Passage planning basics
John Scott describes the ABC of passage planning on paper in the age of GPS
In the age of GPS John Scott shares his simple and logical system for passage planning on paper
For some, passage planning consists of a few jottings related to the weather forecast and some reference to tidal streams and strengths. For me it is a labour of love, a challenge for me and my boat against the elements, and a test of how accurately I can forecast our arrival time given all the information at my disposal. Over the last quarter of a century it has served me well, and on passages of 40-80 miles, we almost always arrive very close to the ETA.
My system is simple and logical and is no doubt similar to many other people’s approach to planning. Input is derived from the following sources:
■ Relevant charts
■ Reeve-Fowkes Tidal Atlases
■ Reeds Tidal information (or other)
■ Weather Forecast
■ Pilot books (in certain cases)
All my passage plans are done in a hardback A4 notebook, which is useful for reference when doing the same passage at some later date. In some cases, I have discovered the magic formula for a particular passage which has been repeated despite having to get up at some ungodly hour to achieve the same results.
Plans are done for all passages in excess of 20 miles, or less if a favourable tide cannot be relied upon throughout.
For the purpose of demonstrating the planning system, I have taken as an example a 60-mile passage made last year from Gosport to Eastbourne on the way back from the Solent to the East Coast. The plan was done about six weeks beforehand so that my crew had advance knowledge of start and arrival times and could organise themselves accordingly. A copy of the full plan is shown below.
The planning process was as follows:
Step 1
Reference was made (in this instance) to the book South North Sea and Eastern English Channel Tidal streams and heights by Michael Reeve-Fowkes (available on amazon.co.uk). I have found this series of tidal atlas to be an invaluable planning guide as it not only gives tidal rates and direction hourly, but also a conversion table providing the adjusted rate of flow based on the height and time of high water Cherbourg (hereafter referenced as CH) on that day.
The almanac showed that on 29 August, the date of my intended passage, High Water Cherbourg was 1028
BST – height 6.2m and then again at 2241 BST – height 6.3m.
Step 2
It was necessary first to consider what key challenges there would be confronting us on the passage. In this instance there were three:
n The Looe Channel off Selsey Bill n Beachy Head n Depth at the entrance to Sovereign Harbour, Eastbourne.
All were particularly critical on this occasion as it was spring tide. The following was therefore noted on the passage plan: n 13 miles from Gosport to the entrance of the Looe channel n the easterly flow through the channel was CH HW -4 to HW+1 n 53 miles from Gosport to Beachy Head – low water Eastbourne (Shoreham +015) = 2018 BST
Step 3
A passage plan revolves around minimising the potential adverse impact these challenges pose. The tide at the western end of the Looe Channel (Boulder Bank buoy) flows at up to 6 knots at springs and 2 knots in a westerly direction at Beachy Head. Clearly, ensuring an easterly flow through the Looe Channel was of paramount importance.
The plan is always based on a boat speed of 5 knots so it was going to take just under three hours to reach the entrance to the Looe Channel and on this basis I opted for a start time of 0630 BST.
The passage plan in question is shown in the diagram which has five columns in black pen. The second of these lists times hourly from 0630 onwards. Alongside in column 1 are shown the number of hours before and after high water Cherbourg (HW being at 1030 BST).
Now follows the planning process: using the Reeve-Fowkes tidal information, the tidal direction and rate (using the conversion table to adjust the rate to that shown for 6.2m HW) is entered in column three from 0630 BST onwards.
It is often necessary to make a judgement: for instance in the first hour the tidal chart shows a beneficial south south-east flow at 0.2 knots (0.3 adjusted). However this period entails leaving Portsmouth Harbour, proceeding down the channel and then crossing to the gap in the submarine barrier – hardly a direct course. For this reason, an adjusted speed of 5 knots was estimated for the first hour and entered in column 4. The same figure was entered in column 5 against 0730 BST showing the accumulated mileage after one hour.
Thereafter the tidal direction, adjusted rate and resulting speed over the ground was entered for each hour (this data being based on estimating the yacht’s position on the chart from the accumulated mileage shown in column 5 and reading off the relevant direction and flow at that location).
Despite having 4 hours of adverse tide in the Beachy Head area, the plan resulted in an ETA of 1845 for the 60-mile passage.
Step 4
The plan thus far excluded one vital element, the weather. At 24 hours before departure it was felt safe to revise the plan slightly based on a forecast of 6-10 knot north-west winds and flat seas. This was likely to add at least 1 knot to the boat speed so the plan was revised accordingly.
The revised figures were entered in red in columns 4A and 5A with the result that the new ETA was now 1615 BST.
In the event, we arrived at 1550.