Understanding Waves & Wakes

By: Captain Bill Jennings

Photo- Unsplash

Crossing multiple wakes from other boats can definitely disturb your fun day on the water. Especially if you are in a small boat. To understand how you can best make your ride smoother, you must first understand some of the basic physical characteristics of a boat wake. Once you have a better understanding of boat wake mechanics, you will know how to mitigate their negative affect on the comfort of your ride.

The subject of waves and boat wakes has been long studied by both scientists and engineers. The details and formulas that evolved from their research can boggle the mind of most pleasure boaters. In addition, most of the research on this subject has been aimed at ocean-going commercial ships. However, if we look at this research, but simplify the technical talk and limit our discussion to the size and type of boats that pleasure boaters normally encounter, I believe we can identify important information needed to 'take the beating out of boating.'

Your boat creates two basic waves. A bow wave and a stern wave. Of the two, the stern wave produced by boats under 75 feet are of greatest concern. This is because when lighter weight pleasure boats are on plane, the bow is either out of the water or running high in the water and therefore not the major wave producer.

The stern wake is the V-shaped series of waves created by the displacement of the boat as it passes through the water. How it makes a wake is important. As the boat moves forward, its bottom pushes water down. Once the boat has passed, the water rebounds to a point higher than its normal position. Because water is incompressible, the molecule that has just received the momentum from the passing boat can only move upward into air, where gravity will immediately pull it back down. The moving boat is continually generating at its transom this expanding elliptical wave called the "canonical crest." The height of this crest above the normal water level stores potential energy. In other words, the water above its normal level position wants to fall back down to its normal position, and so the wave (or wake) now has "energy." When it falls back down, it creates the wave we call a 'wake.' The height of that wave above the normal level of the water is called "amplitude". The highest point of that wave is called the "peak". In this process, the moving molecules transfer their energy momentum to the molecules beside them, creating an elliptical motion that moves straight up and down, with each peak being pulled back down by gravity. This gets repeated over and over in a water wave. This sequence of up and down movements makes the wave appear to be moving, but actually the water in waves is not travelling at all. Waves transmit energy, not water, across the surface of the water. Interestingly, if waves are not obstructed by something in the water (like another boat), or wind, they have the potential to travel across large bodies of water for great distances. As you would expect, larger and heavier boats push the water down further, setting off a sequence that shows up as a larger wake.

While this explains wakes, it doesn't tell you how to avoid their transfer of energy to your boat. To learn this, I refer to wake studies by a famous wave scientist, Lord Kelvin. Kelvin calculated that wakes travel at a fixed fraction (81.6%) of the boat's speed. Knowing this, the wake angle of a boat is always the same, because the faster the boat goes the faster the wake spreads, leaving the wake angle constant. Unfortunately, not all researchers agree, saying that wake angles can vary somewhat.

To resolve the dilemma I have compared Lord Kelvin's numbers with my own measurements as they apply to pleasure boats, and included a few scientific opinions to come up with a wake angle average of 21 degrees. You can now use this information to put together a smart driving procedure that will allow you to avoid damaging boat wakes. Take a common example where a large boat is approaching you from directly ahead (as in boats going the opposite direction). When the oncoming boat is almost parallel to you, make a 21 degree turn away from the boat and maintain your boat speed. With this turn you will be running parallel to the wake produced by the oncoming boat and you can "roll" through it without any impact. Once the wake from the other boat has passed under your boat, you can resume your heading.

"Wavelength" is the distance from from one peak to the next peak. The speed or velocity of waves varies with their wavelength. As a boat moves, it creates waves of different speeds, all of which will be slower than the boat itself. But longer wavelengths travel faster than shorter ones. Knowing this gives you the opportunity to judge the speed of an approaching boat and determine the time you have before encountering their wake. Knowing when a wake will arrive and in what size, you can decide how to approach it. Wakes of longer wavelengths can be more safely approached at 90 degrees. Such an approach usually includes reducing speed.

I have discussed situations where you should approach a boat wake at 90 degrees and another where you should run parallel to the wake. What about approaching at 45 degrees? After all, there are still courses that recommend this approach. Well, don't do it, and here is why:

When you enter any wave or a wake at 90 degrees, your boat takes 50% of the impact up your starboard side and 50% up your portside. The impact is evenly distributed. But if you enter a wave at a 45 degree angle, most of the impact strikes one side of your boat, and worse, the angle of the wave face matches the angle of your bow, so it is a flat and jarring hit.

We know that boats travelling at an idle or very slow speeds create very little wake. This is because the combination of low boat impact with the water and a reduced draft at the transom does not impart the momentum necessary to make a significant conical crest (in other words, a wake generator).

A wave's "period," or wavelength, is the distance between the same spot of the next wave, as in crest to crest. Waves with a larger period travel faster than short period waves. This means that waves can travel at different speeds depending on their size. The speed of a wave will be 1.35 times the square root of its period in feet. The speed will be in knots. For example, if you are seeing waves with 4 feet between crests, the square root of 4 is 2, multiplied by 1.35 is 2.7 knots, or 3.1 mph. A 6 foot period would square root at 2.45, and when multiplied by 1.35 gives a wave speed of 3.3 knots, or almost 4 mph. Take a look at the wavelength distance between different wave conditions and use this formula to roughly calculate wave speed. Higher speed waves will impact your ride more severely than low speed waves, simply because you will encounter more of them over a given space of time.

Formulas can certainly help us measure the danger factor of waves. We are told that waves with high "intensity" are dangerous. Therefore, a formula to measure and compare wave intensities can also be useful. Begin with the predicted "amplitude" of waves, or the distance between its centerline (or still position) to the crest (or top) of the wave. Intensity is defined as the square of this number. For example, if amplitude was 2 feet, the intensity number would be the square of 2, or 4. If the amplitude was 3 feet, the intensity number would be 9. This formula demonstrates how 'intensity' can quickly increase with wave size. And this provides you with a useful piece of information when making a "go or no go" decision.

A basic understanding of how waves and wakes work can assist you in minimizing their danger and is key to enjoying safe and comfortable boating.

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