Tidal streams affect your journey, they can push you off course, or into hazards, and areas that are subject to strong tidal streams, the impact can be substantial. After only a short time, you can be far off course and much closer to hazards than you realise.
The changing tides are the result of moving water. As the tide rises, there is in inrush of water, and as the tide falls, the flow of water goes the other way. Any vessel on the
water will be affected by this movement of water. It can push you sideways and send you off course, or it can push you forward or backward through the water.
The tidal streams can be flowing at rates of several knots and any object that is in the tidal stream will be moving with the stream at the same rate.
As navigators, you need to be aware of the impact of the tidal streams on your passage and either work out the effect on your passage, or work out how to offset the effects when planning your passage.
If tidal streams are so important, how do we understand them?
There are three typical representations. Most regions will only show one type for their area, however in this course all are shown to aid in training. However, you can not mix between types in this course, as you will inevitably be confused. Questions will typically be specific on what representation to use to make sure you are comfortable in working with the different representations.
Represented by arrows on a diagrammatic representation of the area, they can be the most intuitive in understanding the movement of the tidal stream.
Represented on the navigation chart using a letter with in a diamond, they can be often the easiest to use in terms of navigation as the direction of the stream is given as a value and does not have to be plotted.
These show the peak springs and neaps rate that occur during each movement.
If the vessel crosses a tidal stream the boat will be pushed sideways, giving a different ground track to the course being steered.
Consider the tide as a moving walkway (travelator).
If you travel in the same direction as the 'flow' you will travel faster or slower if going into the 'flow'.
If you imagine a vessel sailing along at four knots and the tide is flowing the same way at 4 knots, the vessel will be travelling at 8 knots relative to the land. The term used is 'Speed Over the Ground' or SOG.
If the tide is flowing at 4 knots in the opposite direction to the vessel and the yacht is travelling at 4 knots through the water, the vessel will be stationary relative to the land and the SOG will be zero.
Even though, relative to the land, the vessel is stationary, the log (speedometer) will still read 4 knots because it measures the speed through the water.
It is similar to walking on a moving walkway. If the walkway is moving at 2 m/s(metres per second) and you are walking the same way at three m/s you are travelling at 5 m/s in total, so in 10s, you would actually have moved 50m (5m/s * 10seconds).
If you walk the wrong way against the moving
walkway you are travelling at 1 m/s, then after 10s, you would only have moved 10m (1m/s * 10seconds)
If you were to consider these differences over an hour, then it would have a substantial impact.
There are also two speed references that you need to be aware of and understand in order to work with tidal streams correctly.
In terms of navigation, Speed Over Ground is the speed at which the boat is actually travelling when you overlap the track on the chart and includes the effect of drift and leeway.
The direction and strength of the tidal stream depends on:
A navigator needs to know how to work out the speed (rate) and direction of the flow of the tide to plot courses and estimated positions. Speed at sea is measured in knots, the terminology comes from the old practice of determining boat speed by throwing a drogue overboard attached to a line that has knots tied at regularly spaced intervals. By counting how many knots were pulled through a persons fingers in a given time frame as the drogue was pulled backwards.
If you don't have tidal information, there are a few easier ways to work out what the tidal stream rate is.