## Wavelength and Speed

The speed at which a wave travels depends on whether it is in deep or shallow water. Deep water is water deeper than half the wavelength.

 Bft Maximum Duration Duration Duration Fetch with Fetch with Fetch with wind wave of wind to of wind to of wind to unlimited unlimited unlimited force height produce produce produce duration duration duration with 50% of 75% of 90% of of wind to of wind to of wind to unlimited maximum maximum maximum produce produce produce duration wave wave wave 50% of 75% of 90% of and fetch height height height max wave max wave max wave height height height 3 2 ft 1.5 hrs 5 hrs 8 hrs 3 nm 1 3 nm 25 nm 5 8 ft 3.5 hrs 8 hrs 12 hrs 10 nm 30 nm 60 nm 7 20 ft 5.5 hrs 11 hrs 21 hrs 22 nm 75 nm 150 nm 9 40 ft 7 hrs 16 hrs 25 hrs 55 nm 150 nm 280 nm 11 70 ft 9 hrs 19 hrs 32 hrs 85 nm 200 nm 450 nm

20.1 Wave Height and Wind Force, Duration and Fetch

20.1 Wave Height and Wind Force, Duration and Fetch

For deep-water waves the equation is:

WAVELENGTH = 1.56 x wave period squared. The speed of a wave is found by:

WAVE SPEED = 1.25 x square root of wavelength in metres.

### Shallow Water

In shallow water, both wavelength and speed decrease, and height and steepness (wavelength divided by height) increase. Shallow water waves are affected by:

• Reflection: When waves bounce off an obstacle like a rock or a reef back into the oncoming wave front.

Estimating the height of waves is very difficult. A reliable method is to have a volunteer in the crosstrees. When he is level with the crest of a wave you can calculate its height using Pythagorus' theorem. In calmer weather the volunteer can be closer to the deck.

The most important parameters in describing a wave are its Period (P), Speed (S) and Length (L). If you know two of these you can find the third. The basic equation is:

Wave Length = Speed x Period.

Deep water is water over half a wave length deep. Shallow water is under half a wavelength. In shallow water everything except the period changes. Wave Length and Speed decrease and Height increases.

In oceans, waves break when the height to wavelength ratio is about 1:7. In shallow water it is between 1:1.5 to 1:1.25.

20.2

• Diffraction: When waves squeeze together as they pass through the entrance to a bay or a harbour, then spread out again.

• Refraction: Where part of the wave is in shallow water and the rest is in deep water. The part in shallow water slows down but the remainder keeps going with the result that the wave pivots round the portion in shallow water.

Reflection, diffraction and refraction can happen all at once and the result, even on a calm day, can be a very unfriendly sea.

### Breaking Waves

In deep water waves begin to break when the ratio of wave height to wavelength is about 1:7. In shallow water it is usual to assume that they begin breaking when the depth of water is about 1.25 times the wave height.

Swell

Swells are waves created by winds outside your sea area. They are travelling through and have nothing to do with the winds that you are experiencing or the waves those winds create. Unless they hit an obstacle, swells maintain their direction, and in theory, if the helmsman keeps them at a constant angle to the boat, they are a simple aid to maintaining course.

Sadly, in practice this is not the case. Swells often come from more than one direction at the same time and then you have to identify waves generated by local winds. Unless there is one dominant, very obvious wave train, using swells for direction for any length of time is extremely difficult.

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