Sailboats move on the water's surface, in the dynamic area where wind and water interact, exchanging energy and building waves. Winds fill a boat's sails, but it is waves that affect a boat's movement and stability. So waves, more than wind, are really our first interest.
So how do waves form? When wind blows along a consistent direction and with a consonant force for a measured period of time, waves form. We speak of waves in measured terms, calculating their height and period. Height is the distance from the trough to the crest, and period is the time it takes for successive wave crests, or troughs, to pass a fixed point.
Now, here is the key to understanding waves. When you know the wave period you are able to calculate everything else you need to know about waves. Knowing this variable, you can quickly compute these other important wave features:
- A wave length is five times the wave period squared.
- A single wave speed is three times the wave period.
- A group of waves move at a constant speed, which is 1.5 times the wave period.
- A wave feels the bottom and begins to slow (while also growing in height) when water depth is half the wave's length (see item No.1 for determining length).
- A wave breaks when the water depth is five thirds of the wave's height. If you know the water depth, you can work this formula in reverse and calculate what size wave will break in a given water depth. For example, in five feet of water a three-foot wave will break.
So how do you determine the wave period? Well, you can time the passage of wave crests or troughs yourself while observing the ocean around you, and in addition, you can use the Marine Prediction Centers (MPC) wave period charts, which are produced and disseminated several times each day via Weatherfax and the Internet (www.mpc.ncep.noaa.gov).
MPC wave period charts are highly accurate and incorporate global wave energy in determining wave period. A wave model known as the Wave Watch III is now in use by the MPC and has proven itself to be the worldís most accurate and reliable wave model. We are truly fortunate to have this data available to us.
Why do you need to know these relationships concerning waves? Here is why: Letís say a gale (a gale is defined as winds of 34 to 40 knots, which is Force 8 on the Beaufort Scale) is 500 miles away from your position and stationary. These gale-force winds are blowing across 200 miles of open ocean and in doing so they produce waves with a height of 23 feet and a period of approximately nine seconds (data from Table 3302 in American Practical Navigator-Bowditch).
When will these gale-produced waves reach your position? Before we compute an answer letís first consider why we need to know. Knowing when these seas will arrive will allow you to make an appropriate course or anchorage change, keeping your sailboat in comfortable and safe conditions.
So when will these waves arrive? Letís calculate wave speed for the group of waves being produced. The group wave speed is 1.5 times the wave period. The wave period is nine seconds, so the group wave speed is 13.5 knots (1.5 x nine seconds = 13.5 knots). Traveling at 13.5 knots it will take these waves 37 hours, or 1.5 days (500 miles divided by 13.5 knots = 37 hours) to reach your position.
Forerunners of these main waves travel at a faster speed, which is three times the wave period, so in 18 hours the first swells from this gale will reach your position. These first swells are the warning sign of the approaching herd of waves, and should prompt you to take note.
In the days before weather satellites, the approach of hurricanes was monitored by watching ocean swells. Hurricane-force winds produce large waves with long periods, up to 18 seconds, and so move at 27 knots (18 seconds x 1.5 = 27 knots). Since hurricanes mover slower than 27 knots when first developing, the arrival of large swells warns of an approaching tropical cyclone. Better yet, by timing the swell period you can work this math in backwards and determine an approximate distance to the hurricane!
Now let's say for this example that you are anchored in what appears to be a safe cove, where the water depth averages 30 feet. You know that large swells will be arriving within the next 18 to 37 hours and you want to know in what depth these waves will break. Wave height will degrade to some extent during their 500-mile transit to your position, and for now letís assume the wave height upon arrival at your anchorage is 20 feet.
At what depth will these waves begin feeling the bottom and hence slow down? At what depth will these waves break? First, we need to calculate wavelength, which is five times the wave period squared. So 5 x (9 x 9) = 405 feet for wavelength. The waves feel the bottom and begin slowing when the depth is 1/2 of wavelength; so these waves will begin slowing down and growing in height when the water depth is 405/2 = 202 feet. Well, you are OK so far since you are not anchored in 202 feet of water!
Wait though! When will these waves actually break? That occurs when the water depth is the 5/3 wave height (remember, a three-foot wave will break in five feet of water). Do a little new math cross multiplication, and you have depth = 5/3 wave height. Thus, we now know these waves will break in 33 feet of water (5/3 x 20 = 33 feet). Oh no! The water depth in your safe cove is 30 feet! I imagine you had better weigh anchor and move to a different location, but no need to scream, shout, and run about because you have calculated the wave speed and you have 18 hours before the first swells arrive. But don't procrastinate!
Hopefully you now see how waves are a critical factor in sailboat stability and motion, so observe the waves around you, and determine the wave period. Use the MPC wave period chart and always calculate depths where waves will break when choosing an open anchorage. Forewarned is forearmed.