105.6 Tides and Tidal Currents

Question 1

“Tide” refers only to the ___ and ___ of coastal waters

Answer 1

Rise and fall – it strictly refers only to vertical change

Question 2

Where coast waters flow horizontally is call ____ ____, including ___ (in) currents and ___ (out) currents

Answer 2

Tidal currents; flood; ebb

Question 3

Describe a day of tides and current

Answer 3

1. Low tide
2. Flood tidal current –> rising tide level
3. Reaches maximum height –> slack current
4. Ebb tidal current –> lower tide level
5. Reaches minimum height –> slack current
6. Rinse and repeat one more day

Question 4

The maximum tidal current speed is ______ between tide extremes – rising and falling back to zero over time from max high to max low

Answer 4

halfway

Question 5

Slack current can last between ____ and ____ minutes, depending on location

Answer 5

two and twelve

Question 6

Typical time between high and low tides is

Answer 6

6 hours and 13 minutes

Question 7

The entire “daily” tide cycle (including slack water time) takes how much time?

Answer 7

24 hours and 50 minutes (so not quite daily). That’s why tables are needed

Question 8

Use a tide and current table to find:

Answer 8

1. Times of high and low tides
2. Depth of water at each of those points
3. Set and drift, including times of maximum current
4. Times of slack water

Question 9

Tides are caused by the gravity of

Answer 9

the moon

Question 10

Why does an equivalent bulge happen on the side of earth away from moon?

Answer 10

Centrifugal force seeking balance

Question 11

The moon travels around the earth every

Answer 11

24 hours 50 minutes (same a tidal day)

Question 12

When sun and moon aligned, get

Answer 12

spring tides, full or new moon, high tides are higher than average, low tides are lower than average

Question 13

When sun and moon at 90 degree angle to one another, get

Answer 13

neap tides, quarter moons, high tides lower than average, low tides higher than average

Question 14

Actual peak spring and neap tides occur:

Answer 14

slightly after full/new moons, after quarter moons

Question 15

Comparatively, the sun’s influence on tides is only

Answer 15

half that of the moon

Question 16

Tidal CURRENT Table 1 is for ____ and gives

Answer 16

Select Reference Stations (e.g., SFO, SD in CA); Slack water times; and Max Current Velocity Times –

Question 17

Tidal CURRENT Table 2 is for _____ and gives

Answer 17

Subordinate Stations; correction factors to apply to Table 1 data

Question 18

Tidal Current Tables are expressed in _____ _____ Time. You must adjust for _____ _____ time.

Answer 18

Local Standard Time; Daylight Savings Time

Question 19

The Velocity Ratios on Table 2 are a _____ of the velocities shown on Table 1.

Answer 19

percentages (i.e., .4 = 40%)

Question 20

To get actual Tidal Current information at a Subordinate Station you must combine the information from

Answer 20

Table 1 and Table 2

Question 21

Tidal Current Table 3 purpose

Answer 21

Calculate Current velocity of at any time

Question 22

Tidal Current Table 3 x-axis; y-axis; resulting value; its use

Answer 22

X difference between time of slack and time of max from Table 1 (and potentially adjusted for Table 2 Subordinate Stations); Y difference between time of slack and the desired time; table value is a percentage factor (.8 = 80%); apply that to the max current shown on Table 1

Question 23

Tide Tables (not Current) have a similar process to determine water height in between published times similar to Table __ in the ______ ______ tables

Answer 23

3; Tidal Current

Question 24

Main flow of a current is usually at the _____ of the flow, with _____ eddies and _____-currents at the edges.

Answer 24

middle; reversing; counter

Question 25

What is the shoaling effect?

Answer 25

Water speeding up over shallower surfaces; rough water on top

Question 26

Seas are steeper and rougher when the ____ and ____ are in opposition

Answer 26

current and wind

Question 27

Winds can create a current of _____ _____ likely no more than ____ knot.

Answer 27

surface water; 1 knot

Question 28

Storms affect tidal heights. A drop of ____ in barometric pressure will raise tide height by ____.

Answer 28

1-inch; 1 foot

Question 29

Visual indications of current is the buoy ____. In strong current a buoy will also have a

Answer 29

buoy wake; it will have a bow wave;

Question 30

The heel of a buoy can indicate the ___ of the current.

Answer 30

set; buoy heels WITH the direction of current flow

Question 31

An anchored boat will ride bow-to a combination of wind and current. If there is no wind, it indicates the currents

Answer 31

set

Question 32

Wet or dry shoreside objects can also indicate

Answer 32

ebb or flood tide

Question 33

Rule of twelfths

Answer 33

1 hour after high/low, 8% change (I'd round to 10%)
2 hours after, 25% change
3 hours after, 50% change
4 hours after, 75% change
5 hours after, 90% change
6 hours 13 min after, change is 100%

Question 34

Insetting currents are …. and in the book are called the

Answer 34

the tendency of ocean coastal currents to also push inward toward bays and the shore… silent trap.

Question 35

Why can insetting currents catch skippers off-guard?

Answer 35

Sounders might be off if think 1000 feet of water below; Radar reflectors might pick up distant hills or mountains but not the miles of beach and low land leading to them; GPS can be off by a mile or two. Open your eyes!

Question 36

Discolored water can indicate current flows

Answer 36

Bright clear warm water of Gulf Stream vs. green, dark, cold water of Labrador Current

Question 37

Debris in the water can indicate current flows

Answer 37

Generally coming from onshore out to sea

Question 38

Summarize Triangle 1 Steps

Answer 38

1. Draw vector from point of dep
2. From vector tip, draw exactly a 1 hour @ Boat Speed line intersecting back to DR. Mark that point. You now have triangle.
3. Drawn line is the heading to offset current/leeway.
4. Measure Dep to line intersection point for true boat speed

Question 39

Summarize Triangle 2 Steps

Answer 39

1. Plot fix point off the DR line @ t = X. Draw line back to departure.
2. Connect DR plot @ t = X to fix. You now have triangle.
3. Mag of that line is the Set.
4. Bring that mag line back to the 1 hour mark