
Always plan for larger seas
January 22, 2013
U.S. mariners are well acquainted with
the National Weather Service marine weather forecasts that sometimes keep us
company while on watch. But do we get what they’re saying?
It’s not clear whether mariners actually
understand the information that forecasters are trying to convey to us. At the
user end (mariners, dispatchers, port captains, safety managers, etc.) of those
forecasts, the wind portion is generally well understood. Predicted seas are
not, and this can prove dangerous and costly.
It’s important to remember that
seas (the combination of local wind waves and ocean swells, and the effects from
their interaction) are expressed in feet or meters and indicate the predicted
Significant Wave Height. SWH should not be confused with maximum wave height.
The definition of SWH (also called Hs) is the average of the highest one-third of
waves (measured from trough to crest) that occur in the wave spectrum over a
given period of time. There is also Hm (most-probable wave height), H (mean
wave height), H1/10 (highest 10 percent of waves), H1/100 (highest 1 percent of
waves) and Hmax (theoretical maximum wave height). On this scale, Hs falls
above H and below H1/10.
So by using these definitions, one
should plan for seas larger, maybe significantly larger, than the heights
stated in the forecasts. How much larger? They can be bigger than you think. To
calculate H1/10 take Hs and multiply it by 1.27; for H1/100 multiply Hs by
1.67,; and for Hmax multiply Hs by 2. So if the forecast calls for seas of up
to 20 feet you should expect a 10 percent chance of 25.4-foot seas, a 1 percent
chance of 33.4-foot seas, and a slight possibility of 40-foot seas.
In other words, those bigger waves
are always out there somewhere and it’s only a question of whether or not you’ll
both be occupying the same point on the surface of the sea at the same time.
The longer you’re out there the greater the probability.
The quick-and-dirty risk analysis
is to always be prepared for the possibility of encountering individual waves
of roughly twice the predicted SWH. If your vessel, the vessel you’re towing, or
the gear connecting them can’t handle those conditions then you ought not be on
that voyage. And I’m not talking about some one in 10 million freakish monster
rogue wave here. It’s those predictable waves that are routinely predicted and
allowed for in the standard weather forecasts.
An excellent reference is “Significant Wave Height: A closer look at wave forecasts,” by Tom Ainsworth of the NWS office in Juneau, Alaska.
Its last paragraph is well written
and very important: “Prudent mariners know the physical limits of their vessels
with respect to wind speed and wave height. The marine weather forecasts
provide both wind velocity (speed and direction) information and wave height
information. Wave height values, both predicted and observed, are defined as
the significant wave height, Hs. Hs is not a single value but rather a value
which implies a range of heights, from approximately 60% of Hs to 200% of Hs in
the open ocean (narrower range in the inner channels), occurring in a wave
spectrum! Mariners should not focus on the single significant wave value in a
forecast or observation but recognize the concept of the wave spectrum, know
the definition of significant wave height, and be able to determine the expected
range of wave heights.”
Thank you and well done Mr.