Racing To The Lonely Tower And Back
A Bit Of Nab Tower History
For those of you that don’t know the Solent the Nab Tower is a large navigation mark in the Eastern approaches to Portsmouth.
It has an interesting history. It was originally designed as one of a series of forts to be built across the English Channel during World War 1. The structure that makes up the tower was built at Shoreham by a team of 3000 workman. It originally had a concrete floating base with a steel superstructure.
The aim was to tow the seven tower structures out into the English channel, sink them in place in a line, then link the structures with steel boom nets. The idea being to close the English Channel to enemy shipping – particularly submarines. The tower cost £1m to build in 1918. Only one of the towers was actually completed. It was never deployed in anger.
In 1920 (on a very calm day by all accounts) it was towed into position above the Nab Rock off the Eastern End of the Solent. It replaced an existing light ship.
The 20 plus metre tower consisted of a series of water tight compartments that made it float and a steel and concrete upper structure. Flooding the lower compartments caused it to settle in place on the Nab Rock; all be it at an angle of 3 degrees. Something I hadn’t realised until I went looking at the history of what I assumed was just a big lump of concrete. Since 1920 the tower has had a number of upgrades and refurbishments. Originally it was a manned lighthouse, today its automated. The steel structure has been replaced and a helicopter pad installed on the roof.
You can find more about Nab Tower at:
Trinity House – The Nab Tower
Thats enough history for now.
JOG’s Nab Tower Race
This article is a reworking of race summary we originally produced after the 2018 race. It shows how we have advanced the functionality within Rockit to make it a more useful analytical tool.
JOG or the Junior Offshore Group have been running a race out to the Nab Tower as a season open for at least as long as I’ve been a member. Its a great season opener.
JOG’s Nab Tower event in 2018 was our first serious race of the season.
Its a pretty simple idea. Leave Cowes at about 9.30am. Sail East down the Solent with the tide, pass between the forts off Portsmouth then head South East to the Tower. Round the Tower to Port, Sail back west to the Winner Navigation mark on the approaches to Portsmouth then back to Cowes. Remembering to leave Snowdon to starboard on the way out and port on the way back. If the breeze is good then the tide should turn soon after you round the Tower. Its roughly 35 miles there and back. A good early season warm up that if the weather is nice gets a good turnout.
You can sign up to this years Nab Tower Race (now know as the Lonely Tower Race) on the JOG Website using the button below.
This years Nab Tower race is on Saturday 25th March.
In 2018 we were into our 3rd season sailing Jengu; still learning and fixing things that didn’t quite work.
As usual most of the crew met up Friday afternoon to sail Jengu over to Cowes for the race start on the Saturday morning. After rigging the boat up we headed out into the Solent passing Hugo Boss on the way in. After a brief man overboard drill to recover my hat we got down to the serious work of hoisting and dropping Spinnakers. The gentle breeze was ideal for spinnaker practice giving us plenty of time to get the choreography right.
At that point our North A1.5 had been unused in the shed for the preceding 2 years. It came with the boat and I wasn’t quite sure what it was.
According to the North website the A1.5 is ideal for 0-10knots of wind and good when the sea is lumpy up to 12knots true. A lazy Friday with 0-6 knots true looked like the ideal day to take it for a first spin.
It turned out we’d had this beautiful white asymmetric spinnaker sitting in the shed all that time without realising it. It flew beautifully in the light wind and rolls over to windward letting us sail quite deep angles. After much gybing dodging ferries and tankers we headed to Cowes for the evening feeling confident about our spinnaker handling.
Starting Low And Fast
Saturday dawned with none of Fridays sunshine, but a little more breeze from the East. So as predicted we were looking at a long beat to the Tower followed by a deep reach or run back to Cowes.
Our start was OK, safe and close to the Gurnard cardinal in the deep water.
We had learned to be conservative at the JOG start line; with a 2knot tide over the line coming back after an over the line start is really bad news. You can see the 2.25knot East going tide on the start line in the screen grab; look at data point #20.
We were probably row two on the grid, we sailed relatively low and fast on starboard for the first mile East from Cowes.
Rolling a couple of faster starting but slower boats in the process losing a bit of ground to windward on the boats that had started closer to shore but getting into a position where we were in clear air.
We had a couple of benchmark boats around us to compare our progress against; Jybe Talking the J109 of Chris Burleigh and Hot Rats a First 35 from Hamble.
Sailing In High Mode
Once clear of the first group of boats we started to sail in a higher “mode” and this is where it got interesting. Sailing high looks like the data in this image on the right. Data point #56 would be the ideal we have a good VMG and are sailing at a relatively high Apparent Wind Angle (<28degrees).
The VMG is a valid number to take as a indicator of performance as we are on a beat into the wind.
The other factor to consider on a beat is the theoretical hull speed of the boat. The is a limit to how fast a boat will go in displacement mode. Most boats (foiling America’s cup boats are among the exceptions) don’t go faster to windward than the hull speed. The speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel.
Effectively the bow and stern wave stop the boat going any faster … until it starts to plane, which a J109 won’t do or surf which they will definitely do downwind !
You can calculate your boats Hull speed using the formula Velocity in knots = 1.35 x the square root of the waterline length in feet.
For a 35 foot J109 the hull speed is going to be 7.98knots. So for Speed Through The Water (STW) anything over 7knots is going to be approaching that limitation.
We experimented with Jib In-Haulers briefly with the No. 3 in 2017 but didn’t have much success. In 2018 with a new Elvstrom C1 non overlapping Jib we could in-haul the Jib a lot further. The theory is if you in-haul the jib you get to sail a higher angle because the sail is sheeted closer to the centre line; you also get more twist in the jib with the upper leach freer.
Jibe Talking with a similar sail plan, IRC 1.006 versus Jengu’s 1.01 (at the time), were using the windward jib sheet to in haul the Jib.
In 2017 we had a big issue with the Jib back winding the luff of the main sail when in-hauled. We had resolved that problem by sailing with a much tighter outhaul and correspondingly flatter mainsail. The boats balance is very good with that sail plan; we can sail the boat to windward with no hands on the wheel.
With the breeze touching 10-12knots at times we were able to maintain the gap to Jibe Talking sailing between 27 and 30 degrees to the Apparent wind. AWA is Apparent Wind Angle on the screen grab.
How Close To Peak Performance Are We ?
One of the things we can do with Rockit now is find out the the boat configuration for our peak performance.
Rockit takes the True Wind Speed and Angle, trawls its database and finds the point in time when the boat best performed for that combination. Because it holds the corresponding boat configuration it can tell us exactly what was fast in the past. These timeslots with a new out of the box head sail are still some of the best performance stats.
For the first 30 minutes with the breeze staying stronger we were able to maintain and slightly build the gap to Jibe Talking. They started further to leeward, slightly astern and settled a hundred metres or so back.
Then the breeze started to drop off a couple of knots to the 7-8knot range at which point Jibe Talking were able to sail slightly higher and break out of our dirty air. Whilst we could keep them behind by sailing slightly lower, in a straight who can point higher in the light stuff competition we got resoundingly beaten. By the time we got to Gilkicker Point off Portsmouth, Jibe Talking were ahead.
At that point we tacked across the deep water to the Island shore. In reality we had sailed most of the Cowes – Portsmouth leg in 16metres of water at the edge of the deep-water channel.
With hindsight we probably should have tacked further out into the Solent to the north of Ryde Middle to get the best of the East going tide. Its a trade off; the cost of two tacks to get to the deep water versus the extra .5knots we would get from being in the faster tide. Pointing high probably wasn’t the solution. We gave time away to the boats in the deeper water and didn’t gain enough ground to do anything other than stay at the edge of the deep water out of the shallows. Tacking across the East going tide we gained the advantage of the lee bow tide pushing us upwind.
Another tack saw us out through the forts heading South East for the tower. Our other benchmark boat Hot Rats was still behind us so whilst we had lost touch with Jibe Talking we were holding our position against the rest of the fleet. A couple more tacks staying in the deep water away from the island shore saw us reach the Tower.
Rounding The Nab Tower
In reality we probably over stood slightly; we able to came into the tower fast and carried the speed into the mark rounding.
Anyone who has sailed near to the Solent forts will confirm they have a surprisingly large wind shadow. Maybe its due to their round shape but they project a large zone of disturbed air. Add to that 50 or so sailing boats; there is a huge potential to get stuck mid rounding and drop a couple of places.
A lot of other boats were giving the tower a very wide berth, with our boat speed we able to turn relatively tightly around the tower and get a good line on the exit.
WARNING
As a footnote this is Jengu rounding the Tower in 2020. Being very conscious that we (I) had missed the apex of the corner the previous time we did the race we (I) threw caution to the wind and got probably a lot closer that we should have !
Image curtesy of Rick Tomlinson.
A good spinnaker hoist of the A2 not A1.5 set us off on the return leg. The wind had picked up to 12-15 knots, ideal for blasting downwind under the heavier A2. The speed topped out at 10 knots which felt quite fast.
We kept Hot Rats behind us all the way to the Gybe mark at the Winner navigation mark and got the inside line on one of the double handed Fast 3200’s gaining a couple of places going towards the forts. The Gybe looks like this:
We reached the line between the two forts in good shape solidly mid-field; then the wind went very light.
This is a recurring theme; we were quite good in the 7-15knot range, not particularly brave in 20knots plus and lost at sea in anything under 5 knots. We clearly have an issue getting the boat going in the really light stuff. The wind dropped, the spinnaker collapsed, the mainsail flapped in the wash from the passing boats and the 1knot + East going tide pushed us back towards the Tower.
You can see from the track that the boat speed dropped right off.
Its the classic dilemma;
Do you follow the bulk of the fleet that has just made it around the fort on the Island side but parked up; hoping to sail on the Island shore along Ryde Sands in a load of dirty air ?
Or do you go the other side and pick up the earlier change in the tide on the Lee-On-Solent side.
We went north mostly because thats the way the boat wanted to go and to a certain extent once North of the mainland side fort thats the way the tide takes you into the shallows off Portsmouth. We got to maybe 250 metres South East of the new Navigation marks for HMS Queen Elizabeth when the breeze completely shutdown. We were back to drifting around where we had been the previous evening.
The screen grab from the Navionics Web App shows the dilemma. The breeze shutdown when we reached a point more of less equal distance between Horse Sand and No Mans Land Fort.
Time to break out the North A1.5
The great advantage to no wind is it makes spinnaker changes a lot easier. Dump one down the hatch pull the other one out of the bag.
After much gentle coxing and quietly cursing the power boats leaving a wash coming out of Portsmouth we go the boat sailing in the right direction. Once again the big white sail proved more than happy to take us in the right direction.
The tide started to turn as we started moving.
With the big separation between the half of the fleet that went north with us and the bulk that went south it wasn’t clear where we would re-join. We pointed the boat at Gurnard and did our best to keep the boat speed up.
Gently the breeze increased; now we were getting into the theoretical upper range for the A1.5 it being a bit of an unknown quantity we were waiting for it to explode in a million pieces but it held together beautifully.
Somewhere around Norris we re-joined the main fleet; and I think part of the Warsash Spring Championship fleet as well. We had a close port and starboard pass with Night Owl 2; us on Port flying our spinnaker, Night Owl on starboard also flying their spinnaker. We crossed ahead by maybe 50 yards but at one point I thought we might have had to gybe out of the way which wouldn’t have helped our cause.
Coming down to the finish line we struggled to pass North of the Snowdon buoy and had to do a short Gybe onto Starboard to gain some ground to seaward. Gybing back we raced the last mile or so into the finish with one of the Fast 3200 All or nothing.
By the time we got to Snowdon the tide was running fast to the west carrying us quickly towards the final mark.
We had to negotiate a large very slow moving cruising Beneteau that for some reason had decided to raise its cruising chute in the approach to Cowes; their spinnaker handling needed practice, we were away and over the finish line by the time they got the sail flying.
Reviewing the Session Polar Data
Looking at the session level information we could see from the Polars that we were relatively fast.
The number in brackets is the fastest that the boat has sailed on that True Wind direction/True wind speed combination on average during a session. If we are getting it right as a crew we should be matching the figures in brackets which would turn the numbers pink. Green isn’t too shabby either.
Rockit calculates 3 numbers.
What you did this session. Based on an average of the timeslots for a session split according to TWS and TWA.
What you did during the best session. Again looking at the timeslots for a session split according to TWS and TWA. So it finds all the sessions where you have Polar data for a given TWS/TWA and pulls out the best number.
The best ever figure for a given TWS/TWA combination. The number you get when you do a monster surf for half minute and everything goes quiet before you wipe out at the bottom. This isn’t an average number but it does give an aspirational figure for what you might be able to achieve as an average if you didn’t wipe out when you hit the wave in front.
You can prepopulate polar values using VPP data.
Overall the more data we collect the better idea we have of how we can expect the boat to perform.
Looking at the Sail Data for the A2 and A1.5
Using the Boat Configuration functionality within Rockit we can record the sails that are in use at any given point in time. That means that we can then relate sail usage to boat performance and plot the data out.
This graph shows the performance of the boat using the A2 versus the A1.5. The red dots are the A2 the A1.5 is blue. It shows pretty clearly that the A2 is quicker in the stronger breeze. The picture in the lighter breeze is less clear as we have less data.
The image is at the session level; only values recorded for that Nab Tower Race. Its also for a wide True Wind Angle Range; 90-180degrees.
4 years on we have a lot more data and looking at the same combination at the boat level rather than the individual session the pattern is slightly different.
The A1.5 is significantly quicker at the lower wind speed level; higher up the range the picture is less clear but realistically thats beyond the safe wind range of the sail and we probably wouldn’t use it regularly in the plus 10 knots TWS range.
The polar image is from the North Sails website and shows where they target the various sails. The is the current chart our A1.5 is at least 10 years old and sail design definitely moves on.
The other thing to consider is that North’s even numbered sails are runners, designed to sail low whereas the odd numbered sails are reachers design to go higher. So if we were sailing strict windward leeward courses we would leave the A1.5 Reacher at home. The A1.5 is an offshore sail.
It is good to know the A1.5 will survive the higher wind speeds though.
So What Did We Learn ?
We had good boat speed in the 8-15knot TWS range.
Our spinnaker handling was much better and the results were more predictable.
Our tacking was much, much better. We can be up and running on the new tack with the Elvstrom C1 a lot quicker than we ever managed with the huge overlapped No 1 from North.
The A1.5/A2 spinnakers are a good combination of sails to have in the wardrobe.
The depth gauge needed checking. We were getting an extra two metres of water on deck compared to the reading on the chart plotter below. We replaced the Nexus server box that runs the on deck instruments and think that we missed a setting somewhere on the server. The plotter is measuring from the bottom of the keel; the deck displays from the actual transducer. Its a 2 metre difference and something we need to fix before we go to the Western Solent for the next set of races.
We needed to spend more time sailing the boat in the really light airs and work out how to get the boat moving again once its stalled. Something that became apparent during the Spring series race we did on the Sunday.
Out next race was the JOG Cowes – Yarmouth – Cowes weekend the third weekend in May. It would be interesting to see if we can work our way up the fleet then – hopefully we will have a more consistent breeze…putting into practice what we had learned.
Originally published May 2 2018
Footnote
So we went to the JOG Cowes Yarmouth weekend with great expectations…. and came 3rd.