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Moth Worlds – Moth speed uncovered

by Chris Kitchen on 25 Jul 2017
Third Championship win for Chris Rashley & C-Tech at the Moth Europeans 2013. Chris sails the Exocet Moth, the result of three years design during which Ellway Aero-Hydrodynamics have worked closely with C-Tech developing the winning rig. Chris is also the UK National Moth champion and ranks 7th in the World. C-TECH
With the 2017 Moth world championship starting Tuesday, C-Tech’s Tim Willetts takes time to reflect on changes in Moth class rig design and development over the past six years. A lot has happened behind the scenes that is giving the top sailors the edge by staying ahead.

There have been two clear periods of development where ideas were explored and the performance of the class improved. Willetts who is passionate about the Moth says it is a unique and exciting development class to work with.
‘Because the Moth is so sensitive to weight and small design changes, it has been an excellent opportunity to work with the world’s best and learn. The class enables a fast turnaround between new ideas, design and testing which has accelerated progression’ This fast paced environment has kept the Moth at the forefront of international dinghy development, and it remains one of the highest performing classes on foils.

C-Tech has been lucky to work with many top Moth sailors over the years. It’s team of sponsored sailors going into the 2017 Moth World Championships at Lake Garda includes Paul Goodison, Rob Greenhalgh, Dan Ward, Scott Babbage, Rob Gough, Philipp Buhl and Jim McMillan. A significant proportion of the fleet is also using C-Tech components.

2010-2015: Figuring out the Flex Curve – Two Extremes

During this period design trends were extremely varied, every sailor had a different idea and each were chasing a different path for speed. There were two main schools of thought; the first running a mast with a more flexible tip, much like a traditional skiff rig; and the second running an even flex curve where the top and bottom of the mast bent evenly.
C-Tech entered the game with a strong background in skiffs, so it was a good match for us to get started with the flexible tip rigs. The first mast designs were essentially a dumbed-down skiff rig. Although the rigs proved to be very fast through the water, it came at the cost of upwind height and the rigs were only faster in narrow wind ranges.

The other development path was going for a more even mast curve. Because the sails were manufactured using lower grade materials with a reasonable amount of stretch and give, they worked well with these masts and were quite forgiving with varying seam shape, luff curve and mast stiffness. While this set up performed particularly well in the lower half of the wind range, there was a disadvantage in the upper wind range where the sail would open at the lower leech first which was not desirable.

During this time there was not a huge amount of science behind the design trends compared to today, the process was more trial and error. We worked closely with Kevin Ellway from Ellway Aero-Hydrodynamic Design, who is designer of the Exocet Moth and has had a large part to play in development of the Moth class in the UK. ‘I introduced the use of IMCS mast stiffness and flex ratings as an objective method for comparing masts – it comes from windsurfing. It’s not great, but was better than having no standards at all!’ says Ellway. ‘The change from high flex to 0 occurred with the advent of skinny (42mm OD) masts which replaced the 50mm+ diameter masts used on the Bladeriders. Astonishingly, the same sails were used for both types of mast despite the huge difference in flex curve.’

Over time sails tended to move towards less seam shape and more luff curve, and sailors were using multiple sails on the same mast. It was not uncommon for a sailor to turn up to a competition with 1 mast, 4 sails and 2 sets of battens.
With 8 possible combinations it made picking the right gear difficult. And as we know, the less rig options you have the easier the decision is! These decisions were to be made on top of what foils to use. Your day could be over before you even started the race if the wrong gear was selected!

During this period the Moth fleet was on a massive rig development learning curve and eventually we figured out what was working, what wasn’t working, and the science behind it. With the improvement in materials and more sophisticated design software the schools of thought moved towards each other and resulted in the beginning of the current rigs we see today.

2015-2017: The modern 4.5% Flex Curve Era

The move towards a rig with a small flex curve of 4.5% made the most of the even curve mast, yet the softer tip ensured the leech opened first at the head when things got overpowered. The breakthrough for C-Tech came in 2015 at the UK Nationals where Chris Rashley used the brand new C-Tech 15° bent boom and 03 mast and completely dominated the series winning 11 out of 12 races. It was clear the new rig set up was a step forward.

Ellway comments ‘Chris Rashley and I, in conjunction with Lennon Sails, moved to sails which have more luff round and less seam shape – this came from my CFD work. A consequence of this is that the sails are more mast specific now than they used to be.’


As sail membrane materials became stiffer and more stable, so too did the masts. A number of things happened.
This increase in overall stiffness meant there was less give in the whole rig set up so the boats became more difficult and tweaky to sail upwind. The vang purchase increased from 24:1 to 54:1 to get enough leverage to control the leech of the sail. Sails have less luff curve and more seam shape and the cunningham has become a critical control for tuning the rig. The stiff rig is very sensitive to small cunningham adjustments and can significantly affect the performance of the sail.
We have seen definite performance gains upwind particularly in certain wind bands. Up-and-coming sailor Jim McMillan says ‘After switching to the C-Tech O3b UHM mast I have definitely found an improvement in my upwind speed and is the lightest mast on the market’. The trade-off is that the boats have become far more difficult to get them in the groove – and adapt to changes in the conditions without falling out of the groove. In some cases a sailor is at times better off on an older set up that is more forgiving. Because the boats have high average speeds – if you are not moded correctly you can get severely punished.

Downwind however, has seen big performance improvements with the stiff set up. With the rig and sail shape stiffer and more stable you are able to really ‘grunt’ up the boat and the speed shows round the race course.
The other change with the new set up is sailors are now carrying 1 main, 2 masts and sometimes 2 sets of battens. The masts have the same flex curves and just different stiffness’s to accommodate for wind strength or sailor weight. It has led to some changes in battens.

In lower wind ranges battens used are generally the traditional ~40-45% draft position allowing the leech to round up a little to generate power. For upper wind ranges a lot of development has been necessary to get the sail optimally performing. With very high vang loads there is a high tension load through the sail from the tip of the mast down to the clew of the sail effectively driving the sail to fold at that point. To resist this and help with a straighter sail exit, battens have been stiffened in the back half. They now have an effective draft position of ~35%. There is still developments and improvements to be made in these areas but the trends are clear. We are still working on the trade-off in performance downwind with the draft forward battens.

More High Modulus Fibre and Kinked Booms

Since 2015 when the masts and sails got significantly stiffer, booms, wing tubes and mast posts have had to become stiffer as well. We are now using a lot more high modulus (HM) fibre in our products and in more and more cases ultra-high modulus (UHM) fibre. All the wing tubes have become smaller and more aerodynamic, meaning there is no other option but to use (U)HM carbon fibre to maintain stiffness.

In 2015 kinked booms were developed to allow more space for sailors to move and therefore improving boat handling, kinked booms have also allowed a longer leech which provides a small aero benefit.

C-Tech has further developed the boom to incorporate a tapered aft section, optimising the booms stiffness to weight ratio accounting for the changing loads across the boom.

Cleaner Aero

With everything on these boats being pushed to the limit, weight is a massive factor. If you can reduce the weight it means a smaller foil and less drag – therefore more speed. We recently worked on a project with Paul Goodison to clean up the aerodynamics with super light weight wing bar fairings. ‘I was looking at trying to make my Exocet more Aero and the main area was a rear bar fairing,’ says Goodison. ‘C-Tech were able not only to add an aero fairing but also reduce the weight by using UHM carbon.’

Future Developments

C-Tech has worked on a number of development projects with its clients including two Moth wing masts. Both projects showed promise in narrow wind ranges, but the challenges were in the control systems. More development is required to get these rigs up to a viable level; our sailors have chosen to go a different track for now. We are comparing them to the current pocket luff setup that has been seriously refined over time with a huge amount of people and resource involved. The question is who is bold enough to take on the task of developing the wing mast rig to a point it overtakes the current set up.

Another exciting rig we have been working with is that of Rob Goughs. He has developed a wishbone rig that he may be using for the Moth Worlds in Garda. The biggest advantage of this rig is the fact you can achieve a significant end plate effect at the foot of the sail which is what we have seen on the A-Class catamarans and Americas Cup foiling cats. There are other advantages of the rig being spreaderless (less weight and windage) and also getting away with a lighter boom.

With a project like this it is essential to accurately build what has been modelled on the software simulations. Rob comments ‘I have been super impressed with accuracy of the mast production to the spec provided and the level of finish on the section is first class. It's an absolute pleasure working with C-Tech, the team is knowledgeable and very willing to help people who are thinking outside the box.’

We will be watching closely to see what Rob pulls out of the bag, but also keeping in mind that there is a lot more development that can be done on this set up.

After the worlds C-Tech has a development plan to investigate some ideas that improve performance. Either by reducing weight, reducing windage, increasing power or just making the boats easier to sail. This plan includes looking at a better layout with higher hound positions, spreaderless rigs and shorter masts. You only need to look at the latest Exocet production Moth to see the extensive use of C-Tech high modulus carbon tubes and spars to grasp the extent the class is going to for weight and stiffness gains.

C-Tech is excited to continue to helping the development of the Moth class and push boundaries where possible. Willetts says ‘aside from the challenge, it’s the feedback from our customers that really drives the team at C-Tech. When you get comments like that [below] from sailors and designers at the top of the game it really creates a buzz within our team’.
Rob Greenhalgh: ‘I am very happy with all of my C Tech products to date. The spec is always as expected and the finish is superb with no additional work required. Attention to detail and service have always been top level’

Kevin Ellway: ‘I have always been a staunch advocate of C-Tech. This goes back to sailing Cherubs. The C-Tech prepreg, no crimp layup manufacture method and the fact that you don’t end up with steps in the laminate in the mast’s taper area means you get a higher stiffness to weight ratio. Even more important is that C-Tech masts are more consistent. When we measure the bend on other masts, we found big variations in both flex and stiffness.’

Tim Willetts summarises ‘While the team at C-Tech has had the great privilege to work with so many world class sailors, and have made some great steps forwards in the last few years, we are really looking forward to the future of the Moth class as a great test bed for further innovation and development. When you see the calibre of sailors competing at these worlds it is easy to see why we are so passionate about the class.’

Race 1 of the worlds in Malcesine starts today - 25th July. To follow the event at click here

For more information about C-Tech Moth products please contact us at

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