What does it mean when we read forecasts predicting 3-meter waves with a 12-second period?
Is it a dangerous or manageable wave? Let’s see how to interpret the data we’re given!
Sea State
When we read weather forecasts from various apps or weather sites, we are provided with the following parameters:
-
The significant wave height (Hs), which is the vertical distance between the crest and trough of the wave. The significant wave height represents the average height of the highest third of waves.
-
The period is the time in seconds (s) between the passage of two successive crests at a fixed point. The average period is the arithmetic mean of the periods of all waves.
Wave Height
When sailing, we’ve surely observed an anomalous wave, higher than those indicated in the forecasts. This phenomenon is related to the concept of significant wave height, which represents the average height of the highest third of waves. Consequently, it’s entirely normal to occasionally encounter waves higher than this value. But to what extent can they exceed the predicted height?
There are statistical relationships that allow us to estimate the probability of encountering waves of different heights compared to the significant wave height. Below, we’ll look at these relationships to understand how frequently and to what extent we can expect higher waves:
- Hmax = 2 × Hs → occurs approximately every 2000 waves
- H₁/₁₀₀ = 1.67 × Hs → occurs approximately every 100 waves
- H₁/₁₀ = 1.27 × Hs → occurs approximately every 10 waves
Practical Example
If a forecast indicates rough seas with 3-meter waves (Hs), even higher waves might occur at sea:
- 1 wave every 2000 (Hmax) could reach 6 m (if the period is 8s, this happens about every 260 minutes).
- 1 wave every 100 (H₁/₁₀₀) could reach 5 m (if the period is 8s, this means one every 13 minutes).
- 1 wave every 10 (H₁/₁₀) could reach 3.8 m (if the period is 8s, one every 80 seconds).
Long or Short Waves: How to Distinguish Them?
Now that we know waves can be twice as large as predicted, we can continue our reasoning and delve into other aspects of waves.
Waves can be categorized into two main types:
- Long waves, where the distance between two successive crests is much greater than their height. They are gentler and less steep, thus more manageable for navigation.
- Short and steep waves: the distance between crests is closer to the wave height, making them steeper and more difficult to handle, especially with headwinds.
A wave is considered long and navigable if its wavelength is at least 10 times its height. Let’s look at two examples:
- A 3-meter wave with a wavelength of 30 meters or more will be gentler and more manageable in open seas.
- A wave of the same height, but with a wavelength of only 15 meters, will be steeper and potentially breaking.
How to Calculate Wavelength
But where do we get the wavelength (λ)?
The wavelength (λ) can be estimated based on the wave period (T), which is the time between two successive crests passing a fixed point:
λ ≈ 1.56 × T^2
Where:
- λ is the wavelength in meters
- T is the period in seconds
For example, if a wave has a period of 5 seconds, the wavelength will be:
λ = 1.56 × 5^2 = 1.56 × 25 = 39 meters
If instead the period is 3 seconds, the wavelength will be:
λ = 1.56 × 3^2 = 1.56 × 9 = 14 meters
But when does it become dangerous? It depends on the steepness of the wave!
In addition to wavelength, wave height is crucial in determining its dangerousness. The steepness of a wave is calculated by the ratio between height (H) and wavelength (λ):
Steepness = H / λ
IMPORTANT: referring to what we’ve seen before, for H we must take the value Hmax
Waves can be classified based on this ratio:
- If H/λ > 1/7, the wave is very steep, unstable, and may break.
- If H/λ < 1/10, the wave is long and gentle, easier to handle.
But let’s look at some examples: let’s consider two waves of different heights, but with the same period of 10 seconds:
-
3-meter wave – 10-second period
- λ = 1.56 × 10^2 = m
- Steepness = 3/156 → long and manageable wave
-
5-meter wave – 10-second period
- Same wavelength: 156 m
- Steepness = 5/156 ≈ → steeper, but still not critical
However, if a maximum 5-meter wave has a very reduced wavelength, it becomes critical:
-
- 5 meters in height and 35 meters in wavelength → steepness → unstable and dangerous wave.
- 5 meters in height and 100 meters in wavelength → steepness → gentler and navigable wave.
Reference Table
| Wave Height | Period (s) | Wavelength (m) | Steepness H/λ | Danger Level |
|---|---|---|---|---|
| 5 m | 12 s | 225 m | 1/45 | Long and manageable wave |
| 5 m | 8 s | 100 m | 1/20 | Fairly steep, uncomfortable |
| 5 m | 6 s | 56 m | 1/11 | Very steep, difficult |
| 5 m | 5 s | 39 m | 1/8 | Nearly unstable, risk of breaking |
| 5 m | 4 s | 25 m | 1/5 | Dangerous and breaking |
Calcolatore di Ripidità delle Onde
When does a wave start to break?
A wave starts to break when its height exceeds 78% of the water depth:
- If H exceeds 78% of the water depth (H > 0.78 × depth), the wave begins to break (e.g., near shallows or coasts).
- If there is strong wind against current or waves from opposite directions, the crest can become unstable and break.
Risk of Capsizing for a Vessel
When sailing, it’s crucial to consider the effect of waves on the vessel:
- A breaking wave on the beam, with a height greater than 1/3 of the vessel’s waterline length, represents a serious risk of capsizing.
- A wave at least 60% of the waterline length in height will most likely cause the boat to capsize, regardless of its shape.
Conclusion
Even with forecasts of “moderate” waves, sailors should be prepared to occasionally encounter significantly higher waves.
To assess the sea state, one must consider both the maximum wave height and wave length.
A long and high wave can be manageable, while a short and steep wave will be more challenging. The wave becomes dangerous when the height-to-length ratio exceeds 1/7 or when it starts to break. Knowing these parameters helps in planning safe navigation.
Clearly, these are theoretical considerations, and the situation at sea can change due to multiple factors, such as seabed conditions, coastal orography, cross seas, etc.
Do you want to gain experience at sea or obtain a license? Check out the offers from free-time-activities.com!
Subscribe to the newsletter to not miss the next articles!
Last update:
