Tides Flashcards
What causes the tides
The combined graviational pull of the sun and the moon as the earth rotates on its axis
what causes neap tides
for the graviational pull of the sun and moon to be offset by 90 degrees
what causes spring tides
for the sun and moon to be in line and pull together
Why is there a tide on the opsiste side of the planet from the moon
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.
What is a spring tide
a larger tide than average
what is a neap tide
a smaller tide than average
How is mean sea level defined
the average hight of the ocean taken over 20 years
what does MHWS stand for
mean high water springs
what does MLWS stand for
mean low water springs
what does MHWN stand for
Mean high water neaps
what does MLWN stand for
Mean low water neaps
what does HAT stand for
highest astronomical tide
what does LAT stand for
Lowest astronomical tide
What is a tides range
the total difrence between Low water and High water
What are virtical cleareces given as
will normaly be given from HAT but not always so check the chart information
what is chart datum
the 0 depth of the chart, from where the tide is measured from
what is the hight of tide
the level of the tide mesured from chart datum
what is the rise of tide
the increase between the previous low water and the next high water
what is the fall of tide
the decrease in level between the previous highwater to the next low water
what are charted objects eg lighthouses hight measured from
mhws
What limit do depth sounders have
100m
as well as checking depth what can depth sounders be used for
obtaining a position fix
how should you calibrate the depth sounder
doing a manual check with a weight and a line
what positions could the depth sounder be reading from
The waterline, the bottom of the keel, or the transducer location
what are the advantages to setting your depth sounder to read from the water line
it allows you to use the depth to easily obtain a position fix
what are the advantages of setting your depth sounder to read from the bottom of the keel
you know exactly how much water before you run aground.
how do you calculate the depth of water using the tide
total depth = charted depth plus hight of tide
when a depth is written with an underline what does it mean
that it is a drying hight, and it is the hight above chart datum the land is.
Where can you find average tidal heights for areas
on the chart information
What are the standard columns in an average tide table
place, lat, long, HAT, Mhws, Mhwn, Mlwn, mlws
what is a tide table
a sheet displaying the tidal heights for every day
why are there not always 2 hw and lw a day
as the tides advance, sometimes one occurs early in the morning of the next day
what do you somtimes have to take into account reading tide tables
time zone and DST to allow for time conversions
What time are tide tables normaly given in
the local timezone
where do you find infomation on a tide tables time zone
in the top left corner
what are tidal gages and what are they used for
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
What are 3 things found at the top of a tide table
Location, Year and Timezone information
What do you need to do to times in the shaded areas of a tide table
nothing, they are in gmt already
what do you need to do to times in the non-shaded area of the tide table
add one hour to convert to BST to allow for DST
what does U.T. Stand for
universal time
what does DST stand for
daylight savings time
what does GMT stand for
grenwich mean time
what does BST stand for
British summer time
What information is listed on a tided table for a given day
the date, the day of the week, all hight and low tides, and colouration marking it as a neap or a spring or inbetween
what’s the formula for tidal range
High water - low water = range
what time and hight is the hight tide in the afternoon on Thursday the 27th of June at Dunbarton
13.52 dst, 4.1m
what is the time and hight of high water at Dunbarton in the morning of the 6th of Feb
6:04 UT 3.7m
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
neaps 2.4m
what is the range of tide at victoria? is it springs neaps or mid-range? During the afternoon of Thursday the 3rd of January
springs 4.9m
what is the range of tide at victoria? is it springs neaps or mid-range? During the afternoon of Monday the 18th of February
3.6m, mid-range
what is a tidal curve used for
working out hight of the tide at a time between high and low water
what goes in the row of boxes at the base of a tidal curve
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
what does the blue dashed line on a tidal curve represent
the tidal change during a neap tide
what does the red line on the tidal curve represent
a spring range tide
why should you work out the range of the tide in question before using it on a tidal curve
to find out if it is a spring or neap tide
where do the tides hights get added to the graph
at the scales at the top and bottom on the left side of the graph
describe the process of using a tidal curve
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.
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?
the hight of the tide at 10:30 is 3.1m, the minimum depth of water is4.9m
why is using the tidal curve important when anchoring?
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
What is the height of tide 4 hours after HW in the afternoon of Tuesday 29th January at Namley Harbour?
1.4m
What is the height of tide 2 hours before HW in the evening of Sunday 9th of June at Port Fraser?
3m
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?
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
What is a secondary port
a secondary port is a port without a set tidal curve and tide table
how are tides calculated in a secondary port
by converting from the curve and tide tables for the nearest standard port
what direction around the times to the corections move for a secondary port table
in an x shape
if the tide time is halfway between the tide times on the secondary port correction table what is the difrence
halfway between the two corection values
