Boaters love waves, at least when we’re talking about the friendly gesture we give to each other when passing. But that other kind of wave, the one that makes the boat rock and roll? Maybe not so much. Like them or not, waves are a fact of boating, and every boat and every boater has a different tolerance for them.
My first rule for handling our boat in any significant waves is to try not to. When we’re planning a trip, the forecasted wave heights are a critical part of that planning. If we’re uncertain about a forecast, or if the predicted conditions are approaching our tolerance level, we ask ourselves: If we’re going to have a regret, would we rather regret staying in on a day we could have gone out, or going out and wishing we would have stayed in?
New boaters in my Trawlerfest sessions frequently ask, “What are the highest winds or largest waves I’m willing to go out in?” The answer is always, “It depends.” It’s not just about the wind speed or predicted height of the waves. It’s about direction. It’s about the wave period. It’s also about the body of water we’ll be on that day
To make better decisions about when to go out and when to stay in, it helps to understand how waves are formed, and how forecasters predict them. Waves, and subsequently wave heights, are determined by three things: Wind speed, the amount of time the wind has blown, and the distance of open water over which the wind is blowing, or fetch. Combined, these factors work together to create waves. The greater any one of the components, the greater the waves.
A fourth, and less frequently discussed, factor affecting wave height is temperature. When cold (sinking) air moves over the water, it exerts greater pressure on the water surface than warm (rising) air does. Consequently, when a cold front moves across an open body of water, it has the potential to create steeper waves that break sooner than when a warm front is passing over the same water at the same speed.
If you’ve spent any time on the water, you will have noticed that on any given day, waves are not all the same height. When waves are forming, they fall into a pattern of varying size. When weather routers and forecasters report the potential wave height, which can also be referred to as sea state (or, simply, seas), they are referring to the significant wave height, which is the average of the highest third of the waves. Having determined the significant wave height, forecasters then use a mathematical formula to determine the theoretical average height, and the highest waves expected in a given area, but you don’t see those numbers in the forecast. You only see the significant wave height.
Determining if it is safe, or whether it will even be comfortable to venture out, requires an understanding of the full meaning of the forecast. Too many boaters go out not understanding the potential for considerably higher waves built into the forecast. If you encounter waves higher than what was forecasted, it’s not that the forecaster got it wrong; it may just be that higher waves known to be in the forecast existed at the higher end of the predicted range.
Let’s look at how this plays out in an actual forecast. If the prediction calls for seas of 3 to 4 feet, that means you will have some smaller than 3 feet, some in the predicted range, and, most important, some that are twice the significant wave height. That could mean waves as high as 8 feet. No forecast can be exact about where the percentages fall. Just know this: Built into the forecast model is an expectation for some number of waves that could be twice the height predicted.
Understanding wave structure is helpful in understanding how your boat will interact with the waves. If the length of a wave is the distance measured from peak to peak, then the period of a wave is the amount of time it takes for the wave energy to move from peak to peak. (Imagine you were floating stationary, and the top of a wave or swell passed under your boat every four seconds. The period would be four seconds.) Generally speaking, the longer the period, the easier the waves will be to handle.
Waves change shape with time. Large ocean waves that continue beyond the wind that created them become swells. Swells behave differently than wind-waves and affect your boat differently. Waves are a product of conditions at hand, and swells are the remnants of waves that were created somewhere else by conditions different from what you’re experiencing. Swells can be as high or higher than locally produced wind-waves, but are almost always spaced farther apart. It’s also important to note that waves and swells often exist at the same time, and can come from different directions simultaneously.
Realizing the potential to encounter waves that will be higher than the predicted range will help you make better go or no-go decisions. Whenever you head out, determine ahead of time the conditions under which you will call it quits. Identify inlets and know where you can escape to calmer waters if conditions become uncomfortable.
In the next issue, we’ll discuss how different hull forms handle waves, and new technologies available for more accurate predictions.
