Sail A

Question 1

What is apparent wind vs true speed wind?

Answer 1

The true wind is the wind as it blows across the land or water, the wind in the weather forecasts, the wind you feel when you’re standing stationary onshore. However on a boat, you are moving due to wind and current, and so the apparent wind is how much wind you feel when standing on the boat.

Question 2

how are waves formed?

Answer 2

wind, gravity, tidal waves or earthquakes and tsunamis

Question 3

what is wind velocity?

Answer 3

the speed of the wind

Question 4

what is fetch?

Answer 4

the distance over the water that the wind can blow uninterrupted

Question 5

what is duration?

Answer 5

the amount of time the wind blows over the patch of the water

Question 6

what three factors affect wave formation?

Answer 6

wind velocity, fetch and duration.

Question 7

the greater the wind velocity means….

Answer 7

Greater duration, and longer the fetch.

Basically, more energy is converted to waves, and they are large.

Question 8

if the wind velocity is low then….

Answer 8

waves will be small, regardless of the fetch or duration.

Question 9

frictional drag occurs when______.

Answer 9

Wind moves across the water surface.

Drag is a force that acts against the relative motion of one fluid with respect to another fluid.

Question 10

why is drag less over the ocean than on land?

Answer 10

the sea is much smoother than your typical land features.

Question 11

boundary layer?

Answer 11

portion of troposphere where friction drag is felt.

Question 12

the thickness of the atmospheric boundary layer depends on….

Answer 12

temperature, water surface and wind

Question 13

definitions

Answer 13

Crest = Highest point of the wave

Trough = Lowest point of the wave

Wavelength = Distance from one crest/trough to the next (m)

Wave Height = Height from trough to crest (m)

Wave steepness = ratio of wave height to wavelength

Amplitude = distance from the centre of wave to the bottom of the trough (m)

Wave Period = time for one full wavelength to pass a given point (s)

Question 14

what is a wave train?

Answer 14

when waves settle into groups of different size, and velocity while continuously moving away from the source.

Question 15

the speed at which a group of waves travels across the water is known as….?

Answer 15

group velocity

Question 16

the apparent speed of each individual wave in the group is known as…..

Answer 16

phase velocity

Question 17

what are wind-generated waves?

Answer 17

wind-waves are a result of wind disturbing the ocean surface and displacing water. Different types of waves are named based on their restoring force.

Question 18

what are capillary waves?

Answer 18

short-wavelength waves. (small ripples that are around a few cm in length)

Question 19

what are gravity waves?

Answer 19

waves in which gravity acts as the restoring force. (these waves can be up to metres to km long)

Question 20

what are swells?

Answer 20

Waves formed from distant storms. They are capable of travelling long distances.
-Swell with longer wavelengths tend to have lower wave heights and are less susceptible to decay from surface winds. They also carry more energy, and are more likely to form large breakers once they reach the shore.

Question 21

what are rogue waves?

Answer 21

very large waves formed due to wave interference.
destructive interference - waves meet and cancel each other out
constructive interference - the waves add up, and resulting waves are bigger

Question 22

are rogue waves dangerous?

Answer 22

yes they can be

Question 23

what are tsunamis?

Answer 23

very long wavelength waves resulting from seismic events such as earthquakes, under-water landslides and volcanic eruptions. In the open ocean (away from shore) they travel very fast (the same speed as a jet airliner), but have very small amplitude (cm to a meter or so). Thus, they have very small wave slopes, and you might not even notice it in the deep ocean, because the normal wind-waves would catch your attention instead

Question 24

when waves break against the shoreline they have impact on….

Answer 24

erosion and how shorelines are shaped

Question 25

waves info

Answer 25

As waves approach the shore, the bottom of the wave meets the ocean floor. As they drag across the bottom, the front waves slow down, and wavelength is reduced. The following waves start to build up behind the slow ones, and as the wavelengths get shorter, the wave energy gets transferred upwards, increasing wave height. The friction along the bottom slows the base of the wave down while the water at the surface continues forward. When the wave steepness (the ratio between wave height and wavelength) exceeds a ratio of 1:7, it becomes unstable and breaks. The slope of the sea floor greatly influences how quickly the sea floor affects the waves as the waves get closer to shore, and therefore how the waves break.

Question 26

what influences how waves break?

Answer 26

tides and direction of ocean swell
- If ocean swell approaches the shore at an angle, which it typically does, waves reaching the shallow water first will break first.

Question 27

types of breaking waves

Answer 27

• spilling
• plunging
• surging

Question 28

spilling waves

Answer 28

Spilling breakers occur as waves travel across a gently sloping bottom (i.e., gently sloping sea floor near the beach). The wave breaks long and slow, losing its energy as white water spilling from the crest down the front of the wave.

Question 29

plunging

Answer 29

Plunging breakers occur as waves approach moderate to steep bottoms. The wave becomes steeper than a spilling breaker and the crest falls as a well-defined curl, falling forward with considerable energy. The tube that forms as these waves hit the shore at an angle and progress across the shoreline is what surfers love.

Question 30

surging

Answer 30

Surging breakers occur when long wave period, low amplitude waves approach moderately steep shores. The wave doesn’t spill or curl; it builds up and then slides rapidly up the beach with less foam or spray than the other two breakers.