General design notes

Why String?

Membrane sails, sometimes called string sails and are often given all kinds of wonderful names by other lofts, are designed with the structure and shape in mind. The amount of fibre (string) used in the sail is varied by the designer and this variation can be used to get different characteristics from the sail. All sails stretch but the degree of stretch can be controlled to a much higher degree compared to more conventional sails.

Low stretch is not always the end goal and having a progressive feel through the controls is also important - often this is achieved by allowing more stretch. So some stretch here and less stretch there will also lend itself to achieving the desired shape and handling characteristic.

Our string sails for the Moth are 100% Technora fibre with 1.5 mil of film. Our relative high DPI in the loaded areas of the sail is giving great life as a welcome by-product of achieving the desired stretch characteristic.


As we work with some great sailors who are committed to being the best they possibly can be - we want to produce sails to help them - and so we can offer the best product to market. To get to that position we do produce a reasonable number of sails. These sails are often sold on into the general market. This can paint a confusing picture as to the number of designs we offer. We do not release new designs until the testing has reached its goal and the sail has produced the result intended. The new design will then be given a production code like A2m ( All purpose design 2 - membrane cloth). Sails designed for specific conditions like light winds will carry an L pre fix and H for heavy winds. Second hand development sails are exactly that and should not be confused with our standard designs. Ex-development sails will not have a production code.

Non Standard - Standard Sails

Although much of the development for our designs has been done on low stump rigs, we also offer those same designs to fit high stump boats. This change will result in a different looking sail as leech roach is removed when making the leech length longer. However, the luff curve and mould section remain exactly the same. We also offer all our designs with 2 sleeve cut outs or 3 for the upper beak if required.

Low Rig Options

Lennon Sails has been working on lowered rigs since Chris Rast brought his “S Boom” sail to the 2015 Worlds in Australia. Chris got his best result in a Moth Worlds finishing 7th overall. Since then we have made others along the same lines.

In early 2017 Lennon Sails started to experiment with lowered sails for the Thinnair Moth project and produced a double skin sail meant for use at the 2017 Worlds, but due to delays in project this concept was put in hold to concentrate on perfecting the A5m for Paul Goodison’s stunning victory at the Garda Worlds of that year. It became clear to Paul during that event that there might be some milage in pursuing the lowered rig and so over the winter we made several double skin “Deck sweepers” based on the A5m for Paul.

Dylan Fletcher has also wanted to pursue this concept for some time and we also produced a version of the A3.5m in “Deck sweeper” format.



Above: Benoit Marie using an A4m “S Boom” design

 The concept is fairly straight forward. Most of the benefit comes from simply lowering the rig. The mast head typically comes down 200mm for the “normal“ rig height while maintaining max luff length. A small amount of area is borrowed from the back of the sail to make the part that “sweeps” the deck. To actually get this to happen you need a deck fairing but not having one has little effect on getting the benefits.

Managing this extra section of sail is tricky as the boom distorts the area. So it can look untidy but the benefits can be felt as soon as you sheet in for the first time! To make a decksweeper work you need the distance from the top of the foredeck to the mast tip to be around 5200mm. Ideally the boom will be fitted to a mast gooseneck fitting with the bolt countersunk or very close to flush. The opening for the hounds and spreader are usually placed in the same position relative to the top of the foredeck so, for example, if the mast is 200mm shorter the mast fitting are 200 closer to the mast tip.

To work out what mast length you need simply measure the height of your king post and deduct it from 5200mm. Around the lowest batten on the sail above the boom we split the sail into 3 parts. The inner part is in effect a conventional Moth sail with a cunningham strap ready to take your current cunningham set up. The outer layers cover this part of the sail and split over each side of the boom. For boats with wing compression struts we have to make a cut out in the tack area for the struts.

When to go Sweeping or Conventional?

So far it seems as soon as you are solidly on the foils the decksweeper sails are faster. But in marginal foiling conditions conventional sails are earlier up. The combination of the mast tip been higher (in less ground effected air), and having more to lean against in the black art of coaxing the boat in to the air, does offer earlier foiling opportunities. There are also questions around how aero efficient the bit of cloth under the boom is compared to having it on the back of the sail.

There are more options now with lowered non DS or “conventional looking lower aspect designs. These are a good compromise and indications are they offer most of the benefits of the full DS rig with reduced downsides. See “mast appendix” for more details on this.

2018 Moth Worlds Results

1st Paul Goodison A5mDS / A5m Inter
3rd Rome Kirby A5m Standard
4th Brad Funk AH1mDS / A4m
5th Victor Diaz de Leon AH1mDS / A3.5m
7th Benoit Marie A4m “S Boom”


Brad funk using the AH1mDS at the 2018 worlds



Paul Goodison winning the 2018 worlds with an earlier version of the A5m DS


General Features Standard Specification List

Teflon super slide front luff panel
Reinforced window panels.
Leech end batten vernier adjustment
Injection moulded cams - long and short /locking version
Carbon special tapered battens (glass battens alternative available)
Fitted sail numbers
Rolled sail bag
Vernier adjuster key
Cam spacers
Door to door delivery included for the EU and US only. Other areas will incur a shipping charge.

All Standard Type Membrane Moth sails.

Complete sail as listed above.

All Deck Sweeper Type Membrane Moth Sails (DS).

Complete sail as listed above.

For sails supplied with no battens and or no cams please contact us for pricing.

Cams and battens sets when sold separately.

Long or short 3D cams with integral bulkhead. Set £215.40 Individual £39.00 each.

Full carbon battens.

Set £307.00 Individual #1 £29.64, #2 £29.64, #3 £36.37, #4 £44.46, #5 £51.19, #6 £56.58, #7 £59.27
All Prices exclude local tax and VAT.



“Inter rig” 5200mm mast on test - set on a Thinnair platform - CST E22 “inter”and standard15º boom.


Mast Appendix

We work closely with both CST and Ctech who both have several very good mast designs available for the Moth.

The A5m, Ah1mDS and A3.5m designs will work successfully on most of the current popular masts. The standard luff curve works on the CST 21 and 22 and the Ctech 03 and 03b. Mach 2 medium masts also work on this design.

The above mast designs have quite a large difference of total max bend - using the IMCS system they vary from 52 to 67 so having the right mast for your weight is important when looking at this number. However, just as important is the “flex curve”. This is the differential between the bend offset at 25% and 75%. It is this number the luff curve must match.

If the difference is 2% between the upper and lower points - then the flex curve is expressed simply as 2.

This only informs part of the story however. The bend at 50% is expressed as 100% , then the bend at 25% and 75% will be less (normally).
Typically the bend at 25% will be around 67% to 70% of the mid bend and the top bend (75%) will be 72% to 76% of the mid number.

As you can see though, a 69% /73% giving a “flex curve” of 4 won’t be the same as mast with a curve of 80%/84% also having a “flex curve” of 4.

However, having said all that, most of the current popular designs are pretty close to 4 having 69/73 curve.

Mast and Sail Options March 2019.
The current trend is towards low rigs where mast heights are dropping by as much 300mm from the standard rigs of 2017. Below is brief overview of where we are now in early 2019.

Lennon sails are offering sail derivatives to fit the following “off the shelf” masts.

Full Height Standard/Traditional mast.
Range 5290 - 5370mm with standard fittings positions.
Sails to fit this. A5m std / A5m std short leech / A3.5m


Intermediate Rig (Inter).
Range 5200 to 5250 with standard fitting positions (Ctech masts have some fittings variations).

Sails to fit this. A5m “inter”/A5m inter short leech.

The A5m inter has its luff and leech reduced by 70mm from the std sail lengths. The area is added back via a longer foot length.

Deck Sweeper (DS),
Range 5100 and less. Fittings are 200mm closer to the mast tip.

Sails to fit this. A5mDS / AH1mDS / A3.5mDS. These 3 sails are full deck sweeper designs with panels overlapping and dropping below the boom. The A3.5m DS is the most extreme being the flattest and lowest maximising the under boom area to get a bigger endplate effect. We are also supplying non DS type to fit the DS rigs (“low aspect standard type”). So an ultra low standard looking sail. These are not yet fully
tested but offer a reduced downwind and take off deficit compared to a full DS sail set on the same mast.

It’s getting more complex but also offering good options to accommodate a larger weight range. Light weight sailors (sub 70kg) can get on the DS sails early or all the time if very light giving them a wider competitive wind range.

If you only want one rig?
65Kg and under go DS - Lighter sailors may find the flatter based A3.5 design is a better suited.
65kg to 70kg. The low aspect standard type sail fitting the DS mast.
70Kg to 77kg. Intermediate rig.
77 and above - Low standard (5300mm).

All the current sail designs need a longer boom than older length from 3 years ago. Whilst the new standard lengths accommodate the standard and DS sails in having the clew eye close to the leech edge, the low aspect designs are much longer in the foot, requiring the clew to be set well inside the leech edge. So a low aspect sail on an old boom can become problematic as the clew eye ends up way too far forward of the leech edge.
They can however accommodate the DS and standard sails by setting the clew inboard a reasonable amount.

Mast note
We work closely with CST and Ctech and have been providing sails with layout changes to suit personal preferences. Lennon sails won major championships in 2018 with both CST and CTech masts.

Lennon sails provide Ctech Carbon sail battens as standard which are included in the price along with our new injection moulded cams - lighter and stronger than 3d printed.