Keep in mind wave predictions are the averages of the highest 1/3 of waves. However it's not uncommon for waves of twice the predicted height to be mixed in. 5 foot seas, 8 seconds apart on a 40 ft. boat is doable.
I recall last year we went out with a 2-3' wave forecast and 10 knot wind, 25 ft. boat. Nothing uncommon about that, except we were surprised to find about every 7th wave was a five footer. In the Gulf we have a tight wave interval, so that day ended up pretty sporty 25-30 miles offshore.
Waves are measured from trough to crest. Seas are the combination of both locally generated wind waves and distantly generated swell waves. Specifically, seas may be derived using the following formula where S is the height of the swell waves and W is the height of the wind waves.
SEAS =
Seas and waves are expressed in the terms of the significant wave height, the mean or average height of the highest one third of all waves in a swell train or in a wave generating region. It approximates the value an experienced observer would report if visually estimating sea height. When expressed as a range (e.g. seas 3-5 ft), this indicates a degree of uncertainty in the forecast and/or expected changing conditions (not that all waves are between 3-5 ft). Generally, it is assumed that individual wave heights can be described using a Rayleigh distribution.
Example: Significant Wave Height = 10 ft
1 in 10 waves will be larger than 11 ft
1 in 100 waves will be larger than 16 ft
1 in 1000 waves will larger than 19 ft
There are occasional reports of "rogue" waves of an even greater ratio
Therefore, assuming a wave period of 8 seconds, for a significant wave height of 10 feet, a wave 19 feet or higher will occur every 8,000 seconds (2.2 hours).
Wave heights increase dramatically as waves begin to "feel" the bottom near shore or any in shallow water. Wave-current interaction will also dramatically impact wave heights such as in the area of the Gulf Stream or around jetties.
"The seventh wave of the seventh set" An old fisherman's tale? Perhaps, but it does serve to highlight that wave and surf conditions are not always constant. In open waters, the occasional wave may be twice that of the surrounding sea. There are occasional reports of "rogue" waves of an even greater ratio. Near shore waves are even less predictable. So-called "sneaker waves" can grab the unwary who venture too close to the unpredictable sea. Mariners may be drawn too close to the surf zone during periods of relative calm. Proceed cautiously and always be wary of this not uncommon phenomenon, especially in areas where breaking surf is known to occur or appears likely. From the scientific point of view, this phenomena is more likely to occur when several swell systems combine.
The danger presented to a vessel is a function of wave steepness as well as wave height and is unique to each vessel. For small vessels at a given wave height, the danger increases as the wave period decreases. Below is a table used as guidance within the National Weather Service to assist forecasters in identifying sea conditions which may require the issuance of a Small Craft Advisory. See FAQ 13 for definitions of a Small Craft Advisory.
The table was developed for coastal waters (within 60 nm of shore) off the Western US coast from Washington south to southern California and indicates potential danger to vessels with a closed cockpit configuration of ~100 feet or less. The specific values and conditions for the potential danger threats of the table cannot be applied to other waters such as the Gulf Of Mexico, Atlantic Ocean, Alaskan coastal waters or inland waters. However, an important message indicated by the table applies to all waters. That message is... for small vessels, danger increases when the wave period decreases (for a given wave height). The table is intended to be used as forecaster guidance only and not to directly correlate danger to a specific vessel. The danger presented by waves to your own vessel may be quite different.
