Course to Steer
'The Midshipman' has produced an excellent animation which demonstrates how to construct a vector for course to steer on a chart - connect from my links page. Here is a spreadsheet which can solve the same problem:
From your chart, measure the bearing of the required ground track; enter the tide and boat speed. The sheet then calculates a course to steer for the next hour.
It works by making the component of the boat's velocity uptide equal to the tide itself.
This sheet was set up to calculate a course to steer whilst on passage from Ilfracombe to Lundy Island on the morning of August 8th 2005.
I used the diamonds spreadsheet to get the tidal streams for that morning. HW Milford was at 09:02 BST and range of tide 5.2 m. Three hours after HW Milford the tide would be 1.9 kn at 233T This would require a course to steer of 283T in order to achieve a ground track of 270T.
My log book records a compass heading of 289 Magnetic, 285 True, for this time. The tidal diamonds spreadsheet and the cts spreadsheet predict 283 True whilst I actually steered 285T. In truth the chart plotter and the autohelm were doing the real work on that beautiful morning. The compass was got out only for a check on proceedings!
A second example: making for the Wrach channel to enter Cardiff bay on 2nd June 2007. Here are the instrument readings at about 17:00
The boat heading was 275 magnetic, 271true The plotter gave our course over the ground as 336 true. Our position was:
I got the tide data from diamond D on Admiralty chart 1182 and used spreadsheets to get current, height of tide and course to steer:
The spreadsheet gave a course to steer of 271 true in order to get 336 true over the ground. The actual boat heading was 271 true to get 336 true across the ground. The charted depth where the measurements were taken was nearly 6m so with 5m of tide we get the 11m depth recorded on the echosounder.
The projected course over the ground is given by the gps plotter: the dotted line in the photo above.
I steered the boat onto a heading which produced the necessary COG as shown on the plotter screen. The tidal diamond data leads to course predictions which are in good agreement with plotter readings.
The previous examples were taken from my logbook. Both were sunny days in light winds, leeway zero. The following represents a discussion at CYC on a stormy evening, Jan 15th 2008:
From the comfort of armchairs we were discussing making night passage from Minehead to Barry. In particular what course was required to be steered ? The range of tide was 9.1 m and the tidal streams in the vicinity were:
Our discussion was taking place about three hours before HW. Hence the course to steer
The wind was from the West, F5. Given -6 degrees to counteract leeway and +4 degrees magnetic variation then a course of 014(M) would see us safely at Barry in just over two hours.