Set, drift, rate and leeway
1The lesson in plain words
Two things move a boat off the line her bow points along: the water moving under her and the wind pushing her sideways. Name each one exactly.
Stream terms
A tidal stream is the horizontal movement of water caused by the tide. Set is the direction the stream flows towards, as a true bearing: a stream setting 090°T carries you east. Rate is its speed in knots (kn). Drift is the distance it carries you in a given time, in nautical miles (M): a 2.0 kn stream gives 3.0 M of drift in 1.5 hours. Wind is named the other way round: a westerly blows from the west.
Leeway
Leeway is sideways slip caused by wind, always downwind. The leeway angle lies between your heading (where the bow points) and your water track (your path through the water). It is often a few degrees. It is largest close-hauled (sailing as close to the wind as possible) in strong wind, or at low speed with high topsides (high hull sides above the water). Estimate it from the angle between your wake and the direction the bow points.
Three tracks
The heading is where the bow points. The water track adds leeway: your path through the water, at STW (speed through the water). A log is the instrument measuring speed and distance through the water. The ground track adds the stream too: your path over the seabed. GNSS (satellite navigation) shows COG (course over the ground) and SOG (speed over the ground).
Plotting arrows
On the chart, mark the water track with one arrowhead, the ground track with two and the tidal stream with three. This is the convention used on RYA courses and in many textbooks. Whatever convention you use, label every line so another crew member can read your plot.
Where the figures come from
Set and rate come from tidal diamonds and tidal stream atlases (Tidal streams unit), from pilot books, or from your own fixes (the last lesson of this unit). Non-tidal currents work the same way, such as the near-permanent east-going surface current through the Strait of Gibraltar.
2Key data
- Set
- Direction the stream flows TOWARDS, in degrees true
- Rate
- Speed of the stream in knots
- Drift
- Distance carried = rate × time, in M
- Leeway angle
- From heading to water track, always downwind
- Plot arrows
- Water track 1, ground track 2, tidal stream 3 arrowheads
3Key points
- 1
Set is where the stream goes, in degrees true; wind is named for where it comes from.
- 2
Drift = rate × time. A 1.5 kn stream for 2 hours gives 3.0 M.
- 3
Leeway pushes you downwind of your heading, onto the water track.
- 4
Heading + leeway = water track; water track + stream = ground track (COG and SOG).
4Frequent mistakes
Wrong: A stream setting 270°T comes from the west.
Right: Set is where the stream goes. A stream setting 270°T carries you west; it comes from the east.
Wrong: Drift and rate are the same thing.
Right: Rate is a speed in knots; drift is the distance in miles over a stated time.
Wrong: Motorboats make no leeway.
Right: Any boat makes leeway. High topsides and low speed make it worse.
5Practice case & exam keys
You steer 180°T at 5.0 kn STW. The atlas gives a stream setting 270°T at 1.5 kn for the next 2 hours. The wind is from the east and gives about 5° of leeway. A crew member says the wind and stream cancel. Do you agree?
Show the answer
No. Both carry you west. The east wind pushes you west of your heading, so the water track is about 185°T. The stream adds 3.0 M of drift to the west over the 2 hours (1.5 kn × 2 h). Your ground track will lie well west of 180°T. To hold a ground track of 180°T, you must steer east of it. The next lesson shows how to work out that course.
Exam keys
- Set: direction towards, degrees true. Rate: knots. Drift: miles in a stated time.
- Leeway angle: between heading and water track, always downwind.
- Water track + stream = ground track (COG/SOG).
- Arrows: water track 1, ground track 2, tidal stream 3.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
No notes