On the Water

Always plan for larger seas

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. Ainsworth.