We now know how to determine which way a tidal stream will be running and how to estimate how quickly it will be running. Together with the pilotage techniques covered below, this is generally sufficient knowledge to kayak in tidal waters. However, if more ambitious crossings are being contemplated, it is helpful to be able to carry out formal navigation to determine what course to steer and how long the crossing will take.
Imagine a paddler capable of 3 knots paddling speed attempts to paddle across a 1-mile wide tidal strait running at 1.5 knots to the south. If the paddler takes a bearing to a point on the far side and paddles on this bearing, they will not get to where they expect. During the 20-minute crossing, they will drift 0.5 miles southward.
The solution is to angle the boat up tide enough to cancel out the southerly drift. It’s the same principle as a ferry glide on a river, just over a longer distance.
It’s clear that we will need to alter the course that we steer in tidal waters. The question is how much, and how long will a crossing take given the influence of the tide?
For the mathematically minded, this is simply a problem in vector addition to balance the effects of the tide with the paddling speed. For the rest of us, an example is the best way to explain the process.
Estimate course to steer and time taken to cross Bardsey sound from Pen y Cil to the NE tip of Bardsey Island when a 2 knot tide is flowing NW. Your group is capable of paddling at 3 knots in still water.
First draw a line from start point to the destination and well beyond – the green line.
Then draw a line that represents the tidal flow in one hour. In this case, we are told the flow is 2 knots, so a 2-mile long line is needed. Recall that we can use the latitude scale to measure this. We assume that the tide flows in the direction indicated by the arrow on the chart. The blue line is 2 miles long and parallel to the arrow.
Now, set a pair of dividers to the distance paddled in one hour – in this case 3 miles. Place one tip of the dividers at the end of the blue line. Swing the dividers until the other tip lies on the green line. Mark this point and draw a line between it and the end of the blue line – the red line on the chart.
The red line on the chart indicates the direction that you should point the boat – 200˚.
The distance along the green line from the blue line to the red line represents the distance that you would travel if you paddled across this tide for 1 hour – in this case 2.7 miles. It is clear here that it’s going to take less than one hour to get to Bardsey. The distance to Bardsey along the green line is 2 miles. So we expect the crossing to take 60 x 2/2.7 = 45 minutes.
The error often made in this process is drawing the paddling speed (red) line directly to the destination point. Instead, it must be of the correct length to represent 1 hour’s paddling distance and this will determine where it intersects the intended course (green) line.
The method shown here assumes that the tidal stream is constant through the period of the crossing. This is a reasonable assumption for crossings less than one hour. For longer crossings, we must consider the effect of tidal streams that very in strength and direction. These techniques are beyond the scope of this course.
Chart © Crown Copyright and/or database rights. NOT FOR NAVIGATION. Reproduced by permission of the Controller of Her Majesty’s Stationery Office and the UK Hydrographic Office (www.GOV.uk/UKHO)