Awareness of Secondary Ports
Why are there standard and secondary ports?
Standard ports have traditionally been the bigger commercial or military ports used by heavier shipping. These ports have a full set of tidal information, including tide tables and tidal curves like those found in the almanacs. There are various standard ports listed in the index of the Almanac. Generally if a port has a set of tide tables and tidal curves it qualifies as a standard port.
There are plenty of smaller ports and harbour that do not have their own published tide tables or tidal curves. They do not have the same degree of tidal information available for them. These are called secondary ports and only have abridged information provided for them in the almanac. The information for any particular secondary port is linked to a standard port near in close proximity to it. We can therefore borrow a standard ports tide tables and curves to work on a nearby and linked secondary port.
The tidal cycle
As a flood tide floods inshore it can be though of as a huge wave moving relatively slowly (Fig 5.51). As the tide floods northwards along the Farlow Channel the high-water will arrive first at Farlow ‘A’, then Rampton ‘B’, then Whale Bay ‘C’ as it floods and works it way northwards. Eventually reaching the inner recesses of Namely Harbour ‘D’ and esturine harbour of Emsbourne ‘E’. As the tide ebbs and flows in the opposite direction we will experience low tide at Namely harbour first then Whale bay, Port Rampton and Farlow.
The geomorphology of the seabed and land around where the tides flood and ebb have a huge influence on the times and rate of the tide and combine to produce the difference in tidal heights and times of the tides.
Secondary Port Information Box
Fig 5.52 shows a typical secondary port information box. The name and exact position of the secondary port are prominent ‘A’ at the top of the box. The Standard Port, in this case Namely Harbour is usually highlighted in bold ‘B’. In most almanacs there is a little printed arrow indicating which direction you should look in the book to find the Standard Port tide tables and information quickly. We can see from the secondary port information that when the High Wide is 0000 UT or 1200 UT that the High Water at the secondary port, Port Rampton, will be 35 minutes before the standard port, Namely Harbour ‘C’. When the Mean High Water Spring levels are a 4.0 metres the MHWS level at Port Rampton will be 0.5 metres higher. Other differences are indicated in the box, including the different times of High Water and time differences at Low Water. There are also height differences depending on whether it is Low Water Springs or during Neap Tides.
There may be times when we are experiencing mid tides or or the times of the tides are not exactly as printed in the secondary port information box. In this case we must extrapolate the differences. We shall have a quick look below how to do this but not part of the Day Skipper theory course.
Secondary port Differences
We can work out the differences between standard and secondary ports using secondary port information boxes in the Training Almanac. The secondary port differences information can be found on the relevant page for the harbour in question and highlighted in Fig 5.53.
We can see form the secondary port information that
High-water gets to Farlow River first and is between between 40 and 18 mins earlier than namely harbour ‘A’.
High-water at Port Rampton is 35 to 16 mins before HW at Namely harbour ‘B’.
High water at Whale Bay is between 10 and 5 mins before HW ay Namely Harbour.
High-water at Emsbourne is 10 mins after High-water at Namley Harbour.
Fig 5.53 makes it is a bit easier to visualize the highwater progressing northwards towards the standard port at Namely Habour, then proceeding up the estuary to Emsbourne. If we want to know the time of high or low water at a secondary port we can consult the secondary port information boxes and apply them to the standard port tide tables and curves. The resulting adjusted times and heights can then be used directly on the ‘borrowed’ curve of the standard port to find tidal heights and times for its secondary ports.
Interpolation of secondary port differences
We now need to apply the differences to calculate the times and heights of tide at Port Rampton.
The proforma graphs are used for interpolation to avoid mental arithmetic. If you make mistake it is easier to see the error using a graphical approach and less likely to make significant mistakes.
Secondary ports are defined by the RYA as a Yachtmaster level skill and for Day Skipper level you are only required to have an awareness of secondary ports and the differences involved.
It is important to note the differences are given in 24 hour notation, so 0110 is one hour and ten minutes and not 110 minutes.
We can use a proforma to help calculate the time and height of tides at secondary ports. The converted times and heights can then be used directly on the tidal curves for the standard ports. Remember, whenever we are working with tide tables, tidal curves and secondary ports differences etc need to work in UT and not in Daylight Saving TIme.
To calculate the time of HW and LW at secondary port of Port Rampton on the 19th March we need to examine the secondary port information box for Port Rampton (Fig 5.54) and the tide tables for its Standard Port, Namely Harbour (Fig 5.55).
HIgh Water Secondary Port Times
If we want to determine the high water time on the afternoon of the 19th March at Port Rampton we look at the tide tables for Namely Harbour and see that the high tide in afternoon there is at 1452 UT.
The High Water at Namely Harbour is 1452 UT so we have to interpolate between the 1200 UT and 1800 UT secondary port differences which should be somewhere between 16 and 35 minutes before the High Water at Namely Harbour.
Low Water Secondary Port Times
We can do the same for Low Water and see from the secondary port differences that when it is Low Water at Namely Harbour
When Namely Harbour Low Water is at 1800 UT, Port Rampton Low Water is 10 minutes before (-10)
When Namely Harbour Low Water is 0000 UT, Port Rampton Low Water is 6 minutes before.
The Low Water we are looking for 2031 UT so we have to interpolate between the 1800 UT and 0000 UT secondary port differences and should be somewhere between 6 and 10 minutes before Low Water at Namely Harbour.
Secondary Port Heights of High and Low Water
The Secondary Port information box also tells us the differences of tidal height between Namely Harbour and Port Rampton. On the 19th March the height of High Water at Namely Harbour is 3.7 metres and can see from Port Rampton secondary port information that when we havea tidal hieght of 4.0m at namely Harbour the tide in Port Rampton will be 0.5 metres higher (+0.5)
However, the height of High Water we are looking for on the 19th March is 3.7 metres and therefore must interpolate to find the difference at Port Rampton.
The height of tide at Namely Harbour is 3.7 metres which is a mid range tide, with a high water level between springs and neaps and must therefore interpolate between the MHWS +0.5 and MHWN +0.6 differences
For the 19th March tide table LW of 0.7 metres at Namely harbour we can see that it is a mid range Low Water and must interpolate between the MLWS +0.2 and MLWN +0.6 differences.