The 1D Moth according to the Moth Guru

April 7, 2020
April 7, 2020 dori

Kevin Ellway is the Guru of Moth sails. He is the owner and expert behind Ellway Aero-hydrodynamic Designs. His area of expertise is the analysis and design of high performance sail boats, with a special emphasis on foiling boats and sailboards. He is focused on science and number when he talks about design, shapes, wind and water, however this is probably his cover story and we think that he is some sort of shaman, guru, god or supernatural human who controls these forces. He just simply knows too much!

All in all, we’ve invited him to the design of our 1D Moth Sail to help with the aerodynamic design and performance optimisation. Here is what Kevin has to say on the end result!

I’ve been invited to the design process of the 1D Sails team for the design and prototype production of the 1D Moth Sail.

The aerodynamic design process is to determine the optimum sail shape, in terms of camber and twist, for any given point of sailing and wind strength.

In most conditions, Moths are fully powered (righting moment limited), even downwind.

The task is, therefore, to optimise the shape to produce the maximum forward drive force for a given heeling moment.

The forward drive force depends on the drag. When the sail becomes over powered upwind, drag from flow separation (areas where the tell tales don’t fly) becomes of increasing importance. A smooth efficient luff tube helps to reduce the windward separation drag.

The heeling moment depends strongly on the centre of effort. As the wind increases, or as you feather the boat in a gust, we want the CoE to move down

This requires the shape of the sail to change considerably from upwind to downwind, and also to become more bladed upwind as the wind increases.

There are obviously design constraints. There are minimum and maximum sail cambers that can be achieved in practice on a soft sail. In addition, the bend characteristics of the mast also determine how much the sail shape can be changed with the controls. Also, we’ve decided to base the sail to be compatible with existing masts (e.g. CTECH 3B, 3C, 4C, CST Tow Pro).

Given these factors and constraints, the ideal sail flying shapes were determined using a combination of 2D and 3D CFD techniques coupled with a proprietary Moth VPP (velocity prediction program). Upwind performance has been prioritised, while the sail remains fast and efficient downwind.

Click the image to see the flow moving

Determining the optimum flying shapes, although complex, is only half of the equation. Next, we have to achieve the same results on the water. And that is where the skill and experience of Marton Balázs, chief designer at 1D sails comes in.

I had long been frustrated by the luff tubes on moth sails. Compared with racing windsurf sails, which have really efficient wing masts like luff sleeves, moth sleeves tend to be aerodynamically poor and little better than a bolt rope, in many cases.

In addition, many sails rig board flat in the dinghy park, but blow full as they become loaded. This makes the sails hard to use, especially in gusty conditions. This is because as the wind lulls, the sail goes flat and can stall, dumping the sailor into the windward.

So I wanted a sail with a more locked and stable shape and with a really efficient luff sleeve. This is where Marton’s experience with both high performance dinghy and windsurf sails has been essential.

We have achieved a sail that maintains its shape under load and with a super-efficient luff tube which improves drive and reduces drag, especially upwind when overpowered. On the water, this results in a wing like sail that drives forward in every gust. 

As test sailor Carlo d’Ambrosi commented, the sail allows to ‘point higher and go faster than I’ve ever gone before’.

The aero sleeve improves performance upwind, especially when overpowered. The aero sleeve acts like a wing mast – note that all the tell tales on the luff tube are flying indicating fully attached flow.

Want to know more about the 1D EMD20 moth sail? Read ore HERE.

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