105.5 Correcting for Current and Leeway Flashcards
“Waypoint”
point on a chart with precise L&L
“Route”
series of waypoints connected
“COG”
Course Over Ground – actual course boat has gone from last waypoint
“CTE” or “XTE”
Cross-Track Error – usually in feet then miles, left or right of desired course btw waypoints
“SMG” or “SOG”
Speed Made Good or Speed Over Ground – actual speed you are making over ground through the water
“VMG”
Velocity Made Good – speed directly towards next waypoint regardless of course changes
“CTS”
Course to Steer – GPS determined course to arrive at next waypoint
“Course”
intended direction of travel
“CMG” or “COG”
“Track Made Good” or “Course Made Good” actual path of boat over the sea bottom. “Track” includes the effects of current
Heading or “CTS”
direction you must steer in order to correct for current and stay on the DR line
“Set”
direction TOWARD WHICH current flows. Set = 180 degrees, water is flowing toward 180 degrees.
What is difference between “Set” and Wind Direction
Set is expressed as TOWARD; Wind is expressed as FROM. Set of 180 degree, wind of 360 degrees are both moving in the same direction
“Drift”
Is the speed of the current
“Leeway” is the combined influence of (not current)
wind and waves
“Boat Speed”
is the speed of the boat through the water, and ignores current
Boat Speed ignores what?
current
SMG or SOG is actual speed boat moves over
the ground under the water – so takes into effect current
Knot speed reads 6. From ahead, a 2 knot current. What’s happening?
4 knots of true speed or SOG
Triangles (vector diagrams) can be used for two purposes related to currents
- Determining heading to offset current
2. Determine set and direction of current that pushed you off speed
Explain using vector / triangle diagram to determine course to take across current
- Plot basic DR line
- Vector line from departure point using DIRECTION from the set in degrees; and using LENGTH measured in MILES from the drift.
- From tip of new vector, use calipers set to your boat speed (use miles from longitude scale for knots) to find intersection with DR line.
- Connect those points; that is course to head
- The distance between the departure point and the intersection point represents true boat speed
The output of a vector / triangle diagram to determine heading to sail is a ___-____ situation.
one-hour. But heading and speed will be correct for entire length of trip provided current doesn’t change
A “big” deviation between a DR position and a three-point fix would be ___ % and likely due to ____
15% or more; current
Explain using vector / triangle diagram to correct course heading when off DR line
- Get fix at a certain time and plot DR position for that same time
- Line between those two gives you the SET only of the current, not drift
- Draw line from fix back to origin point and find the 1 hour DR position. Connect a vector to the newly drawn line at same SET angle.
- The distance of that Vector is the Set.
- Now use other vector triangle method to get new course
- new line to destination
- advance the vector to current fix position
- line from tip of vector through the one-hour point ahead
- direction of that new line is the heading to destination
- on the new direction line can calculate true boat speed
Always use ___ speed when plotting a current triangle
Boat
The current set and drift in a triangle represents cumulative effects of (4 things)
ocean currents, tidal currents, leeway and minor steering errors
Tidal Current Tables can give you ___ and ____ at different locations. Can be used in _______ _______.
set and drift at different locations. Can use in triangle calculations
River currents are affected by (two weather aspects) and can change based on channel ___, ____ and ____.
rainfall; melting snow/ice; depth, width, shape (curves)
Tidal Current Tables, cruising guides and other sources of current information usually give SET of currents as a
TRUE direction, rather than a Magnetic. (WHAT?) So plot the set using the outer rose.
Even in areas of no tidal, river or ocean current, you may want to adjust heading to accommodate
leeway
To measure just leeway, use handheld on the ___ line, add ___ degrees, compare to boat heading get difference. To compensate, steer the boat the amount of difference ____ ____ the wind.
wake; 180; higher into
