So, here is part II of my tidal planning guide using tidal diamonds. In part I of my tidal planning guide I covered using a tidal stream atlas to make a crossing from the Isle of Man to Scotland.
For this exercise I am planning on paddling from Bamburgh Castle Beach to Megstone on the 8th July 2017. It's not the biggest open crossing but as there can be a fair current running, it is a crossing where you could find yourself caught out without prior planning.
The first item you will need for this crossing is the Admiralty chart covering the Farne Islands and coast.
If you take a look at the chart you will see there is a tidal diamond C just down the coast, between Bamburgh and Seahouses. On the back of the Admiralty charts for the area there will be a table corresponding to the tidal diamond C.
The chart tidal diamond table will provide you with the direction and rate, at both Springs and Neaps, of the tidal flow at that specific point. As tidal Diamond C is not too far away, it will be good enough to help us plan our journey from Bamburgh Castle Beach to Megstone.
The other piece of information you will get from tidal Diamond C is the port on which these timings are based on. In this case it is North Shields. You will therefore need the tide times for North Shields on 8th July 2017. From my Reeds Small Craft Almanac (don't forget to add 1 hour for BST):
Mean Ranges are 4.3m springs and 2.1 neaps
As HW is at 16:15. Counting back 6 hours brings us to 10:15, which is probably not a bad time to set off, as you get to have a bit longer in bed! Obviously, if you wanted too set off an hour or so earlier you would have to count forwards from the 03:50 HW to the required hour. But as I am the leader and I want everyone to feel fresh and not rushed, we will be setting off at 10:15 am. Meet at 9:30 to allow for lots of faffing. For those of us coated in anti-faff, tough! Be ready to set off at 10:15am!
Next I am going to work out the height difference between LW 10:12 and HW 16:15. This gives us a difference of (4.6 - 1.2) = 3.2m
By either using the computation of rates table (see my Tidal Planning Part 1) or by using the computation of rates formula, I can calculate the actual drift rates, for each hour on the day.
I am now going to start working out my tidal vector to paddle on to reach Megstone. To do this I start to enter the values on the computation of rates table, starting off with the starting hour (10:15 - 6 hours before HW) and adding rows for each hour. Note there is a row between each hour. This row between each hour is for the calculation of the average flow and direction over the hour. The tidal diamond gives me the speed and direction for each each hour, but I need to know what the average flow and direction will be over the hour.
Having written the hour rows, I can then enter from the tidal diamond information, the direction and speed. I can also enter the drift rates for the day. All I then need to do is to work out the average flow over the hour and average direction.
Rather than get carried away making lots of table entries, I stop after two hours as I am only interested in calculating the tidal vector for my paddle out. Coming back later is much easier as you have a big target to hit and you can use transits to help maintain your course. If you cannot see transits due to fog, heading west will mean you are going to hit land sooner or later (hopefully Britain)!
If we set off at 10:15 we can use our calcs to work out the tidal vector using our chart. First of all draw a straight line using a soft pencil (2B) from our departure point and to and through our destination target. This is the intended line of travel.
Now, using the values we have calculated in our tidal flow table, if we launched at Bamburgh and didn't paddle we would drift 0.8 nm on a bearing of 318 degrees. So lets add that drift line to our chart.
Having marked where we would be after an hour on our chart, we can then open up our dividers to 3Nm (assuming a paddle speed of 3 kts) and placing one end on our drift position after one hour, swing the dividers across our intended line of travel and mark the point where the arc crosses this line. This marks our position after one hour. A common mistake is to draw a line to the target (megstone) - don't do this!
Draw a line from our drift point to our position after one hours paddling (3nm). This is our paddle vector line.
As you can see our position after one hour is way past Megstone. So how long will it take us to get to Megstone? To find out draw another line parallel to the vector line that passes through Megstone.
Measure the length of this line from where it crosses the drift line to where it crosses the line of intended travel. You can do this easily on a chart by using the dividers and the lines of latitude printed on the vertical side of the chart. I make this to equal 1.8nm. As it take us 60 minutes to travel 3 nm, you can easily extrapolate how many minutes it will take to paddle 1.8nm
1.8 / 3 = min / 60
min = 1.8 * 60 / 3
min = 1.8 * 20
min = 36
So there you have it. Setting off at 10:15 it will take us 36 minutes to paddle out to Megstone following a bearing(vector) of 72 degrees.
Obviously for longer crossings you simply repeat the above, using tidal diamond information and/or tidal stream atlas details as necessary, using the total tidal drift or one hour drift method as described in part one.
In the next Tidal Planning post (Part 3) I will look at the Lundy Crossing.
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