Safe Boat Operations – Estimating Wave Height

Source: Mariners Learning System,
By Captain Bob Figular

An accurate estimate of wave height is subjective and sometimes difficult to accomplish, but there are a number of methods that, with practice, will give good results:

Height of eye or freeboard – With the boat in the trough and on a level and even keel, any wave that obscures the horizon is greater than the height of a person’s eye. One can also compare a wave to the deck edge or a structure such as the handrail. The wave face is observed while bowing into it and in the trough on an even keel. This is also generally the best method for judging surf.

Comparison with floating structures or vessels - This technique is most useful when observing from land, but may be applied while underway. If the freeboard of a buoy is known to be 13 feet, that information can be used to determine the height of the waves passing it. A buoy can also be used to determine the wave period. One can observe a vessel underway and by estimating the freeboard of the vessel and observing its motions on the water, he or she can gain a fair estimate of the seas in which it is operating.

Comparison with a fixed structure - Observation of waves as they pass a fixed structure, such as a break-wall, jetty, or pier, can be very accurate and can also provide wave period.

Depth Sounder - Using a digital finder with a fast update speed can be very accurate for determining wave height. By comparing the depth in the trough on even keel with the depth at the crest on even keel, an accurate measurement can be obtained.

All of these methods can be useful and fairly accurate for determining wave height, but they require practice and experience. By comparing a local Weather Service buoy report with the crew’s observations, they can fine-tune their sense of wave height. With enough practice, you should be able to judge wave heights simply by looking at the waves themselves.