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day skipper > Tides > Flashcards

Tides Flashcards

1
Q

What causes the tides

A

The combined graviational pull of the sun and the moon as the earth rotates on its axis

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2
Q

what causes neap tides

A

for the graviational pull of the sun and moon to be offset by 90 degrees

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3
Q

what causes spring tides

A

for the sun and moon to be in line and pull together

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4
Q

Why is there a tide on the opsiste side of the planet from the moon

A

As the Moon circles the Earth it not only pulls the sea towards it but the land and the Earth itself. It also pulls the centre of gravity of the Earth towards it.

This results in a slightly reduced gravity on the opposite side of the Earth allowing centrifugal force to pull more on the sea causing a second tide.

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5
Q

What is a spring tide

A

a larger tide than average

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6
Q

what is a neap tide

A

a smaller tide than average

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7
Q

How is mean sea level defined

A

the average hight of the ocean taken over 20 years

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8
Q

what does MHWS stand for

A

mean high water springs

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9
Q

what does MLWS stand for

A

mean low water springs

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10
Q

what does MHWN stand for

A

Mean high water neaps

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11
Q

what does MLWN stand for

A

Mean low water neaps

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12
Q

what does HAT stand for

A

highest astronomical tide

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13
Q

what does LAT stand for

A

Lowest astronomical tide

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14
Q

What is a tides range

A

the total difrence between Low water and High water

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15
Q

What are virtical cleareces given as

A

will normaly be given from HAT but not always so check the chart information

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16
Q

what is chart datum

A

the 0 depth of the chart, from where the tide is measured from

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17
Q

what is the hight of tide

A

the level of the tide mesured from chart datum

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18
Q

what is the rise of tide

A

the increase between the previous low water and the next high water

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19
Q

what is the fall of tide

A

the decrease in level between the previous highwater to the next low water

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20
Q

what are charted objects eg lighthouses hight measured from

A

mhws

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21
Q

What limit do depth sounders have

A

100m

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22
Q

as well as checking depth what can depth sounders be used for

A

obtaining a position fix

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23
Q

how should you calibrate the depth sounder

A

doing a manual check with a weight and a line

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24
Q

what positions could the depth sounder be reading from

A

The waterline, the bottom of the keel, or the transducer location

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25
Q

what are the advantages to setting your depth sounder to read from the water line

A

it allows you to use the depth to easily obtain a position fix

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26
Q

what are the advantages of setting your depth sounder to read from the bottom of the keel

A

you know exactly how much water before you run aground.

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27
Q

how do you calculate the depth of water using the tide

A

total depth = charted depth plus hight of tide

28
Q

when a depth is written with an underline what does it mean

A

that it is a drying hight, and it is the hight above chart datum the land is.

29
Q

Where can you find average tidal heights for areas

A

on the chart information

30
Q

What are the standard columns in an average tide table

A

place, lat, long, HAT, Mhws, Mhwn, Mlwn, mlws

31
Q

what is a tide table

A

a sheet displaying the tidal heights for every day

32
Q

why are there not always 2 hw and lw a day

A

as the tides advance, sometimes one occurs early in the morning of the next day

33
Q

what do you somtimes have to take into account reading tide tables

A

time zone and DST to allow for time conversions

34
Q

What time are tide tables normaly given in

A

the local timezone

35
Q

where do you find infomation on a tide tables time zone

A

in the top left corner

36
Q

what are tidal gages and what are they used for

A

they are found at harbour entrances and other depth-sensitive areas. they are basically large meter sticks that show the real hight of the water

37
Q

What are 3 things found at the top of a tide table

A

Location, Year and Timezone information

38
Q

What do you need to do to times in the shaded areas of a tide table

A

nothing, they are in gmt already

39
Q

what do you need to do to times in the non-shaded area of the tide table

A

add one hour to convert to BST to allow for DST

40
Q

what does U.T. Stand for

A

universal time

41
Q

what does DST stand for

A

daylight savings time

42
Q

what does GMT stand for

A

grenwich mean time

43
Q

what does BST stand for

A

British summer time

44
Q

What information is listed on a tided table for a given day

A

the date, the day of the week, all hight and low tides, and colouration marking it as a neap or a spring or inbetween

45
Q

what’s the formula for tidal range

A

High water - low water = range

46
Q

what time and hight is the hight tide in the afternoon on Thursday the 27th of June at Dunbarton

A

13.52 dst, 4.1m

47
Q

what is the time and hight of high water at Dunbarton in the morning of the 6th of Feb

A

6:04 UT 3.7m

48
Q

what is the range of tide at victoria? is it springs neaps or mid-range? During the morning/early afternoon of Saturday the 6th of April

A

neaps 2.4m

49
Q

what is the range of tide at victoria? is it springs neaps or mid-range? During the afternoon of Thursday the 3rd of January

A

springs 4.9m

50
Q

what is the range of tide at victoria? is it springs neaps or mid-range? During the afternoon of Monday the 18th of February

A

3.6m, mid-range

51
Q

what is a tidal curve used for

A

working out hight of the tide at a time between high and low water

52
Q

what goes in the row of boxes at the base of a tidal curve

A

the high water time in DST and then the times hours before and after up to the low water before and after the high in question

53
Q

what does the blue dashed line on a tidal curve represent

A

the tidal change during a neap tide

54
Q

what does the red line on the tidal curve represent

A

a spring range tide

55
Q

why should you work out the range of the tide in question before using it on a tidal curve

A

to find out if it is a spring or neap tide

56
Q

where do the tides hights get added to the graph

A

at the scales at the top and bottom on the left side of the graph

57
Q

describe the process of using a tidal curve

A

first, find the tide information for the nearest standard port, and look up the day’s tides in it. Then find the high water time and hight closest to when you are sailing. Next, convert the tide time into DST if necessary and write it in the high water tide time in the bottom of the chart. Next populate the time boxes 1 hour forwards and back of high water time all the way to low water on either side of high water. Next, add the high water and low water heights to the scale bars on the top and bottom of the left section of the tidal curve. Join these up to form the tidal hight line. Now to work out the tidal hight, draw a line vertically up the chart till it intersects the tidal curve. here turn 90 degrees and draw along the chart until the line hits the tidal hight line drawn earlier. lastly, turn 90 degrees upwards until the line reaches the scale bar at the top of the chart. the hight of the tide at the time can then be read off the scale.

58
Q

a skipper anchoring in port Fraser at 1030 wishes to know the hight of the tide when the high tide is 4.2m at 1230DST and the lows are both 0.4m. What is the height of the tide at this time? If the vessel has a draught of 1.2m and the skipper wants a clearance of 1m, what is the minimum depth of water to anchor in at 10:30?

A

the hight of the tide at 10:30 is 3.1m, the minimum depth of water is4.9m

59
Q

why is using the tidal curve important when anchoring?

A

it allows the skipper to calculate the rise in tide from the current depth, allowing it to be added on the reading from the depth sounder, and finding the maximum amount of water and thus chain needed for the precise location

60
Q

What is the height of tide 4 hours after HW in the afternoon of Tuesday 29th January at Namley Harbour?

A

1.4m

61
Q

What is the height of tide 2 hours before HW in the evening of Sunday 9th of June at Port Fraser?

A

3m

62
Q

On Sunday 25th August at Victoria.

7a) What is the time and height of daytime HW and the height of the following LW?
7b) What will be the height of tide at 1545 DST?

A

daytime hight is at 12.45 dst with hight 5.2m
evening low is at 18:45 with a hight of `1.1m
the hight of tide at 15:45 Dst will be 3.1m

63
Q

What is a secondary port

A

a secondary port is a port without a set tidal curve and tide table

64
Q

how are tides calculated in a secondary port

A

by converting from the curve and tide tables for the nearest standard port

65
Q

what direction around the times to the corections move for a secondary port table

A

in an x shape

66
Q

if the tide time is halfway between the tide times on the secondary port correction table what is the difrence

A

halfway between the two corection values

day skipper (16 decks)

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